JP6483389B2 - Fast neutron core design method - Google Patents

Description

Embodiments of the present invention relates to a method of designing a high-speed neutrons core.

  FIG. 21 is a plan view showing a configuration example of a core of a conventional fast reactor. The fast reactor core 103 includes a core fuel assembly 16 disposed in the center and containing a large amount of fissile material, and further includes a large amount of fissile parent material disposed on the outer side in the radial direction and converted into a fissile material by neutron absorption. A blanket fuel assembly 19 is included. The core fuel assembly 16 includes an inner core fuel assembly 17 disposed in the central region, and an outer core fuel assembly 18 disposed radially outside thereof. Further, the control rod guide tube 20 is arranged without being adjacent to each other in the region where the inner core fuel assemblies 17 are arranged.

  In FIG. 21, the inner core fuel assembly 17 is represented by a regular hexagon, the outer core fuel assembly 18 is a circle in the hexagon, and the blanket fuel assembly 19 is a double circle in the hexagon. Reference numeral 20 indicates a hexagonal shape with C added.

  FIG. 22 is an elevational sectional view showing the structure of a conventional fuel assembly. FIG. 23 is a horizontal sectional view taken along line XXIII-XXIII in FIG. Each of the inner core fuel assembly 17 and the outer core fuel assembly 18 has a plurality of fuel elements 27 disposed in a tubular trumpet tube 24 as shown in FIGS. 22 and 23.

  Below the trumpet tube 24, there is provided an entrance nozzle 22 for constituting the coolant flow path by fixing and supporting the core fuel assembly 16 to a core support plate (not shown). A coolant inflow hole 21 is formed in the side wall of the entrance nozzle 22. An upper portion of the trumpet tube 24 is provided with a handling head 25 that serves as a grip when the core fuel assembly 16 is loaded into the fast reactor core 103 or taken out from the fast reactor core 103.

  The coolant 10 flowing out from the coolant inflow hole 21 to the lower part of the core fuel assembly passes through the entrance nozzle 22 and flows into the fuel element 27 that is a heat generation source. The coolant 10 is heated by the fuel element 27 and then flows out from the coolant outlet hole 26 in the center of the handling head 25.

  FIG. 24 is an elevational sectional view of a fuel element of a conventional fuel assembly. 25 is a horizontal sectional view taken along line XXV-XXV in FIG. As shown in FIGS. 24 and 25, in the fuel element 27, a core fuel 28 containing a large amount of fissile material is disposed in the center of the fuel, and the lower and upper portions of the core fuel 28 are converted into fissile material by neutron absorption. A lower blanket fuel 29 and an upper blanket fuel 30 containing a large amount of fissile parent material are disposed. These fuels are stored in the cladding tube 34, the lower portion of the cladding tube 34 is sealed with the lower end plug 35, and the upper portion is sealed with the upper end plug 36. A gap is provided between the core fuel 28, the lower blanket fuel 29 and the upper blanket fuel 30, and the cladding tube 34. In the case of an oxide fuel core, this gap is filled with helium gas. Further, a lower plenum 32 and an upper plenum 33 for accumulating gas discharged by burning the fuel are formed below the lower blanket fuel 29 and above the upper blanket fuel 30 in the cladding tube 34, respectively. ing.

  In a nuclear reactor using fast neutrons, reducing the combustion reactivity contributes to improving the economy by extending the cycle length and improving the safety by reducing the insertion reactivity of the control rod erroneous pulling. Here, the combustion reactivity is a change in reactivity accompanying combustion of fuel from the start to the end of the operation cycle.

  In a conventional nuclear reactor, a method of reducing the plutonium (Pu) enrichment, improving the internal conversion ratio, and reducing the combustion reactivity has been adopted. In this method, it is known that when the Pu isotope composition varies, the combustion reactivity also varies greatly (see, for example, Patent Document 1).

  In addition, in the case of a uranium-free fuel for improving the efficiency of transuranium (TRU) combustion including Pu, the combustion reactivity is greatly increased. Therefore, a method that does not depend on fluctuations in Pu isotope composition or the size of uranium is desired. There is also a demand for an accelerator-driven nuclear reactor to suppress the increase in the accelerator beam current due to the combustion reactivity.

  In order to solve such a demand, there are several design examples in which a flammable poison is applied in a nuclear reactor using fast neutrons (see, for example, Patent Document 2, Non-Patent Document 1, and Non-Patent Document 2). However, a systematic design method is not shown in these design examples.

JP 2008-216009 A JP 2005-24346 A

Feasibility of Using Burnable Poisons for Reduction of Coolant Void Reactivity in LMR for TRU Transmutation (PHYSOR 2004) A STUDY ON BURNABLE ABSORBER FOR A FAST SUB-CRITICAL REACTOR HYPER (OECD / NEA 2001)

  By the way, in the case of applying a flammable poison in a reactor using fast neutrons as described above, the combustion conversion speed is slow just by mixing a neutron absorber, so the moderator is used to reduce the energy of the neutron. A method of increasing the conversion speed by decelerating from a high speed is known.

  An object of the embodiment of the present invention is to further increase the burning rate of the neutron absorber in the system in which the moderator and the neutron absorber are arranged.

Further, the fast neutron core design method according to the present embodiment includes a moderator thickness determining step for setting the thickness of the neutron moderator, and an absorber setting step for setting the material of the neutron absorber and the thickness of the neutron absorber. And a neutron flux calculation step for calculating a neutron flux distribution of the system after the moderator thickness determination step and the absorber setting step, and a neutron set in the absorber setting step after the neutron flux calculation step Based on the neutron absorption cross section by the material of the absorber, the neutron flux obtained in the neutron flux calculation step, and the operation period, the absorber remaining amount calculation step for calculating the remaining amount of the neutron absorber at the end of the operation period; the percentage relative to the initial amount of the remaining amount of neutron absorbing portion is closed and a determination step of determining whether less or not than a specified value, the thickness D1 of the neutron moderating portion, reduced Distance larger than √Tau, characterized in that.

  According to the embodiment of the present invention, in the system in which the moderator and the neutron absorber are arranged, it is possible to further increase the burning rate of the neutron absorber.

It is a sectional elevation showing the composition of the fast neutron nuclear reactor concerning a 1st embodiment. It is a top view which shows the structure of the fast neutron core which concerns on 1st Embodiment. It is an elevation sectional view showing the composition of the flammable poison assembly of the fast neutron core according to the first embodiment. FIG. 4 is a horizontal sectional view taken along line IV-IV in FIG. 3. It is an elevation sectional view of a neutron moderator element of a fast reactor core concerning a 1st embodiment. FIG. 6 is a horizontal sectional view taken along line VI-VI in FIG. 5. It is an elevation sectional view of the neutron absorber element of the fast reactor core according to the first embodiment. FIG. 8 is a horizontal sectional view taken along line VIII-VIII in FIG. 7. It is explanatory drawing which showed the combustible poison aggregate | assembly conceptually. It is explanatory drawing of the setting of the thickness of a neutron absorption part. It is a graph which shows the example of the change of the energy spectrum of a neutron. It is a flowchart explaining the procedure of the fast neutron core design method which concerns on 1st Embodiment. It is a graph explaining the example of the effective multiplication factor change of the fast neutron core which concerns on 1st Embodiment, and the conventional core. It is a horizontal sectional view showing the composition of the flammable poison assembly for the fast neutron core according to the second embodiment. It is a horizontal sectional view showing a combination of a neutron moderator assembly for a fast neutron core according to a third embodiment and a plurality of first composite fuel assemblies. It is a top view which shows the structure of the fast neutron core which concerns on 4th Embodiment. It is a horizontal sectional view showing the composition of the 2nd compound fuel assembly of the fast neutron core concerning a 4th embodiment. It is an elevation sectional view showing the composite fuel element of the second composite fuel assembly of the fast neutron core according to the fourth embodiment. It is an elevation sectional view showing a modification of a composite fuel element of a second composite fuel assembly of a fast neutron core according to a fifth embodiment. It is a top view which shows the structure of the fast neutron core which concerns on 6th Embodiment. It is a top view which shows the structural example of the core of the conventional fast reactor. It is an elevational sectional view showing the configuration of a conventional fuel assembly. FIG. 23 is a horizontal sectional view taken along line XXIII-XXIII in FIG. 22. It is a sectional elevation view of a fuel element of a conventional fuel assembly. FIG. 25 is a horizontal sectional view taken along line XXV-XXV in FIG. 24.

Hereinafter, with reference to the accompanying drawings, a description will be given fast neutron core design method engaged Ru to an embodiment of the present invention. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted. In the following description, each element constituting the core expresses a posture such as a vertical relationship in a state where each element is arranged in the core.

[First Embodiment]
FIG. 1 is an elevational sectional view showing the configuration of the fast neutron reactor according to the first embodiment. The fast neutron reactor 1 includes a fast neutron core (hereinafter simply referred to as a core) 3, a cylindrical reactor vessel 2 that includes the core 3 and has a bottom portion and extends in the vertical direction, and an upper portion of the reactor vessel 2. A shielding plug 11 is provided so as to close the opening. The core 3 is supported by a core support plate 4.

  Above the core 3, a core upper mechanism 7 supported by a shielding plug 11 is provided. Below the core support plate 4, a reactor internal structure 5 having a function of distributing coolant flow to the core 3 and a core catcher 6 for holding molten fuel in the event of a core melting accident are provided in the same reactor vessel 2. Yes. The core 3 is formed with a plurality of core fuel assemblies 16 and the like described later as constituent elements.

  A control rod driving device 15 is provided above the core 3 of the shielding plug 11 so that a control rod (not shown) can be inserted into the control rod guide tube 20 (FIG. 2) of the core 3. Moreover, the coolant inlet pipe 8 and the coolant outlet pipe 9 penetrate the reactor vessel 2 in an airtight manner, and a part thereof is similarly arranged in the reactor vessel 2. The coolant 10 that has flowed out from the coolant inlet pipe 8 into the lower space in the reactor vessel 2 turns upward and passes through the in-core structure 5 and the core support plate 4, and the core 3, which is a heat generation source. Flow into. After the coolant 10 is heated in the reactor core 3, it flows out from the coolant outlet pipe 9 for heat exchange outside the reactor vessel 2.

  Although FIG. 1 shows an example of a loop reactor, the present invention is not limited to this. For example, in the case of a tank-type nuclear reactor in which a primary cooling system including an intermediate heat exchanger (not shown) is housed in a reactor vessel that houses the core if the core has the characteristics described below. It's okay.

  FIG. 2 is a plan view showing the configuration of the fast neutron core according to the first embodiment. The entire core 3 is substantially circular in plan view. The core 3 has a plurality of inner core fuel assemblies 17 extending in the vertical direction at the center and arranged in parallel to each other, and has a substantially circular shape as a whole.

  The outer core fuel assemblies 18 are arranged in two layers in parallel with the inner core fuel assemblies 17 so as to surround the inner core fuel assemblies 17 on the radially outer side of the inner core fuel assemblies 17. Further, on the outer side in the radial direction, blanket fuel assemblies 19 are arranged in almost three layers in parallel with the outer core fuel assemblies 18 so as to surround the outer core fuel assemblies 18. Here, the number of layers of the outer core fuel assembly 18 and the blanket fuel assembly 19 is an example, and is not limited to the case of two layers and three layers, respectively.

  In the region where the inner core fuel assemblies 17 are arranged, a plurality of control rod guide tubes 20 are arranged in parallel with the inner core fuel assemblies 17 without being adjacent to each other. Further, in the region where the inner core fuel assemblies 17 are arranged, a plurality of burnable poison assemblies 37 are not adjacent to each other and are parallel to the inner core fuel assemblies 17 in a plurality. Arranged locally.

  Each side of the regular hexagonal column of the core fuel assembly 16, the blanket fuel assembly 19, the control rod guide tube 20, and the combustible poison assembly 37 is parallel to the side of the other assembly to which each of these assemblies is adjacent. Are adjacent to each other.

  The inner core fuel assembly 17 of the core fuel assembly 16 includes a plurality of inner fuel elements (not shown) and a trumpet cylinder tube that houses them. The inner fuel element has a lower blanket fuel, a core fuel and an upper blanket fuel arranged from the lower side in the vertical direction, and a fuel part cladding tube for storing these. The core fuel, which is a part that generates neutrons by fission (hereinafter referred to as a fuel part) includes, for example, fissile plutonium (Pu), uranium (U), minor actinides (Np, Am, Cm), or a mixture thereof. .

  Similarly, the outer core fuel assembly 18 of the core fuel assembly 16 similarly includes a plurality of outer fuel elements (not shown) and a trumpet-shaped trumpet tube. Similarly to the inner fuel element, the outer fuel element has a lower blanket fuel, a core fuel and an upper blanket fuel arranged from the lower side in the vertical direction, and a fuel part cladding tube for storing these. The core fuel of the inner fuel element and the core fuel of the outer fuel element differ in Pu enrichment or U enrichment, that is, the concentration of fissile material.

  Although FIG. 2 shows the case of a two-region core, the present invention is not limited to this. As long as the flammable poison assembly 37 is disposed at various locations in the core region, for example, one region or three or more regions may be used.

  FIG. 3 is an elevational sectional view showing the configuration of the combustible poison assembly. 4 is a horizontal sectional view taken along the line IV-IV in FIG. The combustible poison assembly 37 includes a plurality of neutron moderator elements 38 and neutron absorber elements 39 that are arranged in parallel to each other and extend in the vertical direction, and a trumpet tube 24 that houses these and extends in the vertical direction. The neutron moderator element 38 and the neutron absorber element 39 are supported at their lower portions by a disk-shaped element support grid 23 attached to the inner wall of the trumpet tube 24. A plurality of flow holes are formed in the element support grid 23 so that the coolant can pass therethrough.

  In the trumpet tube 24, the neutron moderator element 38 and the neutron absorber element 39 are accommodated in a triangular arrangement in a plane. Further, in the trumpet tube 24, the neutron moderator element 38 is arranged in a substantially circular shape in a plane in the center, and the neutron absorber element 39 is formed in two layers so as to cover the radially outer side of the neutron moderator element 38. It is arranged. The number of layers is not limited.

  The trumpet tube 24 has a cylindrical shape whose outer shape is a hexagonal column. The lower part of the entrance nozzle 22 of the trumpet tube 24 is formed in a thin cylindrical shape, and the lower end is closed. A plurality of coolant inflow holes 21 are formed on the side surface of the cylindrical portion. The upper end of the trumpet tube 24 is opened and a coolant outflow hole 26 is formed. In addition, a handling head 25 is formed on the upper portion of the trumpet tube 24 to be handled by, for example, a fuel changer from above. The coolant 10 flows in from the coolant inflow hole 21 of the entrance nozzle 22, rises in the trumpet tube 24, passes through the radially outer side of the neutron moderator element 38 and the neutron absorber element 39, and reaches the coolant outflow hole 26. From above.

  FIG. 5 is a sectional elevation view of the neutron moderator element. 6 is a horizontal sectional view taken along the line VI-VI in FIG. The neutron moderator element 38 includes a neutron moderator 40 that is a portion for decelerating neutrons (hereinafter referred to as a neutron moderator), and a neutron moderator cladding tube 34 a that houses the neutron moderator 40.

  The neutron moderator 40 is supported at its lower end by a disc-like support plate 31a attached to the inner wall of the neutron moderator cladding tube 34a. The neutron moderator cladding tube 34a has a cylindrical shape whose upper and lower sides are open, the lower end opening is closed by a lower end plug 35a, and the upper end opening is closed by an upper end plug 36a to form a sealed space. In the sealed space, a lower plenum 32 a is formed below the neutron moderator 40, and an upper plenum 33 a is formed above the neutron moderator 40. The neutron moderator may be packed in the neutron moderator element in liquid or powder form.

The material of the neutron moderator 40 includes any one of compounds containing hydrogen, lithium, beryllium, boron, and carbon (single element, oxide, carbide, hydroxide), or a combination thereof. For example, zirconium hydride, lithium (concentrate Li ⁶ ), lithium oxide (concentrate Li ⁶ ), lithium hydroxide (concentrate Li ⁶ ), beryllium, beryllium oxide, graphite, boron carbide (concentrate B ¹¹ ), carbonized Silicon and the like.

  FIG. 7 is a sectional elevation view of the neutron absorber element. FIG. 8 is a horizontal sectional view taken along line VIII-VIII in FIG. The neutron absorber element 39 includes a neutron absorber 41 that is a portion that absorbs neutrons (hereinafter referred to as a neutron absorber), and a neutron absorber cladding tube 34 b that houses the neutron absorber 41.

  The lower end of the neutron absorber 41 is supported by a disk-shaped support plate 31b attached to the inner wall of the neutron absorber-covering tube 34b. The neutron absorber covering tube 34b has a cylindrical shape whose upper and lower sides are open, the lower end opening is closed by a lower end plug 35b, and the upper end opening is closed by an upper end plug 36b to form a sealed space. In the sealed space, a lower plenum 32 b is formed below the neutron absorber 41, and an upper plenum 33 b is formed above the neutron absorber 41. The neutron absorber may be packed in the neutron absorber element in the form of liquid or powder.

The material of the neutron absorber 41 includes any one of compounds containing lithium, boron, indium, cadmium, samarium, gadolinium, hafnium, europium and erbium (single element, oxide, carbide, hydroxide), or a combination thereof. Shall be. For example, lithium oxide (natural or concentrated Li ⁷ ), lithium hydroxide (natural or concentrated Li ⁷ ), boron carbide (natural or concentrated B ¹⁰ ), samarium oxide, gadolinium oxide, europium oxide Erbium oxide and the like.

  FIG. 9 is an explanatory diagram conceptually showing a flammable poison assembly. In the combustible poison assembly 37 according to the present embodiment, as described with reference to FIG. 4, the neutron moderator element 38 that is a neutron moderator is arranged at the center, and the neutron absorption that is a neutron absorber at the outside in the radial direction. A body element 39 is arranged. Further, an inner core fuel assembly 17 (refer to FIG. 2), which is a fuel portion, is disposed on the radially outer side of the combustible poison assembly 37.

  In this way, in the basic configuration in which the fuel part and the neutron moderation part are arranged with the neutron absorption part interposed therebetween, the thickness D1 of the neutron moderation part, the thickness D2 of the neutron absorption part, and the material of the neutron absorption part The method of selection of will be described below. Here, the thickness D1 of the neutron moderating portion and the thickness D2 of the neutron absorption portion are thicknesses as viewed from the fuel portion side at the closest position. When the neutron moderator and the neutron absorber are not arranged concentrically from the center, the radial thickness D1 of the neutron moderator and the thickness D2 of the neutron absorber are the neutron absorber element 39 and the neutron moderator element 38. It is an equivalent value based on the arrangement of.

  In other words, an equivalent thickness D1 in the radial direction of the neutron moderating portion and an equivalent thickness D2 of the neutron absorbing portion are determined under the conditions described below, and based on the determined thicknesses D1 and D2 in FIG. The specific arrangement of the neutron moderating element 38 arranged at the center in the combustible poison assembly 37 shown and the neutron absorber element 39 arranged outside in the radial direction may be determined.

  First, the thickness D1 of the neutron moderator is determined from the condition that fast neutrons are decelerated to a predetermined energy region in the neutron moderator. Here, the cross-sectional area with reaction such as neutron absorption becomes larger as the energy of neutron is lower. Therefore, in order for neutrons to be effectively absorbed by the neutron absorber, it is necessary to be in a region having a predetermined energy or less. Hereinafter, this energy region is referred to as a low energy region.

The neutron absorption cross section where neutron absorption is effectively performed is, for example, about 100 barn (1 barn = 10 ⁻²⁴ cm ² ) or more. The energy of the neutron corresponding to this absorption cross section is about 1 eV or less. Therefore, in this case, the low energy region is an energy region of about 1 eV or less.

  In order to effectively absorb neutrons in the low energy region in the neutron absorption part, it is necessary to decelerate fast neutrons generated in the fuel part to the low energy region in the neutron moderation part. For this purpose, it is effective to secure the thickness D1 of the neutron moderator in order to sufficiently slow down the fast neutrons generated in the fuel unit in the neutron moderator.

From the above, the thickness D1 of the neutron moderator is made larger than the slowing-down length. That is, D1 is determined on condition of the following formula (1).
D1> √τ (1)
Here, τ is the Fermi age, and √τ is the deceleration distance.

  Next, a method for setting the thickness D2 of the neutron absorbing portion will be described. FIG. 10 is an explanatory diagram for setting the thickness of the neutron absorber. In FIG. 10, the horizontal axis is the distance from the surface of the neutron absorber that faces the neutron moderator. Consider a state where neutrons in the low energy region flow into the neutron absorber from the surface facing the neutron moderator.

The vertical axis represents the reaction rate of the absorption reaction of neutrons to the neutron absorption part. If the neutron absorption cross section of the material substance in the neutron absorbing portion is σ _a and the neutron flux at each position is Φ, the reaction rate of the neutron absorption reaction is σ _a Φ. Usually, the reaction rate is given as an energy integral value of the neutron absorption cross section and neutron flux at each energy.

  Neutrons flow from the surface of the neutron absorber that faces the neutron moderator, that is, from the left side of FIG. The lower the neutron energy, the more neutrons are absorbed by the neutron absorber, so the reaction of low energy neutrons decreases as it moves to the right in FIG. 10, and only the reaction of high energy neutrons remains. It becomes a shape and gradually decreases like a solid A curve, that is, a neutron absorption reaction curve. The distribution of neutrons decreases with decreasing distance from the surface. That is, the thicker the neutron absorption portion, the less the reaction amount of neutrons, but the effect decreases even if the thickness is increased. For example, if the diameter of the core is increased, the diameter of the reactor vessel and the diameter of the shielding structure outside the reactor vessel are increased. As a result, it also affects the layout of the entire nuclear reactor facility.

  Therefore, when setting the thickness D2 of the neutron absorbing portion, it is effective to secure the thickness D2 within a range where the effectiveness of securing the thickness is large. In the neutron absorption reaction rate curve A, the neutron absorption part has the maximum inclination on the surface (distance zero) facing the neutron moderating part. When this inclination is extended as shown by the broken line B, the distance L at which the neutron absorption reaction rate becomes zero is obtained. This distance L is effective as a measure of the thickness that can effectively utilize the absorption function of the neutron absorbing portion. Therefore, D2 is set to a thickness equal to or greater than L.

  Note that the thickness D1 of the neutron moderating portion and the thickness D2 of the neutron absorbing portion may be set as the thickness of the shortest portion viewed from the fuel portion.

  FIG. 11 is a graph showing an example of changes in the energy spectrum of neutrons. In the fuel part, neutrons generated by fission have a high-speed spectrum having a peak around 100 keV as shown by curve F. As the fast neutrons are decelerated, the energy spectrum of the neutrons changes to curves M1 and M2, and becomes a neutron energy spectrum that is almost flat and has few high-speed components as shown by curve M3.

  FIG. 12 is a flowchart for explaining the procedure of the fast neutron core design method according to the first embodiment. First, conditions for the operation period T are set (step S01). Here, the neutron absorber is assumed to finish its role when the operation period T ends. Next, the thickness D1 of the neutron moderating portion is determined by equation (1) (step S02).

  Next, the material substance of the neutron absorber and the thickness D2 of the neutron absorber are set (step S03). Since system conditions are determined in steps S01 to S03, the distribution of neutron flux Φ in this system is calculated (step S04).

When the distribution of the neutron flux Φ is calculated in step S04, the ratio r to the initial amount of the remaining amount of neutron absorber at the end of the operation period T is calculated by the following equation (2) (step S05).
r = e ^−σaΦT (2)

Next, it is determined whether or not the remaining amount ratio obtained in step S05 is equal to or less than a specified value (step S06). For example, if the specified value is 10%, the following expression (3) is obtained, and as a result, the condition of expression (4) is satisfied.
r = e− ^σaΦT <0.1 (3)
σ _a ΦT> log _e 10 (4)
Here, log _e 10 indicates a natural logarithm of 10, which is approximately 2.3.

That is, when the neutron flux is Φ and the operation period is T, the neutron absorption cross section σ _a is 2.3 / ( Needs to be larger than (ΦT).

  When the remaining amount of the material substance in the neutron absorption part is larger than the specified value (NO in step S06), the process returns to step S03, the selection of the material substance in the neutron absorption part, and the setting of the thickness D2 of the neutron absorption part are performed again. Steps S03 and after are repeated. If the remaining amount of the material substance in the neutron absorber is smaller than the specified value (YES in step S06), the thickness D1 of the neutron moderator, the material substance of the neutron absorber, and the thickness D2 of the neutron absorber at that time Use to determine the system.

  FIG. 13 is a graph for explaining an example of the effective multiplication factor change of the fast neutron core according to the first embodiment and the conventional core. The horizontal axis of the graph is the number of days of combustion (days), and the actual number of days of operation is converted to the number of days in the case of rated output. The vertical axis is the effective multiplication factor. The effective multiplication factor in this case includes the effect of BP when flammable poison (BP) is included.

  As shown in FIG. 13, in the case of a BP assembly core without flammable poison, the effective multiplication factor decreases monotonously due to the consumption of fissile material in the fuel assembly and the generation of poison by fission as shown by the broken line. To do. As a result, as shown in the figure, the difference in effective multiplication factor before and after the combustion period is about 0.06.

  On the other hand, in the case of the fast neutron core according to the present embodiment indicated by the solid line indicated as the BP assembly core, the decrease due to the consumption of nuclear fuel material and the decrease due to the consumption of poisonous substance are well matched, and before and after the combustion period The difference in effective multiplication factor is about 0.02.

  When converted to combustion reactivity, it is 5.8% Δρ in the case of a BP assembly core without flammable poison, and 2.2% Δρ in the case of a BP assembly core with flammable poison. In this embodiment, the combustion reactivity is greatly reduced.

  As a result, the burden on the reactivity value that the control rod should bear is reduced, and the number of control rod assemblies is reduced or the reactivity value of one control rod is reduced as in the fast neutron core shown in FIG. By doing so, the conditions of the control rod pull-out accident can be greatly eased.

[Second Embodiment]
FIG. 14 is a horizontal sectional view showing a configuration of a combustible poison assembly for a fast neutron core according to the second embodiment. This embodiment is a modification of the first embodiment. The combustible poison assembly 37 a in the second embodiment includes a plurality of neutron moderator elements 38 and neutron absorber elements 39, similarly to the combustible poison assembly 37 in the first embodiment.

  In the combustible poison assembly 37a in the present embodiment, a neutron moderator element 38 is disposed at the center of the horizontal cross section, a neutron absorber element 39 is disposed on the radially outer side, and the neutron moderator element 38 is further disposed on the radially outer side. The neutron moderator elements 38 and the neutron absorber elements 39 are alternately arranged in layers.

  In the present embodiment, each layer of the neutron moderator element 38 does not satisfy the condition of D1, but a plurality of flammable poison assemblies 37a are viewed from the radially outer fuel portion, The layer D1 may be equivalently satisfied as a whole. Similarly, the neutron absorber element 39 may satisfy the condition of D2 as a whole of the plurality of layers.

  According to the present embodiment configured as described above, absorption in the neutron absorbing portion can be performed even in the energy region where the neutron energy is reduced to the low energy region. This is particularly effective in utilizing the resonance absorption region.

[Third Embodiment]
FIG. 15 is a horizontal sectional view showing a combination of a neutron moderator assembly for a fast neutron core according to the third embodiment and a plurality of first composite fuel assemblies. This embodiment is a modification of the first embodiment. In the core 3, a neutron moderator is locally arranged, a neutron absorber is arranged around each neutron moderator, and a fuel unit is arranged around the neutron absorber. The basic arrangement to do is embodied in another form.

  A neutron moderator assembly 43 is provided as a neutron moderator. The neutron moderator assembly 43 has a plurality of neutron moderator elements 38.

  Six first composite fuel assemblies 42 are arranged around the radial direction of the neutron moderator assembly 43. Each first composite fuel assembly 42 includes a plurality of basic fuel elements 27 a and a plurality of neutron absorber elements 39. The neutron absorber elements 39 are arranged in layers on one side. In addition, the six first composite fuel assemblies 42 are arranged in such a direction that the neutron absorber elements 39 included therein are adjacent to the neutron moderator assembly 43.

  In the present embodiment having the above-described configuration, the neutron moderator assembly 43 serves as a locally disposed neutron moderator, and the six first composite fuel assemblies 42 serve as neutron absorbers disposed around the neutron moderator. Each of the two layers of neutron absorber elements 39 and a fuel part disposed around the two neutron absorber elements 39 are configured in the form of a basic fuel element 27a which is the remaining component of the first composite fuel assembly 42. The basic fuel element 27a has a fuel part and a basic fuel part cladding tube containing the fuel part. The neutron absorber element 39 is not limited to two layers.

  According to the present embodiment configured as described above, the same effects as those of the first embodiment can be obtained.

[Fourth Embodiment]
In the fourth embodiment, a basic arrangement in which the neutron absorbing portion is arranged around the neutron moderating portion and the fuel portion is arranged around the neutron absorbing portion is realized in the axial direction.

  FIG. 16 is a plan view showing a configuration of a fast neutron core according to the fourth embodiment. Instead of the inner core fuel assembly 17 in the central region, a second composite fuel assembly 44 is arranged. The second composite fuel assembly 44 may be provided in place of all the inner core fuel assemblies 17 and the outer core fuel assemblies 18.

  FIG. 17 is a horizontal sectional view showing the configuration of the second composite fuel assembly 44. The second composite fuel assembly 44 includes a plurality of composite fuel elements 45 and a plurality of basic fuel elements 27 a arranged in parallel in the trumpet tube 24. In the horizontal section, the composite fuel element 45 is arranged in the central region. A basic fuel element 27 a is disposed so as to surround the radially outer side of the composite fuel element 45. The composite fuel element 45 may be provided in place of all the basic fuel elements 27a.

  FIG. 18 is an elevational sectional view showing the composite fuel element of the second composite fuel assembly. A composite fuel element 45 disposed in the central region of the second composite fuel assembly 44 includes a neutron moderator 40, a neutron absorber 41 disposed above and below the neutron moderator 40, and a core disposed above and below the insulator 46. It has the fuel 28 and the composite part cladding tube 45a which accommodates these. A lower blanket 29 is disposed below the core fuel 28 disposed below. An upper blanket 30 is disposed on the upper portion of the core fuel 28 disposed on the upper portion. A lower plenum 32 is formed between the lower blanket 29 and the lower end plug 35. An upper plenum 33 is formed between the upper blanket 30 and the upper end plug 36.

  In the fourth embodiment configured as described above, the thickness D1 of the neutron moderator corresponds to the axial height of the neutron moderator 40 in the composite fuel element 45. Further, the thickness D2 of the neutron absorbing portion corresponds to the axial height of the neutron absorber 41 in the composite fuel element 45.

  As described above, according to the fourth embodiment, the realization of the basic arrangement in which the neutron absorbing part is arranged around the neutron moderating part and the fuel part is arranged around the neutron absorbing part can be realized not only in the radial direction but also in the axial direction. By realizing it, the selection range of the combination of the components of the fast neutron core can be expanded, and the design flexibility such as the improvement of safety can be improved.

[Fifth Embodiment]
FIG. 19 is an elevational sectional view showing a modification of the composite fuel element of the second composite fuel assembly of the fast neutron core according to the fifth embodiment. This embodiment is a modification of the fourth embodiment.

  In the composite fuel element 45 of the second composite fuel assembly 44 in the fifth embodiment, the neutron absorber 41 is disposed above and below the neutron moderator 40, and the neutron moderator 40 is disposed above and below the neutron moderator 40. Thus, the neutron moderator 40 and the neutron absorber 41 are alternately arranged in the axial direction.

  In this way, by alternately arranging the neutron moderator 40 and the neutron absorber 41 in the axial direction, as in the second embodiment, the neutron absorbing portion also in the energy region where the neutron energy is reduced to the low energy region. Can be absorbed, and the resonance absorption region can be used.

[Sixth Embodiment]
FIG. 20 is a plan view showing a configuration of a fast neutron core according to the sixth embodiment. This embodiment is a modification of the fourth embodiment. In the fourth embodiment, the second composite fuel assembly 44 is arranged in place of the inner core fuel assembly in the central region. On the other hand, in the sixth embodiment, the second composite fuel assemblies 44 are dispersed at a plurality of locations. The range to be dispersed is not limited to the region of the inner core fuel assembly 17. For example, it may be distributed in a range that extends to the region of the outer core fuel assembly 18.

  The present embodiment configured as described above can provide diversity in the arrangement plan of the fast neutron core, can expand the range of selection, and can improve the flexibility of design in ensuring safety.

[Other Embodiments]
As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. For example, as a core using fast neutrons, a fast reactor is shown as an example, but the core is not limited to a fast reactor. For example, a fast reactor core without a blanket such as an axial blanket or a radial blanket, or a core called a reduced fast core in which the amount of moderator is limited and the neutron spectrum is shifted to the high speed side, or an accelerator driven atom The core of the reactor may be used.

  Moreover, you may combine the characteristic of each embodiment. Furthermore, these embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention.

  These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... Fast neutron reactor, 2 ... Reactor vessel, 3 ... Fast neutron core, 4 ... Core support plate, 5 ... Core structure, 6 ... Core catcher, 7 ... Core upper mechanism, 8 ... Coolant inlet piping, DESCRIPTION OF SYMBOLS 9 ... Coolant outlet piping, 10 ... Coolant, 11 ... Shield plug, 15 ... Control rod drive device, 16 ... Core fuel assembly, 17 ... Inner core fuel assembly, 18 ... Outer core fuel assembly, 19 ... Blanket Fuel assembly, 20 ... Control rod guide tube, 21 ... Coolant inflow hole, 22 ... Entrance nozzle, 23 ... Element support grid, 24 ... Trumpet tube, 25 ... Handling head, 26 ... Coolant outflow hole, 27 ... Fuel element 27a ... Basic fuel element, 28 ... Core fuel, 29 ... Lower blanket fuel, 30 ... Upper blanket fuel, 31 ... Support plate, 32, 32a, 32b ... Lower plenum, 33, 33a, 33b ... Upper RENAM, 34 ... cladding tube, 34a ... neutron moderator coating tube, 34b ... neutron absorber coating tube, 35 ... lower end plug, 36 ... upper end plug, 37, 37a ... flammable poison assembly, 38 ... neutron moderator Element 39 ... Neutron absorber element 40 ... Neutron moderator 41 ... Neutron absorber 42 ... First composite fuel assembly 43 ... Neutron moderator assembly 44 ... Second composite fuel assembly 45 ... Composite fuel element, 45a ... Composite part cladding tube, 46 ... Insulator, 103 ... Fast reactor core

Claims (1)

A moderator thickness determining step for setting a thickness of the neutron moderator,
Absorber setting step for setting the material of the neutron absorber, the thickness of the neutron absorber,
A neutron flux calculating step for calculating a neutron flux distribution of the system after the deceleration portion thickness determining step and the absorbing portion setting step;
After the neutron flux calculation step, based on the neutron absorption cross section by the material of the neutron absorber set in the absorber setting step, the neutron flux obtained in the neutron flux calculation step, and the operation period, the end of the operation period Absorber remaining amount calculating step for calculating the remaining amount of the neutron absorbing portion of,
A determination step of determining whether a ratio of the remaining amount of the neutron absorber to an initial amount is smaller than a specified value;
Have
The thickness D1 of the neutron moderator is larger than the deceleration distance √τ,
A fast neutron core design method characterized by that.

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