US20120288052A1 - Nuclear reactor - Google Patents
Nuclear reactor Download PDFInfo
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- US20120288052A1 US20120288052A1 US13/560,275 US201213560275A US2012288052A1 US 20120288052 A1 US20120288052 A1 US 20120288052A1 US 201213560275 A US201213560275 A US 201213560275A US 2012288052 A1 US2012288052 A1 US 2012288052A1
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- fuel
- graphite
- reactor
- reflector
- reactor core
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- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21C—NUCLEAR REACTORS
- G21C1/00—Reactor types
- G21C1/04—Thermal reactors ; Epithermal reactors
- G21C1/06—Heterogeneous reactors, i.e. in which fuel and moderator are separated
- G21C1/07—Pebble-bed reactors; Reactors with granular fuel
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- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21C—NUCLEAR REACTORS
- G21C11/00—Shielding structurally associated with the reactor
- G21C11/06—Reflecting shields, i.e. for minimising loss of neutrons
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- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21C—NUCLEAR REACTORS
- G21C7/00—Control of nuclear reaction
- G21C7/06—Control of nuclear reaction by application of neutron-absorbing material, i.e. material with absorption cross-section very much in excess of reflection cross-section
- G21C7/08—Control of nuclear reaction by application of neutron-absorbing material, i.e. material with absorption cross-section very much in excess of reflection cross-section by displacement of solid control elements, e.g. control rods
- G21C7/10—Construction of control elements
- G21C7/107—Control elements adapted for pebble-bed reactors
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E30/00—Energy generation of nuclear origin
- Y02E30/30—Nuclear fission reactors
Definitions
- the present invention relates to a nuclear reactor, and more particularly to a nuclear reactor that allows for equalization and/or optimization of power distribution in a reactor core.
- Pebble bed reactors have a flow path (fuel region) that is surrounded by a reflector(s). Sphere-shaped fuel bodies (fuel pebbles) containing nuclear fuel material undergo nuclear reaction inside the flow path thereby producing power.
- the nuclear reactors in general to have equalized power distribution in a reactor core.
- the fuel body goes down the flow path and is repeatedly removed and reloaded, whereby the power distribution in the nuclear reactor is equalized in an axial direction (i.e., direction in which the fuel bodies flow in the flow path).
- replacement (removal and reloading) of the fuel bodies is continuously performed, i.e., removed fuel bodies are inspected, and new fuel bodies are loaded.
- a 170,000 kilowatts (kW) class nuclear reactor uses 415,000 fuel bodies, among which 6,000 are removed from the nuclear reactor for inspection every day. Among the removed 6,000 fuel bodies, approximately 600 are replaced with new ones. New fuel bodies are loaded into the nuclear reactor together with the approximately 5,400 remaining fuel bodies. Generally, average life of the fuel body is about two years, and one fuel body is reloaded nine times on average during its life.
- the conventional reactor includes a detecting unit that receives plural fuel spheres discharged from a reactor core and detects burnup of the fuel spheres, and a sorting unit that determines a radial loading position from which the fuel sphere is reloaded into the reactor core according to the result of detection by the detecting unit.
- a fuel body having a hexagonal columnar shape houses fuel compacts.
- Base material of the fuel compact is graphite.
- the fuel compact is filled with coated fuel particles (of approximately 1-mm diameter).
- Nuclear fuel material contained in the fuel particle undergoes nuclear reaction thereby producing power.
- enriched uranium is prepared in twelve different levels of enrichment, for example. Preparation or manufacture of enriched uranium at various levels of enrichment in a large amount pushes up the manufacturing cost of the fuel.
- one object of the present invention is to provide a nuclear reactor which allows for equalization and/or optimization of power distribution in a reactor core.
- a nuclear reactor pebble bed reactor
- a volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflector is determined based on a power distribution of a reactor core in the direction of flow of the fuel pebbles.
- a part of the reflector located at a position with a higher power density contains a higher volume fraction of the moderator having a smaller moderating power than the graphite than a part of the reflector located at a position with a lower power density.
- the reflector contains a higher volume fraction of the moderator having a smaller moderating power than the graphite in a portion located at an upstream side of the flow path of the fuel pebbles than in a portion located at a downstream side of the flow path of the fuel pebbles.
- the reflector includes an inner reflector and an outer reflector that surrounds a reactor core, the flow path of the fuel pebbles is surrounded by an outer circumference of the inner reflector and an inner circumference of the outer reflector, and a volume fraction of the moderator having a smaller moderating power than the graphite is set equal to or higher than 25% in at least an approximately 1 ⁇ 3 portion of the inner reflector from the upstream side of the flow path of the fuel pebbles.
- the volume fraction of the moderator having a smaller moderating power than the graphite is set equal to or higher than 25% in an approximately 2 ⁇ 3 portion of the inner reflector from the upstream side of the flow path of the fuel pebbles.
- the reflector includes an inner reflector and an outer reflector that surrounds a reactor core, the flow path of the fuel pebbles is surrounded by an outer circumference of the inner reflector and an inner circumference of the outer reflector, and the volume fraction of the moderator having a smaller moderating power than the graphite is set equal to or higher than 75% in at least an approximately 1 ⁇ 3 portion of each of the inner reflector and the outer reflector from the upstream side of the flow path of the fuel pebbles.
- the reflector includes an inner reflector and an outer reflector that surrounds a reactor core, the flow path of the fuel pebbles is surrounded by an outer circumference of the inner reflector and an inner circumference of the outer reflector, and the volume fraction of the moderator having a smaller moderating power than the graphite is set equal to or higher than 75% in an approximately 1 ⁇ 3 portion of the inner reflector from the upstream side of the flow path of the fuel pebbles, equal to or higher than 25% in an approximately 1 ⁇ 3 portion at the center of the inner reflector, and equal to or higher than 25% in an approximately 1 ⁇ 3 portion of the outer reflector from the upstream side.
- criticality of the reactor core is adjusted by enrichment of the fuel pebbles.
- a nuclear reactor (block reactor) according to still another aspect of the present invention includes a reactor core, a fuel block arranged in the reactor core, the fuel block having a coolant flow path through which a coolant flows to cool the fuel block, and a reflector arranged inside the reactor core and sectioned into plural parts along a flow direction of the coolant, the reflector containing graphite and a moderator having a smaller moderating power than the graphite, and a volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflector is determined based on a power distribution in the reactor core in the flow direction of the coolant.
- a volume fraction of the moderator having a smaller moderating power than the graphite in each part of the reflector is determined so that the volume fraction increases from an upstream side to a downstream side of the coolant flow path.
- FIG. 1 is a schematic diagram of a nuclear reactor according to the first embodiment of the present invention
- FIG. 2 is a schematic view of an outer reflector and an inner reflector of the nuclear reactor shown in FIG. 1 ;
- FIG. 3 is a schematic diagram of fuel pebble employed in the nuclear reactor of FIG. 1 ;
- FIG. 4 is a schematic diagram of a coated fuel particle employed in the nuclear reactor of FIG. 1 ;
- FIG. 5 is a schematic diagram illustrating a general operation of the nuclear reactor shown in FIG. 1 ;
- FIG. 6 is a table of a first set of specific examples of composition of reflectors in the nuclear reactor of FIG. 1 ;
- FIG. 7 is a graph illustrating axial power distribution in examples of the nuclear reactor shown in FIG. 6 ;
- FIG. 8 is a table of a second set of specific examples of composition of reflectors in the nuclear reactor of FIG. 1 ;
- FIG. 9 is a graph illustrating power distribution in examples of the nuclear reactor shown in FIG. 8 ;
- FIG. 10 is a perspective view of a nuclear reactor according to the second embodiment of the present invention.
- FIG. 11 is a schematic diagram of a reactor core of the nuclear reactor shown in FIG. 10 ;
- FIG. 12 is a perspective view of a block-type fuel block (multi-hole-type fuel body) employed in the nuclear reactor shown in FIG. 10 ;
- FIG. 13 is a plan view of the block-type fuel block (multi-hole-type fuel body) employed in the nuclear reactor shown in FIG. 10 ;
- FIG. 14 is a sectional view of the block-type fuel block (multi-hole-type fuel body) employed in the nuclear reactor shown in FIG. 10 along a line A-A of FIG. 13 ;
- FIG. 15 schematically shows a reactor core of a conventional nuclear reactor
- FIG. 16 schematically shows a specific example of the reactor core of the nuclear reactor shown in FIG. 10 ;
- FIG. 17 is a graph of power distribution in the nuclear reactor shown in FIG. 16 ;
- FIG. 18 is a graph of neutron flux distribution in a radial direction in the conventional (pebble bed) nuclear reactor.
- FIG. 19 is a graph of neutron flux distribution in an axial direction in the conventional (pebble bed) nuclear reactor.
- FIG. 1 is a schematic diagram of a nuclear reactor according to the first embodiment of the present invention.
- FIG. 2 is a schematic view of an outer reflector and an inner reflector of the nuclear reactor shown in FIG. 1 .
- FIG. 3 is a schematic diagram of a fuel pebble and
- FIG. 4 is a schematic diagram of a coated fuel particle.
- FIG. 5 is a schematic diagram illustrating how the fuel bodies move within the nuclear reactor shown in FIG. 1 for fuel replacement.
- FIG. 6 is a table showing a first set of specific examples of composition of the reflectors of the nuclear reactor shown in FIG. 1 .
- FIG. 7 is a graph showing power distribution in the examples of the nuclear reactor shown in FIG. 6 .
- FIG. 8 is a table showing a second set of specific examples of composition of the reflectors of the nuclear reactor shown in FIG. 1 .
- FIG. 9 is a graph showing power distribution in the examples of the nuclear reactor shown in FIG. 8 .
- FIGS. 18 and 19 are graphs of neutron flux distribution in a conventional nuclear reactor.
- a nuclear reactor 1 is applied, for example, to a pebble bed reactor (see FIG. 1 ).
- the nuclear reactor 1 includes an outer reflector 2 and an inner reflector 3 .
- the reflectors 2 and 3 surround a flow path (fuel region) R.
- the nuclear reactor 1 induces nuclear reaction (nuclear fission or capture, for example) of nuclear fuel materials contained in fuel pebbles (fuel bodies) 4 in the flow path R, thereby generating power.
- nuclear reaction nuclear reaction
- the fuel pebbles 4 circulate within the flow path R, the power distribution is equalized in an axial direction of a reactor core (direction of flow of the fuel pebbles 4 ).
- the fuel pebbles 4 While the nuclear reactor 1 is running, the fuel pebbles 4 are continuously replaced (removed and reloaded), and accordingly an inspection of the removed fuel pebbles 4 and reloading of new fuel pebbles 4 are performed.
- helium gas is employed as a coolant.
- the outer reflector 2 is a cylindrical member of graphite (C) and silicon carbide (SiC), and constitutes an outer wall of the reactor core (see FIGS. 1 and 2 ).
- the outer reflector 2 has a function of reflecting and shielding neutrons emitted from the fuel pebbles 4 at an outer circumference of the reactor core.
- the inner reflector 3 is a column-like member of graphite (C) (and silicon carbide (SiC)), and is arranged on a central axis of the outer reflector 2 .
- the inner reflector 3 has a function of reflecting the neutrons emitted from the fuel pebbles 4 at a center of the reactor core.
- the inner reflector 3 also functions as a moderator to moderate the neutrons.
- the flow path R of the fuel pebbles 4 is between the outer reflector 2 and the inner reflector 3 .
- the flow path R is formed of gaps between the spherical fuel pebbles, and extends in the axial direction of the
- the fuel body (fuel pebble) 4 includes a graphite shell 41 and plural coated fuel particles 42 embedded in the graphite shell 41 (see FIG. 3 ).
- the graphite shell 41 is a sphere of approximately 3 centimeters (cm) in radius.
- the coated fuel particle 42 is a fuel particle of approximately 0.46 millimeter (mm) in radius.
- Plural coated fuel particles 42 are dispersed in a central portion (a portion within a range of approximately 2.5 cm in radius) of the graphite shell 41 .
- One graphite shell 41 contains many coated fuel particles 42 .
- the coated fuel particle 42 includes a fuel kernel 421 of approximately 0.25 mm in radius, a gap 422 , a low-density pyrolytic carbon (PyC) buffer layer 423 , a sealing layer 424 , a high-density pyrolytic carbon (PyC) inner layer 425 of approximately 0.040 mm in thickness in radial direction, a silicon carbide layer 426 of approximately 0.035 mm in thickness in radial direction, and a high-density pyrolytic carbon (PyC) outer layer 427 of approximately 0.040 mm in thickness in radial direction (see FIG. 4 ).
- the gap 422 , the low-density pyrolytic carbon (PyC) buffer layer 423 , and the sealing layer 424 together form a layer of approximately 0.095 mm in thickness in radial direction.
- the coated fuel particle 42 has the fuel kernel 421 as a core and the layers 421 to 426 stacked in the above mentioned order thereon.
- the fuel kernel 421 is uranium-235 ( 235 U) enriched to approximately 8 wt %.
- the fuel pebbles 4 While the nuclear reactor 1 is running, the fuel pebbles 4 are continuously loaded to the reactor core from an upper side of the nuclear reactor 1 (see FIG. 5 ). Then, the fuel pebbles 4 , while flowing through the flow path R inside the reactor core, undergo nuclear reaction, thereby generating thermal energy. After passing through the flow path R, the fuel pebbles 4 are removed from the nuclear reactor 1 from a lower side thereof. Then, a predetermined inspection is carried out on the removed fuel pebbles 4 to check burnup, condition, or the like. If reloadable, the fuel pebble 4 is reloaded to the reactor core from the upper side of the nuclear reactor 1 .
- FIG. 5 shows batches of the fuel pebbles 4 (assembly of fuel pebbles) move from the upper side to the lower side of the nuclear reactor 1 along zones inside the flow path R. Each of arrows in FIG. 5 indicates a travel distance of the fuel pebbles 4 per unit time.
- a predetermined level for example 80 GWd/t
- the power distribution (depended on distribution of neutron flux) in a radial direction and in an axial direction inside the reactor core of the pebble bed reactor.
- a peak of power density appears in the neighborhood of a graphite region due to neutron flux distribution in the radial direction in the reactor core as shown in FIG. 18 . If the distribution of neutron flux is taken along the axial direction of the reactor core (flow direction of the fuel pebbles 4 ), the peak appears at a center of the reactor core (see FIG. 19 ).
- peaks appear as described above because uranium-235 contained in the fuel pebble 4 tends to undergo fission reaction more actively on colliding against soft-spectrum neutron flux than hard-spectrum neutron flux, and also because that the reflector consisting of graphite makes the neutron flux spectrum soft (i.e., thermalized).
- the reflector contains a high volume fraction of silicon carbide and a low volume fraction of graphite (SiC/C reflector), the neutrons are hardly moderated and hence spread afar. Contrarily, when the reflector contains a high volume fraction of graphite (C reflector, or SiC/C reflector), the neutrons are well moderated and easily thermalized. The thermalized neutrons (thermal neutrons) react well with the nuclear fuel material to easily undergo fission reaction.
- the outer reflector 2 and the inner reflector 3 (or at least the inner reflector 3 ) contain graphite and a moderator which has a smaller moderating power than the graphite (silicon carbide, for example), and the reflectors 2 and 3 (or at least 3 ) are sectioned into plural parts in the direction of the flow path R of the fuel pebbles 4 .
- the volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflectors 2 and 3 is determined based on a power distribution in the reactor core in the direction of the flow path R of the fuel pebbles 4 (see FIG. 2 ).
- the inner reflector 3 is sectioned into an upper part 31 , a central part 32 , and a lower part 33 in the direction of flow path R of the fuel pebbles 4 , and the volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each of the parts 31 to 33 is determined based on the power distribution in the reactor core (see FIGS. 6 to 9 ).
- the volume fractions of elements in the reflectors 2 and 3 are determined based on the power distribution in the reactor core, so that a degree of moderation of the neutrons at each position in the flow path R of the fuel pebbles 4 is adjusted. For example, at a position where the volume fraction of the moderator having a smaller moderating power than the graphite is set high, the degree of moderation of the neutrons is low, whereby the power density is suppressed.
- the arrangement as described above is advantageous in that the power distribution in the reactor core can be adjusted, and that the power distribution in the reactor core can be equalized.
- the above described arrangement is advantageous in that the operational costs accompanying the replacement of the fuel pebbles 4 (costs of inspection and replenishment of the fuel pebbles 4 ) can be reduced. Further, the above described arrangement is advantageous in that the cost of power generation can be reduced, since the above described arrangement allows for an economical operation of the nuclear reactor 1 .
- the above described arrangement is advantageous in that the reliability of the nuclear reactor 1 can be enhanced since the number of reloads of the fuel pebbles 4 is decreased, i.e., since there is less possibility of entrainment of impurity into the fuel, which may occur at the time of reloading.
- silicon carbide is employed as the moderator having a smaller moderating power than the graphite.
- the moderator is not limited to silicon carbide.
- tungsten, molybdenum, or one of carbides that have an excellent high temperature resistance other than silicon carbide may be employed as the moderator having a smaller moderating power than graphite.
- low density graphite may be employed.
- the reflectors 2 and 3 are preferably formed so that portions located in positions with high power density contain a higher volume fraction of silicon carbide (i.e., the moderator having a low moderating power than graphite) than portions located in positions with low power density.
- the volume fraction of silicon carbide in each of the reflectors 2 and 3 is set high (i.e., the volume fraction of graphite is set low).
- the volume fraction of graphite in each of the reflectors 2 and 3 is set high (i.e., the volume fraction of silicon carbide is set low, or to zero) at positions where it is desirable to increase the power density (i.e., where the power density is low). Then, the neutron spectrum (distribution of neutron energy) shifts towards a high side of energy. Then, the nuclear reaction becomes difficult to occur, whereby the output of the portion decreases.
- the power distribution in the reactor core can be adjusted, and the power distribution in the reactor core in the direction of flow of the fuel pebbles 4 can be effectively equalized.
- the nuclear reactor 1 is advantageous in that the power distribution in the reactor core in the flow direction of the fuel pebbles 4 can effectively be equalized.
- the volume fraction of the moderator (silicon carbide) having a smaller moderating power than the graphite be set equal to or higher than 25% at least in an approximately 1 ⁇ 3 portion from the upstream side of the flow path R of the fuel pebbles 4 (i.e., at least in the upper part 31 of the inner reflector 3 ; i.e., approximately 1 ⁇ 3) (see FIGS. 6 and 7 ).
- the reflectors have such composition, the degree of moderation of neutrons is decreased at the upstream side of the flow path R, to suppress the maximum power density.
- the nuclear reactor 1 is advantageous in that the power distribution in the reactor core in the flow direction of the fuel pebbles 4 is effectively equalized.
- the volume fraction of the moderator having a smaller moderating power than the graphite be set equal to or higher than 25% in the inner reflector 3 in a portion within the range of approximately 2 ⁇ 3 from the upstream side of the flow path R of the fuel pebbles 4 (i.e., the upper part 31 and the central part 32 of the inner reflector 3 ) (see FIGS. 6 and 7 ).
- Such composition is advantageous in comparison with the above described composition in that the power distribution in the reactor core in the direction of flow of the fuel pebbles 4 is more effectively equalized.
- the composition of the reflectors 2 and 3 is defined in terms of volume.
- both the inner reflector and the outer reflector consist only of graphite.
- the inner reflector 3 is trisected into the upper part 31 , the central part 32 , and the lower part 33 from the upstream side of the flow path R in this order.
- the volume ratio of the silicon carbide (moderator having a smaller moderating power than graphite) to the graphite in the upper part 31 and the central part 32 in the inner reflector 3 is set to 25:75.
- the lower part 33 of the inner reflector 3 and the outer reflector 2 consist of graphite.
- the volume ratio of the silicon carbide to the graphite in the upper part 31 and the central part 32 in the inner reflector 3 is set to 50:50.
- the lower part 33 of the inner reflector 3 and the outer reflector 2 consist of graphite.
- the volume ratio of the silicon carbide to the graphite in the upper part 31 and the central part 32 in the inner reflector 3 is set to 75:25.
- the lower part 33 of the inner reflector 3 and the outer reflector 2 consist of graphite.
- the upper part 31 and the central part 32 of the inner reflector 3 consist of silicon carbide.
- the lower part 33 of the inner reflector 3 and the outer reflector 2 consist of graphite.
- the power density at the upstream side of the flow path R is suppressed in comparison with the conventional nuclear reactor, while the power density at the downstream side of the flow path R is increased by the suppressed amount.
- the maximum power density of the example 4 is suppressed to approximately 1 ⁇ 3 of the maximum power density of the conventional example.
- the results of calculations in FIG. 7 show relation between the positions in the axial direction in the reactor core and the maximum power density near a position radially inside the reactor core.
- the volume fraction of the moderator having a smaller moderating power than graphite in each of the inner reflector 3 and the outer reflector 2 be set equal to or higher than 25% in a 1 ⁇ 3 portion from the upstream side of the flow path R of the fuel pebbles 4 (see FIGS. 8 and 9 ).
- the degree of moderation of neutrons decreases at the upstream side of the flow path R to suppress the power density near the upstream side.
- the degree of moderation of neutrons is adjusted by both the inner reflector 3 and the outer reflector 2 , the power density can effectively be suppressed.
- the output at the downstream side of the flow path R is, then increased owing to the suppressed amount at the upstream side under normal power.
- the above described composition is advantageous in that the power distribution in the reactor core in the flow direction of the fuel pebbles 4 can effectively be equalized.
- the volume fraction of the moderator having a smaller moderating power than graphite in the inner reflector 3 be set equal to or higher than 25% in an approximately 2 ⁇ 3 portion from the upstream side of the flow path R of the fuel pebbles 4 (see FIGS. 8 and 9 ).
- the composition is advantageous in comparison with the above described composition in that the power distribution in the reactor core in the flow direction of the fuel pebbles 4 can more effectively be equalized.
- the criticality is adjusted through the adjustment of enrichment (uranium enrichment) of the fuel pebbles 4 .
- the inner reflector 3 is trisected into the upper part 31 , the central part 32 , and the lower part 33 from the upstream side of the flow path R in this order.
- the outer reflector 2 is also trisected into the upper part 21 , the central part 22 , and the lower part 23 , in this order from the upstream side of the flow path R.
- the volume ratio of the silicon carbide (moderator having a smaller moderating power than the graphite) to the graphite in each of the upper part 31 of the inner reflector 3 and the upper part 21 of the outer reflector 2 is set to 75:25.
- the central part 32 and the lower part 33 of the inner reflector 3 and the central part 22 and the lower part 23 of the outer reflector 2 consist of graphite.
- the upper part 31 of the inner reflector 3 consists of the moderator having a smaller moderating power than graphite.
- the volume ratio of the silicon carbide to the graphite in the upper part 21 of the outer reflector 2 is set to 75:25.
- the central part 32 and the lower part 33 of the inner reflector 3 and the central part 22 and the lower part 23 of the outer reflector 2 consist of graphite.
- the volume ratio of the silicon carbide to the graphite in each of the upper part 31 of the inner reflector 3 and the upper part 21 of the outer reflector 2 is set to 75:25. Further, the volume ratio of the silicon carbide to the graphite in the central part 32 of the inner reflector 3 is set to 25:75. Further, the lower part 33 of the inner reflector 3 and the central part 22 and the lower part 23 of the outer reflector 2 consist of graphite.
- the criticality is maintained through the adjustment of uranium enrichment in the fuel pebbles 4 .
- the power density at the upstream side of the flow path R is suppressed while the power density at the downstream side of the flow path R is increased owing to the suppressed amount at the upstream side under normal power in the nuclear reactors of examples 5 to 8 in comparison with the conventional nuclear reactor.
- the maximum power density of the example 9 is suppressed to a level equal to or lower than approximately 1 ⁇ 3 the maximum power density of the conventional nuclear reactor.
- the power distribution shown in FIG. 9 illustrates relation between the position in the axial direction of the reactor core and the maximum power density near a position radially inside the reactor core.
- FIG. 10 shows a schematic structure of a nuclear reactor according to the second embodiment of the present invention.
- FIG. 11 is an explanatory view of a reactor core of the nuclear reactor shown in FIG. 10 .
- FIGS. 12 to 14 are a perspective view ( FIG. 12 ), a plan view ( FIG. 13 ), and a sectional view along line A-A of FIG. 13 ( FIG. 14 ) of a fuel block (fuel body) employed in the nuclear reactor of FIG. 10 , respectively.
- FIG. 15 shows a schematic structure of the reactor core of the conventional nuclear reactor.
- FIG. 16 shows a schematic structure of a specific example of a reactor core of the nuclear reactor shown in FIG. 10 .
- FIG. 17 is a graph showing results of power distribution of the nuclear reactor shown in FIG. 16 .
- a nuclear reactor 10 is, for example, applied to a block-type high temperature gas reactor (see FIG. 10 ).
- the nuclear reactor 10 includes a steel nuclear reactor vessel 11 , a reactor core 12 , reactor internals 13 that support the reactor core 12 , and a piping 14 for a cooling system. Coolant flows through the piping 14 .
- the reactor core 12 and the reactor internals 13 are housed inside the nuclear reactor vessel 11 .
- Helium gas is employed as the coolant, for example.
- the reactor core 12 includes outer reflectors 121 , fuel blocks (fuel bodies) 122 , control rod guide columns 123 , irradiation test columns 124 , inner reflectors (graphite reflector columns) 125 , and the like (see FIGS. 11 and 12 ).
- the outer reflector 121 is made of graphite in a block-shape. Graphite blocks are arranged in a circle to form an outer peripheral portion of the reactor core 12 .
- the fuel block 122 , the control rod guide column 123 , the irradiation test column 124 , and the inner reflector 125 are each hexagonal column-shaped, and arranged inside the outer reflector 121 with their longitudinal directions (axial directions) in line with the axial direction of the reactor core 12 .
- the reactor core 12 is sectioned into four regions in the radial direction, and the fuel loading is performed independently for each region (see FIG. 11 ). Thus, the distribution of fuel temperature is optimized, and the maximum fuel temperature is minimized.
- the fuel block 122 includes a hexagonal columnar graphite block 1221 , fuel rods 1222 that are inserted into the graphite block 1221 and held thereby, burnable poison rods 1223 , and coolant flow paths 1225 (see FIG. 13 ). Further, the fuel block 122 is formed with a cylindrical graphite sleeve and a fuel compact sealed therein.
- the fuel compact has a cylindrical shape.
- plural coated fuel particles are dispersed in a graphite base material (graphite substrate). Further, the coated fuel particle includes a fuel kernel of uranium dioxide (UO 2 ) and three thin layers of pyrolytic carbon (PyC) or silicon carbide (SiC) formed on the fuel kernel.
- plural coolant flow paths 1225 run in the longitudinal direction of the fuel block 122 (see FIG. 14 ).
- the coolant helium gas
- FIG. 14 shows the flow direction of the coolant by an arrow. After cooling the fuel block 122 , the coolant flows through the piping 14 and is recovered outside the nuclear reactor 11 (see FIG. 10 ).
- the power distribution (depended on distribution of neutron flux) in the axial direction in the reactor core be optimized to reduce the maximum fuel temperature.
- fuel is employed at various degrees of uranium enrichment in the reactor core (in the example described herein, fuel is employed at twelve different degrees of uranium enrichment) for the optimization of the power distribution in the axial direction (see FIG. 15 ).
- the enrichment of the nuclear fuel in each portion of the reactor core 12 is determined based on the power distribution in the reactor core, whereby the nuclear reaction of the nuclear fuel material in the fuel blocks 122 at each position of the reactor core 12 is adjusted. For example, at a position where the enrichment of the nuclear fuel is set high, the nuclear reaction is accelerated to enhance the power density. Contrarily, at a position where the enrichment of the nuclear fuel is set low, the nuclear reaction is decelerated to suppress the power density. Thus, the output of the reactor core 12 can be adjusted.
- Such an arrangement is advantageous in that the power distribution in the reactor core 12 can be optimized.
- the enrichment of the nuclear fuel in each portion of the reactor core 12 is determined so that the enrichment lowers from the upstream side of the coolant flow path 1225 towards the downstream side thereof (i.e., from the upper side to the lower side of the reactor core 12 ) (see FIG. 15 ).
- the enrichment of the nuclear fuel is set high at the upstream side of the reactor core 12 (upper side of the reactor core 12 ) to enhance the power density
- the enrichment of the nuclear fuel is set low at the downstream side of the reactor core 12 (lower side of the reactor core 12 ) to suppress the power density.
- the power distribution in the reactor core in the flow direction of the coolant is effectively optimized and the maximum fuel temperature is suppressed to a low level.
- numerical values shown inside the portions of fuel blocks 122 indicate the enrichment of the nuclear fuel.
- the inner reflector (graphite reflector column) 125 that reflects the neutrons be arranged closer to the center of the reactor core 12 in the radial direction than the fuel blocks 122 in the reactor core 12 (see FIG. 15 ).
- the inner reflector 125 arranged closer to the center of the reactor core 12 in the radial direction reflects and moderates the neutrons.
- the structure is advantageous in that the neutrons are effectively moderated, and that the maximum fuel temperature which is reached at the center of the reactor core at the depressurization accident, for example, can be suppressed to a low level.
- numerical values shown inside the portions of fuel blocks 122 indicate the uranium enrichment of the nuclear fuel.
- the enrichment of the nuclear fuel in each portion of the reactor core 12 is set so that the enrichment decreases from the upstream side toward the downstream side of the reactor core 12 (i.e., from the upper side toward the lower side of the reactor core 12 ).
- the inner reflector 125 is arranged inside the reactor core 12 closer to the center of the reactor core 12 in the radial direction than the fuel blocks 122 .
- the reflector 126 which contains graphite and moderator that has a smaller moderating power than the graphite (for example, silicon carbide) be arranged inside the reactor core 12 ; that the reflector 126 is sectioned into plural parts along the flow direction of the coolant; and that the volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflector 126 is determined based on the power distribution in the reactor core 12 in the flow direction of the coolant (see FIG. 16 ).
- the graphite for example, silicon carbide
- the power density at each position in the reactor core 12 is adjusted. For example, at a position where the volume fraction of the silicon carbide is set high, the degree of moderation of neutrons becomes low, whereby the power density is suppressed. Contrarily, at a position where the volume fraction of silicon carbide is set low, the moderation of neutrons is accelerated to enhance the power density.
- the structure is advantageous in that the power distribution in the reactor core 12 can be adjusted, and that the power distribution in the reactor core 12 can be optimized.
- the volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in respective portions of the reflector 126 in the nuclear reactor 10 be determined so that the volume ratio increases from the upstream side toward the downstream side of the reactor core 12 (from the upper side toward the lower side of the reactor core 12 ) (see FIG. 16 ).
- the volume fraction of the graphite is set high at the upstream side of the reactor core 12 (upper side of the reactor core 12 ) to enhance the output of the reactor core 12 .
- the volume fraction of the moderator having a smaller moderating power than the graphite is set high at the downstream side of the reactor core 12 (lower side of the reactor core 12 ) to suppress the power density.
- the structure is advantageous in that the power distribution in the reactor core 12 can be adjusted, and that the power distribution in the reactor core 12 in the flow direction of the coolant can effectively be optimized.
- the inner reflector 126 in an example 11 shown in FIG. 16 is sectioned into five parts along the flow direction of the coolant (axial direction of the reactor core 12 ), so that the volume fraction of silicon carbide increases by 25% in each part from the upper side toward the lower side of the reactor core 12 .
- the reflector 126 is arranged in the second layer and the fifth layer from the center of the reactor core 12 in the radial direction.
- the plural reflectors 126 are arranged so as to sandwich the fuel blocks 122 from the inner side and the outer side in the radial direction of the reactor core 12 , and so as to be located near the fuel blocks 122 .
- the numerical values shown in the fuel blocks 122 in FIG. 16 indicate the enrichment of the nuclear fuel, whereas the numerical values in the inner reflectors 126 indicate the volume fraction of the moderator (silicon carbide) having a smaller moderating power than the graphite. For example, if the numerical value in the reflector 126 is 25%, it means that the volume ratio of the moderator having a smaller moderating power than the graphite to the graphite is 25:75.
- the composition of the inner reflector 126 is defined by volume.
- composition of the reflector in particular, the volume fraction of the moderator which has a smaller moderating power than graphite is determined based on the power distribution in the reactor core.
- a degree of moderation of neutrons at each position in the flow path of the fuel pebbles is adjusted, accordingly.
- the nuclear reactor according to one aspect of the present invention is advantageous in that the power distribution in the reactor core can be adjusted, and that the power distribution in the reactor core can be equalized accordingly.
- the volume fraction of the moderator, which has a smaller moderating power than the graphite, in a portion of the reflector is set high, when the portion is located in a region with high power density. Therefore, the neutron spectrum (neutron energy distribution) shifts towards a side of a higher energy level, to suppress the nuclear reaction, whereby the power density of the portion is decreased.
- the nuclear reactor according to another aspect of the present invention is advantageous in that the power distribution in the reactor core is effectively equalized in the flow direction of the fuel pebbles.
- the degree of moderation of neutrons is decreased at the upstream side of the flow path to suppress the power density.
- the power density at a downstream side of the flow path increases owing to the suppressed amount of output at the upstream side.
- the nuclear reactor according to the still another aspect of the present invention is advantageous in that the power distribution in the reactor core is effectively equalized in the flow direction of the fuel pebbles.
- the degree of moderation of neutrons at the upstream side of the flow path is decreased to suppress the power density.
- the power density at the downstream side of the flow path is increased owing to the suppressed amount of output at the upstream side.
- the nuclear reactor according to the still another aspect of the present invention is advantageous in that the power distribution in the reactor core is effectively equalized in the flow direction of the fuel pebbles.
- the nuclear reactor (pebble bed reactor) according to the still another aspect of the present invention is advantageous in comparison with the nuclear reactor according to the other aspects in that the power distribution in the reactor core is more effectively equalized in the flow direction of the fuel pebbles.
- the degree of moderation of neutrons is decreased at the upstream side of the flow path, to suppress the power density. Further, since the neutrons are moderated by both the inner reflector and the outer reflector, the power density is effectively suppressed. Under the condition set to keep the total power of the reactor core constant, the output is increased at the downstream side of the flow path owing to the suppressed amount of output at the upstream side of the flow path.
- the nuclear reactor according to the still another aspect of the present invention is advantageous in that the power distribution in the reactor core is effectively equalized in the flow direction of the fuel pebbles.
- the nuclear reactor (pebble bed reactor) according to the still another aspect of the present invention is advantageous in comparison with the nuclear reactor according to the other aspects in that the power distribution in the reactor core is more effectively equalized in the flow direction of the fuel pebbles.
- the criticality of the reactor core is adjusted by the enrichment of the fuel pebble, the output can be maintained.
- the volume ratio of the graphite to the moderator which has a smaller moderating power than the graphite in each portion of the reflector is determined so as to optimize the power distribution in the reactor core. Therefore, the power density of the reactor core at each position in the coolant flow path is adjusted.
- the nuclear reactor according to the still another aspect of the present invention is advantageous in that the power distribution of the reactor core can be adjusted, and that the power distribution in the reactor core can be equalized and/or optimized accordingly.
- the volume fraction of the moderator which has a smaller moderating power than graphite is set high in the downstream side of the coolant flow path (lower side of the reactor core) to suppress the maximum power density and/or the maximum fuel temperature.
- the nuclear reactor according to the still another aspect of the present invention is advantageous in that the power distribution in the reactor core can be adjusted and that the power distribution in the reactor core in the flow direction of the coolant can be effectively optimized so that the power density decreases towards the downstream side.
- the composition of the reflector (in particular the volume fraction of the moderator that has a smaller moderating power than graphite) is determined so as to optimize the power distribution in the reactor core. Therefore, the degree of moderation of neutrons at each position in the flow path of the fuel bodies is adjusted.
- the nuclear reactor according to the present invention is advantageous in that the power distribution in the reactor core can be adjusted, and that the power distribution in the reactor core can be optimized accordingly.
- the nuclear reactor according to the present invention since the volume ratio of the graphite to the moderator which has a smaller moderating power than the graphite in each portion of the reflector is determined so as to optimize the power distribution in the reactor core, the power density of the reactor core at each position in the coolant flow path is adjusted.
- the nuclear reactor according to the present invention is advantageous in that the power distribution in the reactor core can be adjusted, and that the power distribution of the reactor core can be equalized and/or optimized accordingly.
- the nuclear reactor according to the present invention is useful in that the power distribution in the reactor core can be equalized and/or optimized without the need of repetitious reloading of the fuel pebbles (in the pebble bed reactor core) and without the need of preparation of fuel in various different uranium enrichment levels (in the block-type reactor core).
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Abstract
A nuclear reactor includes a reflector and a flow path. The reflector reflects neutrons, contains graphite and a moderator having a smaller moderating power than the graphite, and is sectioned into plural parts along a direction of flow of fuel pebbles. The flow path is surrounded by the reflector, and the fuel pebbles flow through the flow path and undergo nuclear reaction to generate power. Volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflector is determined based on a power distribution in the reactor core in the direction of flow of the fuel pebbles.
Description
- This application is a divisional of U.S. application Ser. No. 13/115,517, filed on May 25, 2011, which is a divisional of U.S. application Ser. No. 11/965,378, filed on Dec. 27, 2007 and issued as U.S. Pat. No. 7,978,807 on Jul. 12, 2011, which is based on and claims the benefit of priority from Japanese Patent Application No. 2007-200419, filed Aug. 1, 2007, the entire contents of which are incorporated herein by reference.
- 1. Field of the Invention
- The present invention relates to a nuclear reactor, and more particularly to a nuclear reactor that allows for equalization and/or optimization of power distribution in a reactor core.
- 2. Description of the Related Art
- Pebble bed reactors (PBR) have a flow path (fuel region) that is surrounded by a reflector(s). Sphere-shaped fuel bodies (fuel pebbles) containing nuclear fuel material undergo nuclear reaction inside the flow path thereby producing power. In view of safety, it is preferable for the nuclear reactors in general to have equalized power distribution in a reactor core. In the PBR, the fuel body goes down the flow path and is repeatedly removed and reloaded, whereby the power distribution in the nuclear reactor is equalized in an axial direction (i.e., direction in which the fuel bodies flow in the flow path). Hence, while the nuclear reactor is running, replacement (removal and reloading) of the fuel bodies is continuously performed, i.e., removed fuel bodies are inspected, and new fuel bodies are loaded.
- For example, a 170,000 kilowatts (kW) class nuclear reactor uses 415,000 fuel bodies, among which 6,000 are removed from the nuclear reactor for inspection every day. Among the removed 6,000 fuel bodies, approximately 600 are replaced with new ones. New fuel bodies are loaded into the nuclear reactor together with the approximately 5,400 remaining fuel bodies. Generally, average life of the fuel body is about two years, and one fuel body is reloaded nine times on average during its life.
- However, if the replacement of the fuel bodies becomes more frequent, operational costs (costs for inspection and replenishment of the fuel bodies) increase accordingly, which in turn results in an increased cost for power generation. Hence, it is preferable to equalize the power distribution in the nuclear reactor without increasing the frequency of fuel replacement.
- One conventional technique applicable to the pebble bed reactor is known from Japanese Patent Application Laid-Open No. 2003-222693. The conventional reactor (nuclear reactor facility) described therein includes a detecting unit that receives plural fuel spheres discharged from a reactor core and detects burnup of the fuel spheres, and a sorting unit that determines a radial loading position from which the fuel sphere is reloaded into the reactor core according to the result of detection by the detecting unit.
- In a block reactor, a fuel body having a hexagonal columnar shape houses fuel compacts. Base material of the fuel compact is graphite. The fuel compact is filled with coated fuel particles (of approximately 1-mm diameter). Nuclear fuel material contained in the fuel particle undergoes nuclear reaction thereby producing power. For the equalization or the optimization of the power distribution in an axial direction, enriched uranium is prepared in twelve different levels of enrichment, for example. Preparation or manufacture of enriched uranium at various levels of enrichment in a large amount pushes up the manufacturing cost of the fuel.
- It is an object of the present invention to at least partially solve the problems in the conventional technology. Specifically, one object of the present invention is to provide a nuclear reactor which allows for equalization and/or optimization of power distribution in a reactor core.
- In order to achieve an object as described above, a nuclear reactor (pebble bed reactor) according to one aspect of the present invention includes a reflector reflecting neutrons and containing graphite and moderator having a smaller moderating power than the graphite, the reflector being sectioned into plural parts along a direction of flow of fuel pebbles, and a flow path surrounded by the reflector, and through the flow path the fuel pebbles (fuel bodies) flow and undergo nuclear reaction to generate power. A volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflector is determined based on a power distribution of a reactor core in the direction of flow of the fuel pebbles.
- Further, in the nuclear reactor (pebble bed reactor) according to another aspect of the present invention, when a power density obtained with the reflector consisting of graphite is taken as a reference, a part of the reflector located at a position with a higher power density contains a higher volume fraction of the moderator having a smaller moderating power than the graphite than a part of the reflector located at a position with a lower power density.
- Still further, in the nuclear reactor (pebble bed reactor) according to still another aspect of the present invention, the reflector contains a higher volume fraction of the moderator having a smaller moderating power than the graphite in a portion located at an upstream side of the flow path of the fuel pebbles than in a portion located at a downstream side of the flow path of the fuel pebbles.
- Still further, in the nuclear reactor (pebble bed reactor) according to still another aspect of the present invention, the reflector includes an inner reflector and an outer reflector that surrounds a reactor core, the flow path of the fuel pebbles is surrounded by an outer circumference of the inner reflector and an inner circumference of the outer reflector, and a volume fraction of the moderator having a smaller moderating power than the graphite is set equal to or higher than 25% in at least an approximately ⅓ portion of the inner reflector from the upstream side of the flow path of the fuel pebbles.
- Still further, in the nuclear reactor (pebble bed reactor) according to still another aspect of the present invention, the volume fraction of the moderator having a smaller moderating power than the graphite is set equal to or higher than 25% in an approximately ⅔ portion of the inner reflector from the upstream side of the flow path of the fuel pebbles.
- Still further, in the nuclear reactor (pebble bed reactor) according to still another aspect of the present invention, the reflector includes an inner reflector and an outer reflector that surrounds a reactor core, the flow path of the fuel pebbles is surrounded by an outer circumference of the inner reflector and an inner circumference of the outer reflector, and the volume fraction of the moderator having a smaller moderating power than the graphite is set equal to or higher than 75% in at least an approximately ⅓ portion of each of the inner reflector and the outer reflector from the upstream side of the flow path of the fuel pebbles.
- Still further, in the nuclear reactor (pebble bed reactor) according to still another aspect of the present invention, the reflector includes an inner reflector and an outer reflector that surrounds a reactor core, the flow path of the fuel pebbles is surrounded by an outer circumference of the inner reflector and an inner circumference of the outer reflector, and the volume fraction of the moderator having a smaller moderating power than the graphite is set equal to or higher than 75% in an approximately ⅓ portion of the inner reflector from the upstream side of the flow path of the fuel pebbles, equal to or higher than 25% in an approximately ⅓ portion at the center of the inner reflector, and equal to or higher than 25% in an approximately ⅓ portion of the outer reflector from the upstream side.
- Still further, in the nuclear reactor (pebble bed reactor) according to still another aspect of the present invention, criticality of the reactor core is adjusted by enrichment of the fuel pebbles.
- Still further, a nuclear reactor (block reactor) according to still another aspect of the present invention includes a reactor core, a fuel block arranged in the reactor core, the fuel block having a coolant flow path through which a coolant flows to cool the fuel block, and a reflector arranged inside the reactor core and sectioned into plural parts along a flow direction of the coolant, the reflector containing graphite and a moderator having a smaller moderating power than the graphite, and a volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflector is determined based on a power distribution in the reactor core in the flow direction of the coolant.
- Still further, in the nuclear reactor (block reactor) according to still another aspect of the present invention, a volume fraction of the moderator having a smaller moderating power than the graphite in each part of the reflector is determined so that the volume fraction increases from an upstream side to a downstream side of the coolant flow path.
- The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings.
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FIG. 1 is a schematic diagram of a nuclear reactor according to the first embodiment of the present invention; -
FIG. 2 is a schematic view of an outer reflector and an inner reflector of the nuclear reactor shown inFIG. 1 ; -
FIG. 3 is a schematic diagram of fuel pebble employed in the nuclear reactor ofFIG. 1 ; -
FIG. 4 is a schematic diagram of a coated fuel particle employed in the nuclear reactor ofFIG. 1 ; -
FIG. 5 is a schematic diagram illustrating a general operation of the nuclear reactor shown inFIG. 1 ; -
FIG. 6 is a table of a first set of specific examples of composition of reflectors in the nuclear reactor ofFIG. 1 ; -
FIG. 7 is a graph illustrating axial power distribution in examples of the nuclear reactor shown inFIG. 6 ; -
FIG. 8 is a table of a second set of specific examples of composition of reflectors in the nuclear reactor ofFIG. 1 ; -
FIG. 9 is a graph illustrating power distribution in examples of the nuclear reactor shown inFIG. 8 ; -
FIG. 10 is a perspective view of a nuclear reactor according to the second embodiment of the present invention; -
FIG. 11 is a schematic diagram of a reactor core of the nuclear reactor shown inFIG. 10 ; -
FIG. 12 is a perspective view of a block-type fuel block (multi-hole-type fuel body) employed in the nuclear reactor shown inFIG. 10 ; -
FIG. 13 is a plan view of the block-type fuel block (multi-hole-type fuel body) employed in the nuclear reactor shown inFIG. 10 ; -
FIG. 14 is a sectional view of the block-type fuel block (multi-hole-type fuel body) employed in the nuclear reactor shown inFIG. 10 along a line A-A ofFIG. 13 ; -
FIG. 15 schematically shows a reactor core of a conventional nuclear reactor; -
FIG. 16 schematically shows a specific example of the reactor core of the nuclear reactor shown inFIG. 10 ; -
FIG. 17 is a graph of power distribution in the nuclear reactor shown inFIG. 16 ; -
FIG. 18 is a graph of neutron flux distribution in a radial direction in the conventional (pebble bed) nuclear reactor; and -
FIG. 19 is a graph of neutron flux distribution in an axial direction in the conventional (pebble bed) nuclear reactor. - The present invention will be described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited by embodiments described below. Elements included in the embodiments may be replaceable with or substantially equivalent to alternative elements easily conceived by those skilled in the art. Further, it should be obvious to those skilled in the art that modifications described in relation to the embodiments below may optionally be combined with each other without departing from the scope of the present invention.
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FIG. 1 is a schematic diagram of a nuclear reactor according to the first embodiment of the present invention.FIG. 2 is a schematic view of an outer reflector and an inner reflector of the nuclear reactor shown inFIG. 1 .FIG. 3 is a schematic diagram of a fuel pebble andFIG. 4 is a schematic diagram of a coated fuel particle.FIG. 5 is a schematic diagram illustrating how the fuel bodies move within the nuclear reactor shown inFIG. 1 for fuel replacement.FIG. 6 is a table showing a first set of specific examples of composition of the reflectors of the nuclear reactor shown inFIG. 1 .FIG. 7 is a graph showing power distribution in the examples of the nuclear reactor shown inFIG. 6 .FIG. 8 is a table showing a second set of specific examples of composition of the reflectors of the nuclear reactor shown inFIG. 1 .FIG. 9 is a graph showing power distribution in the examples of the nuclear reactor shown inFIG. 8 .FIGS. 18 and 19 are graphs of neutron flux distribution in a conventional nuclear reactor. - A
nuclear reactor 1 is applied, for example, to a pebble bed reactor (seeFIG. 1 ). Thenuclear reactor 1 includes anouter reflector 2 and aninner reflector 3. Thereflectors nuclear reactor 1 induces nuclear reaction (nuclear fission or capture, for example) of nuclear fuel materials contained in fuel pebbles (fuel bodies) 4 in the flow path R, thereby generating power. In thenuclear reactor 1, since thefuel pebbles 4 circulate within the flow path R, the power distribution is equalized in an axial direction of a reactor core (direction of flow of the fuel pebbles 4). While thenuclear reactor 1 is running, thefuel pebbles 4 are continuously replaced (removed and reloaded), and accordingly an inspection of the removedfuel pebbles 4 and reloading ofnew fuel pebbles 4 are performed. In thenuclear reactor 1, helium gas is employed as a coolant. - The
outer reflector 2 is a cylindrical member of graphite (C) and silicon carbide (SiC), and constitutes an outer wall of the reactor core (seeFIGS. 1 and 2 ). Theouter reflector 2 has a function of reflecting and shielding neutrons emitted from thefuel pebbles 4 at an outer circumference of the reactor core. Theinner reflector 3 is a column-like member of graphite (C) (and silicon carbide (SiC)), and is arranged on a central axis of theouter reflector 2. Theinner reflector 3 has a function of reflecting the neutrons emitted from thefuel pebbles 4 at a center of the reactor core. Theinner reflector 3 also functions as a moderator to moderate the neutrons. The flow path R of thefuel pebbles 4 is between theouter reflector 2 and theinner reflector 3. The flow path R is formed of gaps between the spherical fuel pebbles, and extends in the axial direction of the reactor core. - The fuel body (fuel pebble) 4 includes a
graphite shell 41 and plural coatedfuel particles 42 embedded in the graphite shell 41 (seeFIG. 3 ). Thegraphite shell 41 is a sphere of approximately 3 centimeters (cm) in radius. The coatedfuel particle 42 is a fuel particle of approximately 0.46 millimeter (mm) in radius. Plural coatedfuel particles 42 are dispersed in a central portion (a portion within a range of approximately 2.5 cm in radius) of thegraphite shell 41. Onegraphite shell 41 contains many coatedfuel particles 42. The coatedfuel particle 42 includes afuel kernel 421 of approximately 0.25 mm in radius, agap 422, a low-density pyrolytic carbon (PyC)buffer layer 423, asealing layer 424, a high-density pyrolytic carbon (PyC)inner layer 425 of approximately 0.040 mm in thickness in radial direction, asilicon carbide layer 426 of approximately 0.035 mm in thickness in radial direction, and a high-density pyrolytic carbon (PyC)outer layer 427 of approximately 0.040 mm in thickness in radial direction (seeFIG. 4 ). Thegap 422, the low-density pyrolytic carbon (PyC)buffer layer 423, and thesealing layer 424 together form a layer of approximately 0.095 mm in thickness in radial direction. The coatedfuel particle 42 has thefuel kernel 421 as a core and thelayers 421 to 426 stacked in the above mentioned order thereon. Thefuel kernel 421 is uranium-235 (235U) enriched to approximately 8 wt %. - While the
nuclear reactor 1 is running, thefuel pebbles 4 are continuously loaded to the reactor core from an upper side of the nuclear reactor 1 (seeFIG. 5 ). Then, thefuel pebbles 4, while flowing through the flow path R inside the reactor core, undergo nuclear reaction, thereby generating thermal energy. After passing through the flow path R, thefuel pebbles 4 are removed from thenuclear reactor 1 from a lower side thereof. Then, a predetermined inspection is carried out on the removedfuel pebbles 4 to check burnup, condition, or the like. If reloadable, the fuel pebble 4 is reloaded to the reactor core from the upper side of thenuclear reactor 1. If the burnup has exceeded a predetermined level (for example 80 GWd/t), or if the condition is not good, the fuel pebble 4 is taken away and stored.FIG. 5 shows batches of the fuel pebbles 4 (assembly of fuel pebbles) move from the upper side to the lower side of thenuclear reactor 1 along zones inside the flow path R. Each of arrows inFIG. 5 indicates a travel distance of thefuel pebbles 4 per unit time. - Generally, it is preferable for safety, for example, to equalize the power distribution (depended on distribution of neutron flux) in a radial direction and in an axial direction inside the reactor core of the pebble bed reactor. For example, if the nuclear reactor employs a reflector consisting of graphite, a peak of power density appears in the neighborhood of a graphite region due to neutron flux distribution in the radial direction in the reactor core as shown in
FIG. 18 . If the distribution of neutron flux is taken along the axial direction of the reactor core (flow direction of the fuel pebbles 4), the peak appears at a center of the reactor core (seeFIG. 19 ). The peaks appear as described above because uranium-235 contained in the fuel pebble 4 tends to undergo fission reaction more actively on colliding against soft-spectrum neutron flux than hard-spectrum neutron flux, and also because that the reflector consisting of graphite makes the neutron flux spectrum soft (i.e., thermalized). - Generally, when the reflector contains a high volume fraction of silicon carbide and a low volume fraction of graphite (SiC/C reflector), the neutrons are hardly moderated and hence spread afar. Contrarily, when the reflector contains a high volume fraction of graphite (C reflector, or SiC/C reflector), the neutrons are well moderated and easily thermalized. The thermalized neutrons (thermal neutrons) react well with the nuclear fuel material to easily undergo fission reaction.
- In the
nuclear reactor 1, theouter reflector 2 and the inner reflector 3 (or at least the inner reflector 3) contain graphite and a moderator which has a smaller moderating power than the graphite (silicon carbide, for example), and thereflectors 2 and 3 (or at least 3) are sectioned into plural parts in the direction of the flow path R of thefuel pebbles 4. Further, the volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of thereflectors FIG. 2 ). For example, theinner reflector 3 is sectioned into anupper part 31, acentral part 32, and alower part 33 in the direction of flow path R of thefuel pebbles 4, and the volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each of theparts 31 to 33 is determined based on the power distribution in the reactor core (seeFIGS. 6 to 9 ). - According to the above described arrangement, the volume fractions of elements in the
reflectors 2 and 3 (in particular, the volume fraction of the moderator having a smaller moderating power than the graphite) are determined based on the power distribution in the reactor core, so that a degree of moderation of the neutrons at each position in the flow path R of thefuel pebbles 4 is adjusted. For example, at a position where the volume fraction of the moderator having a smaller moderating power than the graphite is set high, the degree of moderation of the neutrons is low, whereby the power density is suppressed. Contrarily, at a position where the volume fraction of the moderator having a smaller moderating power than the graphite is set low, the degree of moderation of the neutrons is high, whereby the power density is enhanced. Thus, the arrangement as described above is advantageous in that the power distribution in the reactor core can be adjusted, and that the power distribution in the reactor core can be equalized. - Further, when the power distribution in the reactor core is equalized, the number of replacements (the number of reloads and removals) of the
fuel pebbles 4, performed for equalizing the composition of the reactor core in the axial direction, can be reduced. Thus, the above described arrangement is advantageous in that the operational costs accompanying the replacement of the fuel pebbles 4 (costs of inspection and replenishment of the fuel pebbles 4) can be reduced. Further, the above described arrangement is advantageous in that the cost of power generation can be reduced, since the above described arrangement allows for an economical operation of thenuclear reactor 1. Still further, the above described arrangement is advantageous in that the reliability of thenuclear reactor 1 can be enhanced since the number of reloads of thefuel pebbles 4 is decreased, i.e., since there is less possibility of entrainment of impurity into the fuel, which may occur at the time of reloading. - For the reflectors of the
nuclear reactor 1, silicon carbide is employed as the moderator having a smaller moderating power than the graphite. The moderator, however, is not limited to silicon carbide. For example, tungsten, molybdenum, or one of carbides that have an excellent high temperature resistance other than silicon carbide may be employed as the moderator having a smaller moderating power than graphite. Similarly, low density graphite may be employed. - If the power density in the nuclear reactor which employs the
reflectors reflectors reflectors reflectors fuel pebbles 4 can be effectively equalized. - Further, in the
pebble bed reactor 1,new fuel pebbles 4 are loaded from the upper side of the reactor core. Therefore, the power density tends to be higher at the upstream side of the flow path R than at the downstream side of the flow path R. Hence, in thereflectors nuclear reactor 1 is advantageous in that the power distribution in the reactor core in the flow direction of thefuel pebbles 4 can effectively be equalized. - When the
nuclear reactor 1 includes thereflectors outer reflector 2 and theinner reflector 3 as described above (seeFIGS. 1 and 2 ), it is preferable that the volume fraction of the moderator (silicon carbide) having a smaller moderating power than the graphite be set equal to or higher than 25% at least in an approximately ⅓ portion from the upstream side of the flow path R of the fuel pebbles 4 (i.e., at least in theupper part 31 of theinner reflector 3; i.e., approximately ⅓) (seeFIGS. 6 and 7 ). When the reflectors have such composition, the degree of moderation of neutrons is decreased at the upstream side of the flow path R, to suppress the maximum power density. Then, the power density increases at the downstream side of the flow path R owing to a suppressed amount at the upstream side. Thus, thenuclear reactor 1 is advantageous in that the power distribution in the reactor core in the flow direction of thefuel pebbles 4 is effectively equalized. - Further, it is preferable in the
nuclear reactor 1 that the volume fraction of the moderator having a smaller moderating power than the graphite be set equal to or higher than 25% in theinner reflector 3 in a portion within the range of approximately ⅔ from the upstream side of the flow path R of the fuel pebbles 4 (i.e., theupper part 31 and thecentral part 32 of the inner reflector 3) (seeFIGS. 6 and 7 ). Such composition is advantageous in comparison with the above described composition in that the power distribution in the reactor core in the direction of flow of thefuel pebbles 4 is more effectively equalized. In the first embodiment, the composition of thereflectors - In the conventional nuclear reactor with the reflectors having the compositions shown in
FIG. 6 and the reactor core having the power distribution shown inFIG. 7 , both the inner reflector and the outer reflector consist only of graphite. - On the other hand, in an example 1 of the
nuclear reactor 1, theinner reflector 3 is trisected into theupper part 31, thecentral part 32, and thelower part 33 from the upstream side of the flow path R in this order. The volume ratio of the silicon carbide (moderator having a smaller moderating power than graphite) to the graphite in theupper part 31 and thecentral part 32 in theinner reflector 3 is set to 25:75. Thelower part 33 of theinner reflector 3 and theouter reflector 2 consist of graphite. - In an example 2 of the
nuclear reactor 1, the volume ratio of the silicon carbide to the graphite in theupper part 31 and thecentral part 32 in theinner reflector 3 is set to 50:50. Thelower part 33 of theinner reflector 3 and theouter reflector 2 consist of graphite. - In an example 3 of the
nuclear reactor 1, the volume ratio of the silicon carbide to the graphite in theupper part 31 and thecentral part 32 in theinner reflector 3 is set to 75:25. Thelower part 33 of theinner reflector 3 and theouter reflector 2 consist of graphite. - In an example 4 of the
nuclear reactor 1, theupper part 31 and thecentral part 32 of theinner reflector 3 consist of silicon carbide. Thelower part 33 of theinner reflector 3 and theouter reflector 2 consist of graphite. - As shown in results of calculations in
FIG. 7 , in the nuclear reactors of the examples 1 to 4, the power density at the upstream side of the flow path R is suppressed in comparison with the conventional nuclear reactor, while the power density at the downstream side of the flow path R is increased by the suppressed amount. For example, when the conventional example is compared with the example 4, the maximum power density of the example 4 is suppressed to approximately ⅓ of the maximum power density of the conventional example. The results of calculations inFIG. 7 show relation between the positions in the axial direction in the reactor core and the maximum power density near a position radially inside the reactor core. - Further, in the
nuclear reactor 1, it is preferable that the volume fraction of the moderator having a smaller moderating power than graphite in each of theinner reflector 3 and theouter reflector 2 be set equal to or higher than 25% in a ⅓ portion from the upstream side of the flow path R of the fuel pebbles 4 (seeFIGS. 8 and 9 ). With such a composition, the degree of moderation of neutrons decreases at the upstream side of the flow path R to suppress the power density near the upstream side. In addition, since the degree of moderation of neutrons is adjusted by both theinner reflector 3 and theouter reflector 2, the power density can effectively be suppressed. The output at the downstream side of the flow path R is, then increased owing to the suppressed amount at the upstream side under normal power. Thus, the above described composition is advantageous in that the power distribution in the reactor core in the flow direction of thefuel pebbles 4 can effectively be equalized. - Further, in the
nuclear reactor 1, it is preferable that the volume fraction of the moderator having a smaller moderating power than graphite in theinner reflector 3 be set equal to or higher than 25% in an approximately ⅔ portion from the upstream side of the flow path R of the fuel pebbles 4 (seeFIGS. 8 and 9 ). The composition is advantageous in comparison with the above described composition in that the power distribution in the reactor core in the flow direction of thefuel pebbles 4 can more effectively be equalized. - In the
nuclear reactor 1, the criticality is adjusted through the adjustment of enrichment (uranium enrichment) of thefuel pebbles 4. - For example, in an example 5 of the
nuclear reactor 1 having the reflectors with the composition as shown inFIG. 8 and the power distribution as shown inFIG. 9 , theinner reflector 3 is trisected into theupper part 31, thecentral part 32, and thelower part 33 from the upstream side of the flow path R in this order. Theouter reflector 2 is also trisected into theupper part 21, thecentral part 22, and thelower part 23, in this order from the upstream side of the flow path R. The volume ratio of the silicon carbide (moderator having a smaller moderating power than the graphite) to the graphite in each of theupper part 31 of theinner reflector 3 and theupper part 21 of theouter reflector 2 is set to 75:25. Thecentral part 32 and thelower part 33 of theinner reflector 3 and thecentral part 22 and thelower part 23 of theouter reflector 2 consist of graphite. - In an example 6 of the
nuclear reactor 1, theupper part 31 of theinner reflector 3 consists of the moderator having a smaller moderating power than graphite. Further, the volume ratio of the silicon carbide to the graphite in theupper part 21 of theouter reflector 2 is set to 75:25. Further, thecentral part 32 and thelower part 33 of theinner reflector 3 and thecentral part 22 and thelower part 23 of theouter reflector 2 consist of graphite. - In an example 7 of the
nuclear reactor 1, the volume ratio of the silicon carbide to the graphite in each of theupper part 31 of theinner reflector 3 and theupper part 21 of theouter reflector 2 is set to 75:25. Further, the volume ratio of the silicon carbide to the graphite in thecentral part 32 of theinner reflector 3 is set to 25:75. Further, thelower part 33 of theinner reflector 3 and thecentral part 22 and thelower part 23 of theouter reflector 2 consist of graphite. - In an example 8 of the
nuclear reactor 1 which has the same composition as the example 7 of thenuclear reactor 1, the criticality is maintained through the adjustment of uranium enrichment in thefuel pebbles 4. - As shown in the results of calculations of
FIG. 9 , the power density at the upstream side of the flow path R is suppressed while the power density at the downstream side of the flow path R is increased owing to the suppressed amount at the upstream side under normal power in the nuclear reactors of examples 5 to 8 in comparison with the conventional nuclear reactor. For example, the maximum power density of the example 9 is suppressed to a level equal to or lower than approximately ⅓ the maximum power density of the conventional nuclear reactor. The power distribution shown inFIG. 9 illustrates relation between the position in the axial direction of the reactor core and the maximum power density near a position radially inside the reactor core. -
FIG. 10 shows a schematic structure of a nuclear reactor according to the second embodiment of the present invention.FIG. 11 is an explanatory view of a reactor core of the nuclear reactor shown inFIG. 10 .FIGS. 12 to 14 are a perspective view (FIG. 12 ), a plan view (FIG. 13 ), and a sectional view along line A-A ofFIG. 13 (FIG. 14 ) of a fuel block (fuel body) employed in the nuclear reactor ofFIG. 10 , respectively.FIG. 15 shows a schematic structure of the reactor core of the conventional nuclear reactor.FIG. 16 shows a schematic structure of a specific example of a reactor core of the nuclear reactor shown inFIG. 10 .FIG. 17 is a graph showing results of power distribution of the nuclear reactor shown inFIG. 16 . - A
nuclear reactor 10 is, for example, applied to a block-type high temperature gas reactor (seeFIG. 10 ). Thenuclear reactor 10 includes a steelnuclear reactor vessel 11, areactor core 12,reactor internals 13 that support thereactor core 12, and a piping 14 for a cooling system. Coolant flows through thepiping 14. In thenuclear reactor 10, thereactor core 12 and thereactor internals 13 are housed inside thenuclear reactor vessel 11. Helium gas is employed as the coolant, for example. - The
reactor core 12 includesouter reflectors 121, fuel blocks (fuel bodies) 122, controlrod guide columns 123,irradiation test columns 124, inner reflectors (graphite reflector columns) 125, and the like (seeFIGS. 11 and 12 ). Theouter reflector 121 is made of graphite in a block-shape. Graphite blocks are arranged in a circle to form an outer peripheral portion of thereactor core 12. Thefuel block 122, the controlrod guide column 123, theirradiation test column 124, and theinner reflector 125 are each hexagonal column-shaped, and arranged inside theouter reflector 121 with their longitudinal directions (axial directions) in line with the axial direction of thereactor core 12. In thenuclear reactor 10, thereactor core 12 is sectioned into four regions in the radial direction, and the fuel loading is performed independently for each region (seeFIG. 11 ). Thus, the distribution of fuel temperature is optimized, and the maximum fuel temperature is minimized. - The
fuel block 122 includes a hexagonalcolumnar graphite block 1221,fuel rods 1222 that are inserted into thegraphite block 1221 and held thereby,burnable poison rods 1223, and coolant flow paths 1225 (seeFIG. 13 ). Further, thefuel block 122 is formed with a cylindrical graphite sleeve and a fuel compact sealed therein. The fuel compact has a cylindrical shape. In the fuel compact, plural coated fuel particles are dispersed in a graphite base material (graphite substrate). Further, the coated fuel particle includes a fuel kernel of uranium dioxide (UO2) and three thin layers of pyrolytic carbon (PyC) or silicon carbide (SiC) formed on the fuel kernel. - In the
fuel block 122, pluralcoolant flow paths 1225 run in the longitudinal direction of the fuel block 122 (seeFIG. 14 ). Through thecoolant flow path 1225, the coolant (helium gas) flows from the upper side of thereactor core 12 to the lower side of thereactor core 12.FIG. 14 shows the flow direction of the coolant by an arrow. After cooling thefuel block 122, the coolant flows through the piping 14 and is recovered outside the nuclear reactor 11 (seeFIG. 10 ). - Generally, in the block-type high temperature gas reactor, it is preferable for safety, for example, that the power distribution (depended on distribution of neutron flux) in the axial direction in the reactor core be optimized to reduce the maximum fuel temperature. In the conventional nuclear reactor, fuel is employed at various degrees of uranium enrichment in the reactor core (in the example described herein, fuel is employed at twelve different degrees of uranium enrichment) for the optimization of the power distribution in the axial direction (see
FIG. 15 ). - In such an arrangement, the enrichment of the nuclear fuel in each portion of the
reactor core 12 is determined based on the power distribution in the reactor core, whereby the nuclear reaction of the nuclear fuel material in the fuel blocks 122 at each position of thereactor core 12 is adjusted. For example, at a position where the enrichment of the nuclear fuel is set high, the nuclear reaction is accelerated to enhance the power density. Contrarily, at a position where the enrichment of the nuclear fuel is set low, the nuclear reaction is decelerated to suppress the power density. Thus, the output of thereactor core 12 can be adjusted. Such an arrangement is advantageous in that the power distribution in thereactor core 12 can be optimized. - Specifically, the enrichment of the nuclear fuel in each portion of the
reactor core 12 is determined so that the enrichment lowers from the upstream side of thecoolant flow path 1225 towards the downstream side thereof (i.e., from the upper side to the lower side of the reactor core 12) (seeFIG. 15 ). In other words, the enrichment of the nuclear fuel is set high at the upstream side of the reactor core 12 (upper side of the reactor core 12) to enhance the power density, whereas the enrichment of the nuclear fuel is set low at the downstream side of the reactor core 12 (lower side of the reactor core 12) to suppress the power density. Thus, the power distribution in the reactor core in the flow direction of the coolant is effectively optimized and the maximum fuel temperature is suppressed to a low level. InFIG. 15 , numerical values shown inside the portions of fuel blocks 122 indicate the enrichment of the nuclear fuel. - In the above structure, it is preferable that the inner reflector (graphite reflector column) 125 that reflects the neutrons be arranged closer to the center of the
reactor core 12 in the radial direction than the fuel blocks 122 in the reactor core 12 (seeFIG. 15 ). In such structure, theinner reflector 125 arranged closer to the center of thereactor core 12 in the radial direction reflects and moderates the neutrons. Thus, the structure is advantageous in that the neutrons are effectively moderated, and that the maximum fuel temperature which is reached at the center of the reactor core at the depressurization accident, for example, can be suppressed to a low level. InFIG. 15 , numerical values shown inside the portions of fuel blocks 122 indicate the uranium enrichment of the nuclear fuel. - For example, in the example shown in
FIGS. 15 and 16 , in the conventional nuclear reactor, the enrichment of the nuclear fuel in each portion of thereactor core 12 is set so that the enrichment decreases from the upstream side toward the downstream side of the reactor core 12 (i.e., from the upper side toward the lower side of the reactor core 12). At the same time, theinner reflector 125 is arranged inside thereactor core 12 closer to the center of thereactor core 12 in the radial direction than the fuel blocks 122. - In the
nuclear reactor 10, it is preferable that thereflector 126 which contains graphite and moderator that has a smaller moderating power than the graphite (for example, silicon carbide) be arranged inside thereactor core 12; that thereflector 126 is sectioned into plural parts along the flow direction of the coolant; and that the volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of thereflector 126 is determined based on the power distribution in thereactor core 12 in the flow direction of the coolant (seeFIG. 16 ). - In such structure, since the volume ratio of the graphite to the silicon carbide (moderator having a smaller moderating power than the graphite) in each portion of the
reflector 126 is determined based on the power distribution in the reactor core, the power density at each position in thereactor core 12 is adjusted. For example, at a position where the volume fraction of the silicon carbide is set high, the degree of moderation of neutrons becomes low, whereby the power density is suppressed. Contrarily, at a position where the volume fraction of silicon carbide is set low, the moderation of neutrons is accelerated to enhance the power density. Thus, the structure is advantageous in that the power distribution in thereactor core 12 can be adjusted, and that the power distribution in thereactor core 12 can be optimized. - Further, it is preferable that the volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in respective portions of the
reflector 126 in thenuclear reactor 10 be determined so that the volume ratio increases from the upstream side toward the downstream side of the reactor core 12 (from the upper side toward the lower side of the reactor core 12) (seeFIG. 16 ). In other words, the volume fraction of the graphite is set high at the upstream side of the reactor core 12 (upper side of the reactor core 12) to enhance the output of thereactor core 12. Contrarily, the volume fraction of the moderator having a smaller moderating power than the graphite is set high at the downstream side of the reactor core 12 (lower side of the reactor core 12) to suppress the power density. Thus, the structure is advantageous in that the power distribution in thereactor core 12 can be adjusted, and that the power distribution in thereactor core 12 in the flow direction of the coolant can effectively be optimized. - For example, the
inner reflector 126 in an example 11 shown inFIG. 16 is sectioned into five parts along the flow direction of the coolant (axial direction of the reactor core 12), so that the volume fraction of silicon carbide increases by 25% in each part from the upper side toward the lower side of thereactor core 12. Thereflector 126 is arranged in the second layer and the fifth layer from the center of thereactor core 12 in the radial direction. Thus, theplural reflectors 126 are arranged so as to sandwich the fuel blocks 122 from the inner side and the outer side in the radial direction of thereactor core 12, and so as to be located near the fuel blocks 122. - The numerical values shown in the fuel blocks 122 in
FIG. 16 indicate the enrichment of the nuclear fuel, whereas the numerical values in theinner reflectors 126 indicate the volume fraction of the moderator (silicon carbide) having a smaller moderating power than the graphite. For example, if the numerical value in thereflector 126 is 25%, it means that the volume ratio of the moderator having a smaller moderating power than the graphite to the graphite is 25:75. In the second embodiment, the composition of theinner reflector 126 is defined by volume. - As shown by the results of calculations shown in
FIG. 17 , it can be seen that the power distribution in thereactor core 12 in the flow direction of the coolant in thenuclear reactor 10 of the example 11 is further optimized in comparison with the power distribution in the conventional example. - As can be seen from the foregoing, in the nuclear reactor (pebble bed reactor) according to one aspect of the present invention, composition of the reflector (in particular, the volume fraction of the moderator which has a smaller moderating power than graphite) is determined based on the power distribution in the reactor core. A degree of moderation of neutrons at each position in the flow path of the fuel pebbles is adjusted, accordingly. Thus, the nuclear reactor according to one aspect of the present invention is advantageous in that the power distribution in the reactor core can be adjusted, and that the power distribution in the reactor core can be equalized accordingly.
- In the nuclear reactor (pebble bed reactor) of another aspect of the present invention, the volume fraction of the moderator, which has a smaller moderating power than the graphite, in a portion of the reflector is set high, when the portion is located in a region with high power density. Therefore, the neutron spectrum (neutron energy distribution) shifts towards a side of a higher energy level, to suppress the nuclear reaction, whereby the power density of the portion is decreased. Thus, the nuclear reactor according to another aspect of the present invention is advantageous in that the power distribution in the reactor core is effectively equalized in the flow direction of the fuel pebbles.
- In the nuclear reactor (pebble bed reactor) according to still another aspect of the present invention, the degree of moderation of neutrons is decreased at the upstream side of the flow path to suppress the power density. Under a condition set to keep the total power of the reactor core constant, the power density at a downstream side of the flow path increases owing to the suppressed amount of output at the upstream side. Thus, the nuclear reactor according to the still another aspect of the present invention is advantageous in that the power distribution in the reactor core is effectively equalized in the flow direction of the fuel pebbles.
- In the nuclear reactor (pebble bed reactor) according to still another aspect of the present invention, the degree of moderation of neutrons at the upstream side of the flow path is decreased to suppress the power density. Under the condition set to keep the total power of the reactor core constant, the power density at the downstream side of the flow path is increased owing to the suppressed amount of output at the upstream side. Thus, the nuclear reactor according to the still another aspect of the present invention is advantageous in that the power distribution in the reactor core is effectively equalized in the flow direction of the fuel pebbles.
- The nuclear reactor (pebble bed reactor) according to the still another aspect of the present invention is advantageous in comparison with the nuclear reactor according to the other aspects in that the power distribution in the reactor core is more effectively equalized in the flow direction of the fuel pebbles.
- In the nuclear reactor (pebble bed reactor) according to still another aspect of the present invention, the degree of moderation of neutrons is decreased at the upstream side of the flow path, to suppress the power density. Further, since the neutrons are moderated by both the inner reflector and the outer reflector, the power density is effectively suppressed. Under the condition set to keep the total power of the reactor core constant, the output is increased at the downstream side of the flow path owing to the suppressed amount of output at the upstream side of the flow path. Thus, the nuclear reactor according to the still another aspect of the present invention is advantageous in that the power distribution in the reactor core is effectively equalized in the flow direction of the fuel pebbles.
- The nuclear reactor (pebble bed reactor) according to the still another aspect of the present invention is advantageous in comparison with the nuclear reactor according to the other aspects in that the power distribution in the reactor core is more effectively equalized in the flow direction of the fuel pebbles.
- In the nuclear reactor (pebble bed reactor) according to still another aspect of the present invention, since the criticality of the reactor core is adjusted by the enrichment of the fuel pebble, the output can be maintained.
- In the nuclear reactor (block reactor) according to still another aspect of the present invention, the volume ratio of the graphite to the moderator which has a smaller moderating power than the graphite in each portion of the reflector is determined so as to optimize the power distribution in the reactor core. Therefore, the power density of the reactor core at each position in the coolant flow path is adjusted. Thus, the nuclear reactor according to the still another aspect of the present invention is advantageous in that the power distribution of the reactor core can be adjusted, and that the power distribution in the reactor core can be equalized and/or optimized accordingly.
- In the nuclear reactor (block reactor) according to still another aspect of the present invention, the volume fraction of the moderator which has a smaller moderating power than graphite is set high in the downstream side of the coolant flow path (lower side of the reactor core) to suppress the maximum power density and/or the maximum fuel temperature. Thus, the nuclear reactor according to the still another aspect of the present invention is advantageous in that the power distribution in the reactor core can be adjusted and that the power distribution in the reactor core in the flow direction of the coolant can be effectively optimized so that the power density decreases towards the downstream side.
- In the nuclear reactor according to the present invention, the composition of the reflector (in particular the volume fraction of the moderator that has a smaller moderating power than graphite) is determined so as to optimize the power distribution in the reactor core. Therefore, the degree of moderation of neutrons at each position in the flow path of the fuel bodies is adjusted. Thus, the nuclear reactor according to the present invention is advantageous in that the power distribution in the reactor core can be adjusted, and that the power distribution in the reactor core can be optimized accordingly.
- In the nuclear reactor according to the present invention, since the volume ratio of the graphite to the moderator which has a smaller moderating power than the graphite in each portion of the reflector is determined so as to optimize the power distribution in the reactor core, the power density of the reactor core at each position in the coolant flow path is adjusted. Thus, the nuclear reactor according to the present invention is advantageous in that the power distribution in the reactor core can be adjusted, and that the power distribution of the reactor core can be equalized and/or optimized accordingly.
- As can be seen from the foregoing, the nuclear reactor according to the present invention is useful in that the power distribution in the reactor core can be equalized and/or optimized without the need of repetitious reloading of the fuel pebbles (in the pebble bed reactor core) and without the need of preparation of fuel in various different uranium enrichment levels (in the block-type reactor core).
- Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.
Claims (2)
1. A nuclear reactor of a Block-type High Temperature Gas Reactor comprising:
a reactor core;
a fuel block arranged in the reactor core, the fuel block having a coolant flow path through which a coolant flows to cool the fuel block; and
a reflector arranged inside the reactor core and sectioned into plural parts along a flow direction of the coolant, the reflector containing graphite and a moderator having a smaller moderating power than the graphite, wherein
a volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflector is determined based on a power distribution in the reactor core in the flow direction of the coolant, and
a volume fraction of the moderator having the smaller moderating power than the graphite in each part of the reflector is determined so that the volume fraction increases from an upstream side to a downstream side of the coolant flow path.
2. The nuclear reactor according to claim 1 , wherein
the reflector is sectioned into five parts along the flow direction of the coolant, so that the volume fraction of the moderator having the smaller moderating power than the graphite increases by 25% in each part from the upper side toward the lower side of the reactor core.
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US13/560,275 US20120288052A1 (en) | 2007-08-01 | 2012-07-27 | Nuclear reactor |
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JP2007200419A JP2009036606A (en) | 2007-08-01 | 2007-08-01 | Nuclear reactor |
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US11/965,378 US7978807B2 (en) | 2007-08-01 | 2007-12-27 | Nuclear reactor |
US13/115,517 US20110222641A1 (en) | 2007-08-01 | 2011-05-25 | Nuclear reactor |
US13/560,275 US20120288052A1 (en) | 2007-08-01 | 2012-07-27 | Nuclear reactor |
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DE102007056135A1 (en) * | 2007-11-20 | 2009-05-28 | Ald Vacuum Technologies Gmbh | Reactor graphite consisting of isotropic highly crystalline natural graphite as the main component and silicon or zirconium carbide and its preparation |
WO2011040989A1 (en) * | 2009-04-09 | 2011-04-07 | The Regents Of The University Of California | Annular core liquid-salt cooled reactor with multiple fuel and blanket zones |
RU2009127347A (en) * | 2009-07-17 | 2011-01-27 | Олег Николаевич Морозов (RU) | REACTOR INSTALLATION |
RU2501105C1 (en) * | 2012-11-13 | 2013-12-10 | Федеральное государственное бюджетное учреждение "Национальный исследовательский центр "Курчатовский институт" | Method of prolonging life of channel-type graphite nuclear reactor |
JP6473601B2 (en) * | 2014-11-12 | 2019-02-20 | イビデン株式会社 | Core structural material |
JP6473602B2 (en) * | 2014-11-12 | 2019-02-20 | イビデン株式会社 | Graphite block |
US9793010B2 (en) | 2015-02-19 | 2017-10-17 | X-Energy, Llc | Nuclear fuel pebble and method of manufacturing the same |
US10522255B2 (en) | 2015-02-19 | 2019-12-31 | X-Energy, Llc | Nuclear fuel pebble and method of manufacturing the same |
CA3089067A1 (en) * | 2018-01-22 | 2019-08-29 | Ultra Safe Nuclear Corporation | Composite moderator for nuclear reactor systems |
US11075015B2 (en) * | 2018-03-12 | 2021-07-27 | Terrapower, Llc | Reflectors for molten chloride fast reactors |
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US3366549A (en) * | 1966-06-30 | 1968-01-30 | Atomic Energy Commission Usa | Gas-cooled nuclear reactor |
DE3020124A1 (en) * | 1980-05-27 | 1981-12-03 | Hochtemperatur-Reaktorbau GmbH, 5000 Köln | GAS COOLED HIGH TEMPERATURE REACTOR |
JPS622189A (en) * | 1985-06-28 | 1987-01-08 | 株式会社日立製作所 | High-temperature gas furnace |
JPS6291894A (en) * | 1985-10-18 | 1987-04-27 | 日本原子力研究所 | Nuclear reactor |
DE3601747A1 (en) * | 1986-01-22 | 1987-07-23 | Hochtemperatur Reaktorbau Gmbh | SWITCHING OFF A HIGH TEMPERATURE REACTOR |
JP3326759B2 (en) * | 1993-12-27 | 2002-09-24 | 学校法人東海大学 | Plutonium annihilation nuclear reactor using liquid nuclear fuel |
JP3905392B2 (en) | 2002-01-31 | 2007-04-18 | 株式会社日立製作所 | Reactor equipment, reactor core, and operating method of reactor |
US7864913B2 (en) * | 2004-02-19 | 2011-01-04 | Kabushiki Kaisha Toshiba | Fast reactor having reflector control system and neutron reflector thereof |
US7403585B2 (en) * | 2004-07-01 | 2008-07-22 | Battelle Energy Alliance, Llc | Optimally moderated nuclear fission reactor and fuel source therefor |
US20060210011A1 (en) * | 2005-03-16 | 2006-09-21 | Karam Ratib A | High temperature gas-cooled fast reactor |
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2007
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- 2007-12-27 US US11/965,378 patent/US7978807B2/en not_active Expired - Fee Related
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US20110222641A1 (en) | 2011-09-15 |
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