WO2006137845A1 - Reacteur nucleaire a lit de boulets bidisperses - Google Patents

Reacteur nucleaire a lit de boulets bidisperses Download PDF

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Publication number
WO2006137845A1
WO2006137845A1 PCT/US2005/031928 US2005031928W WO2006137845A1 WO 2006137845 A1 WO2006137845 A1 WO 2006137845A1 US 2005031928 W US2005031928 W US 2005031928W WO 2006137845 A1 WO2006137845 A1 WO 2006137845A1
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Prior art keywords
pebbles
reflector
pebble
fuel
column
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PCT/US2005/031928
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English (en)
Inventor
Martin Z. Bazant
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Massachusetts Institute Of Technology
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Publication date
Priority claimed from US10/934,051 external-priority patent/US20060050835A1/en
Application filed by Massachusetts Institute Of Technology filed Critical Massachusetts Institute Of Technology
Publication of WO2006137845A1 publication Critical patent/WO2006137845A1/fr

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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
    • 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

  • VHTR Very High Temperature Gas-Cooled Reactor
  • NGNP Next Generation Nuclear Plant
  • INEEL Idaho National Engineering and Environmental Laboratory
  • the Block Prismatic Reactor in which the uranium is embedded in a solid graphite core, composed of blocks pre-drilled with holes to allow the passage of helium coolant, as well as control rods.
  • a reactor of this type is currently operating in Japan.
  • a disadvantage of the static block design is that the core must be shut down and reassembled as part of a costly and dangerous refueling process.
  • the static core design also makes it difficult to ensure uniform burn-up and prevents any adjustments to the fuel and moderator distribution during operation, both of which can negatively impact fuel efficiency.
  • the second VHTR/NGNP design is the Pebble Bed Reactor (PBR), in which the uranium TRISO microspheres are embedded in graphite fuel pebbles, roughly the size of a tennis ball.
  • PBR Pebble Bed Reactor
  • the fuel pebbles are very slowly cycled through the pebble-bed core, as many as 10-15 times each, which yields rather uniform burn-up and allows continuous operation without the need to shutdown for refueling.
  • PBR was invented by Dr. Rudolf Schulten in the late 1950s, and Germany later successfully operated a pebble-bed reactor for over 20 years. The PBR concept is currently being revived around the world.
  • HTR-IO small pebble-bed reactor
  • PBMR Pebble-Bed Modular Reactor
  • MIT Massachusetts Institute of Technology
  • MPBR Modular Pebble-Bed Reactor
  • pebble-bed reactors are continuously refueled by slowly cycling radioactive fuel pebbles, which resemble billiard balls, through the reactor core.
  • the pebble-bed core is believed to be immune to the "worst-case" nuclear-reactor scenario (i.e., a loss of coolant which would lead to a melting of the uranium fuel and a catastrophic release of radiation).
  • the radioactive material is contained in specially engineered micro-spheres (see below) dispersed in solid graphite, which cannot get hot enough to melt.
  • the graphite encasement should also make the spent fuel pebbles more rugged and resistant to corrosion in long-term storage, which is aided by the lack of radioactive liquid waste, due to the choice of helium gas (rather than water) as a coolant.
  • the graphite reflector pebbles like the graphite lining of the reactor, are substantially nonfissionable (i.e., few, if any, of then nuclei therein undergo fission during typical reactor operation); and they reflect and slow the uranium's neutrons to thereby moderate the neutrons so as to enhance the energy-producing fission process.
  • the reflector pebble need not be pure graphite, but nevertheless is formed mostly of one or more substances of low atomic weight (e.g., beryllium or carbon) that serve to reflect and slow the neutrons with little tendency toward neutron absorption and that do not significantly release neutrons or other nuclear decay products.
  • the reflector pebbles do not include the radioactive micro-spheres and are generally considered non-radioactive; though, at most, free neutrons are generated in the reflector pebbles at a rate decreased several orders of magnitude (e.g., less than 0.01%) from that of the average fuel pebble in the reactor, if free neutrons are generated in the reflector pebbles at all.
  • Each of the uranium-dioxide micro-spheres in the fuel pebbles is typically about 0.5 mm in diameter and is coated with a layer of porous carbon, a layer of high-density pyrolitic carbon, a layer of silicon carbide, and then another layer of high-density pyrolitic carbon (a composite referred to as "TRISO").
  • the uranium is in the form of enriched U-235.
  • the silicon carbide layer is sufficiently dense that no radiologically significant quantities of gaseous or metallic fission products are released from the fuel elements at temperatures up to 1,65O 0 C; this temperature range far exceeds the normal operating temperature (about 1,200 0 C) of a reactor and is further believed to exceed the core temperature response that would arise from a loss of forced cooling in the reactor.
  • the uranium of the uranium-oxide is enriched to provide about 8% U-235, which is the isotope of uranium that undergoes the fission reaction.
  • the encapsulation of the uranium-oxide micro-spheres also reduces or eliminates any risk that they might otherwise pose as a resource for weapons proliferation, which is low to begin with due to the relatively low concentration of U-235.
  • the reflector pebbles 10 are fed through a drop- hole at the end of a central conduit 12 into a reactor core vessel 14.
  • the central conduit 12 leads to a drop hole at the approximate center of the ceiling 15 of the reactor vessel 14, though the central conduit 12 (or set of such conduits) need not enter the ceiling precisely at the center.
  • the reactor core vessel 14 is a cylinder encased in walls of reflecting graphite blocks.
  • the reflector pebbles 10 fill a central reflector column 16 on the axis of the reactor core vessel 14, where the reflector pebbles flatten the neutron flux in the center of the core.
  • fuel pebbles 18 are fed through drop-holes at the ends of a plurality of peripheral conduits 20 passing through the ceiling 15 of the reactor core vessel 14.
  • the fuel pebbles 18 form an annular fuel column 22 between the column 16 of reflector pebbles 10 and the outer vessel wall.
  • the pebbles 10, 18 slowly flow downward through the reactor core vessel 14 to the sorter 26 at its exit.
  • the sorter 26 sorts the reflector pebbles 10 from the fuel pebbles 18 as they exit the vessel 14 and typically redirects the pebbles 10, 18 back to the top of the vessel 14 through conduits 12 and 20, e.g., by applying a pressure differential.
  • the sorter 26 further identifies spent fuel pebbles 28, which it removes from circulation, and introduces fresh fuel pebbles 30 to replace the spent fuel pebbles 28.
  • the sorter 26 can sort the pebbles 10, 18 and identify spent fuel pebbles 28 on the basis of any of a variety of properties, such as the temperature and mass of the pebbles or (primarily) from measurements of the amount of radiation emitted from the pebbles.
  • the central reflector column 16 and the annular fuel column 22 do not share a distinct boundary as mixing of the reflector pebbles 10 and fuel pebbles 18 occurs in what is referred to as an annular mixed column 24 between the central reflector column 16 and annular outer column 22.
  • the mixed column 24 is not directly controlled. Instead, it arises spontaneously through complicated dynamical processes occurring at the upper surface 34 below the drop holes in the ceiling 15 as well as in the subsurface region 16, 22, 24 of bulk granular flow toward the sorter 26.
  • the graphite reflector pebbles 10 in the central column 16 help to moderate the nuclear chain reactions in the core vessel 14 by slowing neutrons released from the fuel pebbles 18 and reflecting the neutrons back into the fuel column 22 where the neutrons can cause more fission events and thus sustain the reaction, hi this process, the graphite does not itself undergo any nuclear fission reactions; it simply redirects the neutrons and absorbs some of their kinetic energy.
  • the moderating function of the graphite reflector pebbles 10 is also performed by the graphite lining of the fuel pebbles 18 as well as by the graphite blocks that form the outer wall of the core vessel 14.
  • the core vessel 14 is roughly 3.5 m in diameter (larger than the German reactor core), and were it filled only with fuel pebbles 18, the central region would experience much larger fluxes than the outer region. This would lead to non-uniform burning and, given the size of the reactor, could make controlling the reaction more difficult.
  • the central reflector column 16 of reflector pebbles 10 thus allows for greater fuel efficiency and more-uniform burning. In a conventional liquid-cooled reactor, control over the bulk neutron- flux profile can be achieved by inserting graphite rods into the core.
  • central graphite column Another purpose of the central graphite column is related to safety and control. As in conventional reactors, the power level is adjusted by control rods, which slow and absorb neutrons, typically with boron carbide. Fully inserting the rods allows rapid shutdown, by making the nuclear chain reaction go subcritical. In a pebble-bed reactor, the control rods cannot penetrate the granular-solid pebble bed, so they are inserted into pre-drilled holes in the outer graphite bricks of the core vessel. Another advantage of the central graphite column, therefore, is that the fuel is confined to a more narrow annular region, closer to the control rods in the outer core walls.
  • the width of the central graphite column must be carefully determined and controlled. If the column is too wide, then the power output of the reactor is overly reduced. On the other hand, if it is too narrow, the neutron flux distribution is overly non-uniform, leading to inefficient fuel burn-up. Moreover, the peak neutron flux could exceed fuel temperature limits.
  • the relative sizes of the graphite reflector column 16 and the fuel column 22 are determined by the placement (in advance) of different dropping points for pebbles at the top of the core vessel (where the various conduits 12, 20 enter the ceiling 15 of the reactor vessel 14). These dropping points cannot be moved, so the steady-state composition of the core cannot be changed once the reactor is built and in operation.
  • helium gas passes through a porous graphite column, where it is not directly heated, as it is in the outer fuel annulus. This can compromise thermal efficiency by mixing the hottest helium from the fuel annulus with cooler helium from the graphite column, as the gas passes from the core to the heat exchanger and power generator. If the gas could be focused mostly on the fuel annulus, then the reactor could be operated at higher power (fewer inserted control rods) because heat could be extracted more quickly, while maintaining the appropriate maximum temperature.
  • the above concerns and drawbacks can be addressed by employing a guide ring apparatus and two different sizes of pebbles (a "bi-disperse" core) in the pebble-bed reactors and associated methods, described above.
  • the pebbles included the larger fuel pebbles, which contain fissionable radioactive material, and the smaller reflector pebbles, which are substantially free of fissionable radioactive material (e.g., consisting essentially of graphite).
  • the fuel pebbles and reflector pebbles in the reactor core vessel are in direct contact along a vertical interface without any physical barrier constraining the pebbles (so as to prevent mixing therebetween) throughout most or all of the volume occupied by the pebbles, hi the bi-disperse model, the fuel and reflector pebbles can share the characteristics of those described in the Background, other than the relative sizes of each.
  • the difficulty in predicting and controlling the pebble composition of the core is that the distribution of pebbles arises dynamically from the very complicated and poorly understood process of slow granular drainage from a silo. Although some simple and reasonably successful continuum models exist for the mean velocity, there is no mathematical model that correctly predicts velocity fluctuations and pebble mixing in this context.
  • This mixing at the free surface can be reduced or eliminated in pebble-bed nuclear reactors, such as those described throughout this disclosure, by a guide ring mounted within the reactor core vessel (as is described in U.S. Patent Application 10/264,098, the entire teachings of which are incorporated herein by reference).
  • Reflector pebbles are fed through a reflector conduit into the reactor core vessel and through the zone defined by the guide ring.
  • Radioactive fuel pebbles are fed through one or more fuel conduits into the reactor core vessel outside the zone defined by the guide ring.
  • the guide ring can be substantially cylindrical and should extend down to within at least one or two pebble diameters of the free surface of the pebble bed in the reactor core vessel such that the fuel pebbles and reflector pebbles flow through the reactor in an annular fuel column and in a central reflector column, respectively. Because very little granular mixing occurs beneath the free surface of the pebble bed, the use of the guide ring can provide a distinct boundary between the fuel column and reflector column throughout the reactor core vessel. Further, the cross-sectional area of the guide ring can be adjustable, or a plurality of nested guide rings can be provided to enable dynamic adjustment of the respective cross sections of the two columns (all sections cited herein being measured in a plane perpendicular to the axis of the reactor core vessel). The guide ring can be the only static part in the reactor core (the pebbles not being static).
  • the adjustable guide ring can essentially eliminate pebble mixing and can give unprecedented control over the core composition (and thus the fuel efficiency, safety, and power output) of a pebble-bed reactor that uses two or more different kinds of pebbles.
  • This guide ring is simple, safe and inexpensive to implement and therefore offers wide applicability in pebble-bed nuclear reactors, particularly in view of the potentially significant benefits in performance, in power variability and in the flexibility of being able to use different types of fuel in the same reactor vessel.
  • a potential disadvantage of the two-pebble design of MPBR is reduction in thermal efficiency caused by the passage of helium through the porous column of graphite pebbles.
  • the helium would flow entirely through the hotter and more radioactive fuel annulus, as in a recently modified PBMR design with a solid central graphite column. Improved helium flow can be achieved using pebbles alone, through a very simple modification, which preserves all of the advantages of MPBR and makes the design even more attractive.
  • This improvement can be effected by making the graphite moderator/reflector pebbles in the central column smaller than the (standard) fuel pebbles in the outer annulus.
  • the primary advantage of bi-dispersity lies in focusing the helium flow on the fuel column, where it is heated the most, which can dramatically improve thermal efficiency and power capability of the reactor. The latter would result from more-efficient cooling of the fuel region, which could allow the removal of more control rods (greater power), while keeping the core temperature at the target value for safety and optimal fuel performance.
  • a second advantage of bi-dispersity is the very easy sorting of fuel and graphite pebbles by size alone, upon exit from the core.
  • FIG. 1 is a partially schematic diagram of an existing design for a pebble-bed nuclear reactor, showing different regions of the reactor core, including a mixing column of reflector pebbles and fuel pebbles.
  • FIG. 2 is a partially schematic diagram of a new design for a pebble-bed nuclear reactor including a guide ring extending slightly below the free surface of the pebble bed in the reactor core vessel; the guide ring suppresses surface mixing in the reactor core vessel, which reduces or eliminates the annular mixed column in the core vessel.
  • FIG. 3 is a sectional view upward from plane 3-3 of the reactor illustrated in FIG. 2; the sectional view illustrates the output of the conduits through which the pebbles are dropped and the guide ring.
  • FIG. 4 is a sectional view of an adjustable guide ring.
  • FIG. 5 is a sectional view of a plurality of nested guide rings.
  • FIG. 6 is a sectional view of a bi-disperse pebble-bed nuclear reactor core, wherein the graphite pebbles (center) are smaller than the surrounding fuel pebbles.
  • FIG. 7 is a schematic illustration of coolant-gas flow through a standard pebble-bed nuclear reactor with only fuel pebbles, offered for comparison.
  • FIG. 8 is a schematic illustration of coolant-gas flow through a mono-disperse, two- pebble pebble-bed nuclear reactor with reflector and fuel pebbles of the same size.
  • FIG. 9 is a schematic illustration of coolant-gas flow through a bi-disperse, two- pebble pebble-bed nuclear reactor, wherein the reflector pebbles are smaller than the fuel pebbles.
  • FIG. 10 is an illustration, partially schematic, showing the walls of a pebble-bed nuclear reactor and other elements of the gas-flow system.
  • the power system of a modular pebble bed reactor includes a reactor, where thermal energy is generated by a nuclear reaction, and a power conversion unit, where the thermal energy is converted to mechanical work and then to electrical energy by a thermodynamic cycle and a generator.
  • a fluid particularly a gas, such as helium
  • Helium is particularly suitable as the heat-transfer medium because of its chemical inertness, its phase stability through normal operating changes in a reactor, and its small nuclear-absorption cross section.
  • a simple way to prevent the mixing of reflector and fuel pebbles and to precisely control the composition of the core is to add a "guide ring" assembly 32 to the top of the core vessel 14, as shown in FIG. 3.
  • the guide ring 32 can be made of graphite or some other strong, solid, neutron-reflecting material used in other parts of the core, and its inner diameter can be approximately half the diameter of the vessel 14 (e.g., 1.75 m ring inner-diameter for a vessel having an inner diameter of 3.5 m).
  • the pebbles 10, 18 typically fill 80 to 95% of the core vessel during operation, with the remaining volume being open space above the free surface of the pebble bed.
  • the guide ring 32 extends down to a position near the pebble-bed free surface (may be slightly above or below), so the guide ring 32 will extend away from the ceiling 15 into the core vessel 14 a distance of about 5% to about 20% of the height of the core vessel 14.
  • the remaining 80- 95% of the core vessel 14 (below the guide ring) is undivided volume for containing the pebbles 10, 18 without any physical separators for segregating the fuel pebbles 18 from the reflector pebbles 10.
  • the height of the guide ring 32 (measured along the axis of the cylinder) can be about 50 to about 250 cm.
  • the guide ring 32 extends down from the ceiling to a position that is, on average, 2-10 pebbles below the evolving free surface on either side (during steady state cycling of the core), although this depth allowance may need to be increased to accommodate larger fluctuations in surface height.
  • the guide ring 32 need not penetrate the free surface and can, in fact, extend down only to a height one to two pebble diameters above the free surface. Fluctuations in the height of the free surface due to avalanches can be reduced by using more drop holes per unit area of the ceiling, which leads to smaller conical piles and thus smaller avalanche events. Fluctuations in height can also arise from statistical variations in the arrival of different types of pebbles from the sorter.
  • the guide ring 32 need not extend directly from the ceiling 15, though the guide ring 32 preferably extends a sufficient height above the surface of the pebbles 10, 18 to ensure that no pebbles 10, 18 can bounce over the guide ring 32. This height can be determined by assuming that the pebbles on the surface behave as an inelastic solid floor that absorbs some fraction of the kinetic energy when a collision occurs. However, to be sure that no pebbles cross the regions separated by the guide ring, it may be preferable for the guide ring to extend directly from the ceiling.
  • the guide ring 32 in this embodiment is cylindrical and is aligned with the axis of the core vessel 14, though other cross-sectional shapes and alignments for the guide ring 32 can be utilized.
  • the guide ring 32 completely blocks avalanches of different kinds of pebbles 10, 18 from mixing at the free surface, but otherwise does not interfere with the bulk granular flow.
  • Pebbles 10, 18 tumbling down the free surface in avalanches settle into randomly packed positions against the wall of the guide ring 32, where they slowly sink into the core as the pebbles drain from the bottom of the core vessel 14.
  • the fuel pebbles 18 that are at the inner edge of the outer annular column 22 directly contact the reflector pebbles 10 that are at the outer edge of the central reflector column 16 without any physical barrier therebetween. Nevertheless, a sharp interface ⁇ i.e., little or no mixing or crossover of pebbles) exists between the columns 16, 20.
  • the moving interface will have roughness only at the scale of a single pebble, until it converges very close to the lower opening leading to the sorter 26.
  • a single ring is fixed in place at a position determined in advance by core-physics calculations.
  • the core composition is determined by drop-holes at fixed positions that typically cannot be practically changed after the reactor is built.
  • a fixed-guide ring would still have the desirable effect of eliminating the mixed column 24.
  • the reflector column 16 is not precisely at the center of the core vessel 14, and there may even be a plurality of reflector columns 16, each at a different position, within the core vessel 14.
  • a guide ring is provided for each reflector column to reduce or prevent intermixing of the reflector pebbles with the surrounding fuel pebbles.
  • FIG. 3 One configuration of drop holes that can be used in the reactor is illustrated in FIG. 3.
  • This configuration includes one centered conduit 12 through which reflector pebbles 10 are fed and eight peripheral conduits 20 through which fuel pebbles 18 are fed from the ceiling 15 of the core vessel 14.
  • the eight conduits 20 for the fuel pebbles 18 are evenly distributed around the periphery of the core. Though, of course, the number and particular positioning of the conduits can be varied. Where a plurality of reflector columns are formed in the pebble bed, corresponding conduits for supplying reflector pebbles are positioned above respective guide rings for each of the columns.
  • the utility of the adjustable guide ring for providing dynamic variation of the core composition is desirable for a number of reasons. First, the adjustability of the guide ring can offer substantial benefits in terms of efficiency and safety.
  • a predetermined fixed design may not (and generally will not) be configured to set the optimal width of the reflector column from the perspectives of fuel efficiency or safety from power peaking. Since the optimal width of the central column is difficult to predict in advance from mathematical models, it would be preferable to measure fuel efficiency, peak temperatures, or other metrics empirically once the reactor is operating and then adjust the width of the central graphite column as desired until an optimal composition is reached. Second, the ability to dynamically vary the core composition affords the operator the flexibility to use different types of fuel, with different performance characteristics and limitations. Since the design of fuel pebbles is an area of active research and development, it is likely that a functioning reactor may need to switch its fuel type.
  • Changing the type of fuel also enables one to control the power output of the reactor in response to various economic considerations.
  • the core composition may not be possible to adjust the core composition to make optimal use of the new fuel and also to stay within its possibly different design limitations, e.g., maximum allowable temperature.
  • the adjustable guide ring makes it easy to control the width of the inner column, and hence the composition of the reactor core, on the fly during reactor operation by adjusting the width of the guide ring 32.
  • the guide ring extends deep into the core, it is not easily moved radially (to directly expand or contract its diameter) because the granular material acts like a hard solid in the bulk packed region. Near the free surface, however, the guide ring can be moved radially without much hindrance by surface pebbles, which are fairly easily displaced. Above the free surface, the guide ring can be moved radially without any trouble.
  • the guide ring 32 when it extends no more than a few pebble diameters (e.g., no more than 10) into the core, can be widened or constricted if, e.g., it is made of a set of overlapping guide-ring members 36, here in the form of cylindrical arcs, which can be offset relative to one another (via, e.g., a motorized displacement mechanism 38) to provide varying degrees of overlap and to consequently circumscribe a greater or lesser volume.
  • any such motor mechanism may need to be outside the reactor vessel 14 to avoid the risks of operating in a high- temperature, high-radiation environment.
  • each of the rings 32 can be lowered one at a time into the core vessel 14 from the ceiling of the vessel (by a mechanism outside the shielding inner wall of the vessel) to the prescribed guide-ring height below the free surface of the pebble bed.
  • the rings 32 can be in a variety of sizes having inner diameters ranging from about one-quarter to about three-quarters the inner diameter of the vessel 14.
  • the core can be drained without refilling while one guide ring 32 is raised and another guide ring 32 is lowered, but this interferes with the normal cycling operation and requires a way of storing some pebbles before reintroducing them into the vessel.
  • a simpler approach would be to switch from one guide ring to a new one of a different size as follows. (1) While the old guide ring is still fixed in its normal operating position, the new ring is lowered from its storage position in the ceiling until it rests under its own weight on the free surface, at some distance above the normal operating depth. (2) The old ring is raised to its storage position in the ceiling, while adjustments are made for pebbles to begin arriving according to the new composition (ratio of fuel to reflector pebbles), consistent with the desired width for the central column set by the new guide ring. (3) The new ring is allowed to slowly sink into the pebble bed under its own weight as the core drains, while new pebbles arriving from the drop holes are blocked from crossing it.
  • the new ring is fixed in place when it sinks to its desired operating height. In this way, the width of the guide ring can be adjusted without interfering with the normal operation of the reactor, aside from any changes needed to modify the ratio of fuel to reflector pebbles in the core vessel.
  • the method just described may lead to temporary height fluctuations in the region being contracted (either the central graphite column or the outer fuel column), which could conceivably be larger than desired (e.g., to avoid possibly blocking a drop-hole), hi that case, very little perturbation of the surface height can be achieved by simply repeating this process for a sequence of closely nested rings, whose diameters may differ by as little as one or two pebble diameters, with concomitant small changes in the fuel to reflector pebble ratio. Since it may also be desirable to make adjustments in composition gradually for other reasons (e.g., to see the effect of the new composition on fuel efficiency), this would most likely be normal operational procedure to change the width of the guide-ring.
  • a new concept for MPBR and related designs is a bi-disperse core in which the two kinds of pebbles, fuel pebbles 18 and graphite moderator/reflector pebbles 10, have different sizes.
  • the reflector pebbles 10 are significantly smaller than the fuel pebbles 18; e.g., the diameter (linear size) of the reflector pebbles 10 can be less than 90% that of the fuel pebbles 18.
  • the diameter of the reflector pebbles 10 is at least 20% smaller (e.g., in the range of about 20% to about 50% smaller) than that of the fuel pebbles 18.
  • the linear size ratio of the reflector pebbles 10 to the fuel pebbles 18 is about 1 :2.
  • the size ratio of the fuel pebbles 18 and reflector pebbles 10 is an important parameter, the optimal value of which may vary depending on the composition (i.e., the number of reflector pebbles 10 versus the number of fuel pebbles 18) and geometry (i.e., reactor core size and shape).
  • the pebble flow and core profile for the bi-disperse MPBR can be very similar to that of the "mono-disperse" MPBR with only one size for graphite and fuel pebbles (as illustrated in FIG. 2).
  • Experiments and simulations at MIT have shown that the degree of bulk mixing during the cycling of a mono-disperse core is very small with pebbles typically deviating from the mean streamlines on the order of one pebble diameter (well below the free surface).
  • the hot helium gas leaves the core through a large number of small holes drilled in the lower walls, mainly in the funnel region 46.
  • the guide ring 32 in MPBR helps somewhat to concentrate gas flow by creating a stagnant zone 48 above the reflector column 16, forcing the gas flow to enter the column 16 further down in the pebble bed, as shown in FIG. 8.
  • the bi-disperse core (with a guide ring 32), however, causes a much stronger expulsion of gas streamlines from the reflector column 16, thus focusing the gas flow on the hotter fuel column 22 as desired, as shown in FIG. 9.
  • a more-detailed view of the wall structure and apparatus associated with the gas flow in an MPBR having a guide ring is shown in FIG. 10.
  • Cooler, high-pressure helium gas is introduced from conduit 56 into a top chamber 58 of the reactor core vessel 14.
  • the gas is driven through the conduit 56 and through the rest of the system by a pump 60.
  • the gas flows through passages 62 in the graphite bricks 63 through the core ceiling 15 outside the inner volume defined by the guide ring 32.
  • the gas flows through the reactor core, where it extracts heat from the fuel pebbles, and exits from the funnel region 46 through passages 64 into a bottom chamber 66.
  • the hotter, lower-pressure helium gas then exits the bottom chamber 66 and flows through conduit 56 through a turbine 68, which is coupled with a generator 70 for harnessing electricity from the energy of the heated gas.
  • the gas is then pumped back through the reactor core vessel 14 again after cooling and compression.
  • control rods 72 inserted through the graphite bricks 63 for controlling the rate of reactions within the core.
  • This focusing of the gas flow can be easily understood in terms of the classical theory of flow in porous media.
  • the flow of helium gas through the pebble bed should obey Darcy's Law to a good approximation, as for other fluids passing through porous media.
  • Darcy's law states that the mean gas velocity for a given pressure gradient is proportional to the local permeability of the porous medium. For mono-disperse random packings of spheres, the permeability varies as the square of the sphere diameter (or pore size).
  • the permeability, and hence the gas flow rate will be smaller in the more-dense reflector column 16 than in the less-dense fuel annulus 22 by a factor of the square of the size ratio of the two kinds of pebbles 10, 18.
  • the permeability of the central reflector column 16 is reduced by a factor of four. This strong sensitivity to the size ratio allows a dramatic reduction in the gas flow through the graphite reflector column 16, instead focusing the helium on the hotter fuel column 22, which is the optimal flow profile.
  • the change in helium-flow distribution is accomplished without significantly changing any other aspects of the pebble-bed core behavior ⁇ e.g., pebble flow and core neutronics).
  • the precise degree of flow focusing depends mainly on geometrical factors, which are easily controlled as part of the reactor design, such as the ratio of radii of the two kinds of pebbles 10 and 18, and the ratio of radii of the two columns (the reflector column 16 and the fuel column 22).
  • the former variable can be easily varied while the reactor is operating.
  • the adjustable guide ring 32 which sets the latter variable, this additional flexibility allows the bi-disperse core to be optimized and controlled during normal operation, in sharp contrast to static core designs (such as the Block Prismatic VHTR).

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  • Devices And Processes Conducted In The Presence Of Fluids And Solid Particles (AREA)

Abstract

La présente invention a trait à une cuve de coeur de réacteur d'un réacteur nucléaire à lit de boulets contenant des boulets combustibles et des boulets réflecteurs alimentés dans la cuve à travers des conduits respectifs. Un anneau de guidage peut être prévu de sorte que les boulets réflecteurs traversent l'anneau de guidage et forment une colonne réflectrice, tandis que les boulets combustibles passent hors de l'anneau de guidage et forment une colonne combustible annulaire entourant la colonne réflectrice. L'anneau de guidage contrôle la taille et la forme de la colonne réflectrice et contrôle le mélange des deux types de boulets. En outre, les boulets combustibles peuvent présenter un diamètre sensiblement supérieur à celui des boulets réflecteurs. Par conséquent, la colonne réflectrice va avoir plus d'espace vide, et un gaz de refroidissement va donc s'écouler de préférence à travers la colonne de boulets combustibles.
PCT/US2005/031928 2004-09-01 2005-08-31 Reacteur nucleaire a lit de boulets bidisperses WO2006137845A1 (fr)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US93256104A 2004-09-01 2004-09-01
US10/932,561 2004-09-01
US10/934,051 US20060050835A1 (en) 2004-09-03 2004-09-03 Bi-disperse pebble-bed nuclear reactor
US10/934,051 2004-09-03

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WO2006137845A1 true WO2006137845A1 (fr) 2006-12-28

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN104756195A (zh) * 2012-07-25 2015-07-01 李正蔚 球形燃料反应堆
EP2518733A4 (fr) * 2009-12-23 2015-07-08 Univ Tsinghua Système de production de vapeur doté d'un réacteur à haute température refroidi au gaz et procédé associé

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR80481E (fr) * 1956-05-17 1963-05-03 Brown Réalisation de réactions nucléaires dans un réacteur surgénérateur
US3287910A (en) * 1963-09-04 1966-11-29 Cornell Aeronautical Labor Inc Nuclear reactor
GB1057527A (en) * 1964-07-01 1967-02-01 Brown Boveri Krupp Reaktor Apparatus for sorting spherical beads
DE2631237A1 (de) * 1976-07-12 1978-01-19 Hochtemperatur Reaktorbau Gmbh Gasgekuehlter kernreaktor mit einer schuettung kugelfoermiger brennelemente
US20040066875A1 (en) * 2002-10-03 2004-04-08 Bazant Martin Z. Guide ring to control granular mixing in a pebble-bed nuclear reactor

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR80481E (fr) * 1956-05-17 1963-05-03 Brown Réalisation de réactions nucléaires dans un réacteur surgénérateur
US3287910A (en) * 1963-09-04 1966-11-29 Cornell Aeronautical Labor Inc Nuclear reactor
GB1057527A (en) * 1964-07-01 1967-02-01 Brown Boveri Krupp Reaktor Apparatus for sorting spherical beads
DE2631237A1 (de) * 1976-07-12 1978-01-19 Hochtemperatur Reaktorbau Gmbh Gasgekuehlter kernreaktor mit einer schuettung kugelfoermiger brennelemente
US20040066875A1 (en) * 2002-10-03 2004-04-08 Bazant Martin Z. Guide ring to control granular mixing in a pebble-bed nuclear reactor

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2518733A4 (fr) * 2009-12-23 2015-07-08 Univ Tsinghua Système de production de vapeur doté d'un réacteur à haute température refroidi au gaz et procédé associé
CN104756195A (zh) * 2012-07-25 2015-07-01 李正蔚 球形燃料反应堆
CN104756195B (zh) * 2012-07-25 2017-07-07 李正蔚 球形燃料反应堆

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