JP6672437B2 - Fast neutron core and fast neutron reactor - Google Patents

Description

  An embodiment of the present invention relates to a fast neutron core and a fast neutron reactor using the same.

  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 at the center and containing a large amount of fissile material, and a large amount of fissile parent material disposed radially outside thereof and being converted into fissile material by neutron absorption. Including a blanket fuel assembly 19. The core fuel assembly 16 has an inner core fuel assembly 17 arranged in the central region and an outer core fuel assembly 18 arranged radially outside. The control rod guide tubes 20 are arranged adjacent to each other in a 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 a hexagon, the blanket fuel assembly 19 is a double circle in a hexagon, and a control rod guide tube. Reference numeral 20 denotes a hexagon with C added.

  FIG. 22 is a vertical sectional view showing the configuration 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 wrapper tube 24, as shown in FIGS.

  At the lower part of the wrapper tube 24, an entrance nozzle 22 for fixing and supporting the core fuel assembly 16 on a core support plate (not shown) to form a coolant flow path is provided. A coolant inflow hole 21 is formed in a side wall of the entrance nozzle 22. At the upper part of the wrapper tube 24, a handling head 25 serving as a grip portion when the core fuel assemblies 16 are loaded into or taken out of the fast reactor core 103 is provided.

  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 which is a heat source. After being heated by the fuel element 27, the coolant 10 flows out of the coolant outlet 26 in the center of the handling head 25.

  FIG. 24 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 of FIG. As shown in FIGS. 24 and 25, the fuel element 27 is provided with a core fuel 28 containing a large amount of fissile material in the center of the fuel, and is converted into fissile material by neutron absorption at the lower and upper portions of the core fuel 28. A lower blanket fuel 29 and an upper blanket fuel 30 each containing a large amount of fissile parent material are provided. These fuels are accommodated in a cladding tube 34, the lower portion of the cladding tube 34 is sealed with a lower end plug 35, and the upper portion is sealed with an upper end plug 36. A gap is provided between the core fuel 28, the lower blanket fuel 29, the upper blanket fuel 30, and the cladding tube 34. In the case of an oxide fuel core, the gap is filled with helium gas. Below the lower blanket fuel 29 and above the upper blanket fuel 30 in the cladding tube 34, a lower plenum 32 and an upper plenum 33 for accumulating gas discharged by burning the fuel are formed, respectively. ing.

  In a reactor using fast neutrons, reducing the combustion reactivity contributes to the improvement of economy by extending the cycle length and to the improvement of safety by reducing the insertion reactivity of control rod erroneous withdrawal. Here, the combustion reactivity is a change in the reactivity accompanying the combustion of the 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 decreasing the combustion reactivity has been adopted. In this method, it is known that when the Pu isotope composition changes, the combustion reactivity also changes greatly (for example, see Patent Document 1).

  Further, in the case of a uranium-free fuel for improving the combustion efficiency of transuranium element (TRU) containing Pu, the combustion reactivity is greatly increased. For this reason, a method that does not depend on the variation of Pu isotope composition or the size of uranium is desired. It should be noted that there is also a demand for an accelerator-driven reactor to suppress an 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 burnable poisons are applied to a nuclear reactor using fast neutrons (for example, see Patent Document 2, Non-Patent Document 1, and Non-Patent Document 2). However, these design examples do not show a systematic design method.

JP 2008-260909 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 a nuclear reactor using fast neutrons as described above, when a burnable poison is applied, simply mixing a neutron absorber causes a slow combustion conversion rate, so the neutron energy is reduced by using a moderator. A method of increasing the conversion speed by reducing the speed from a high speed is known.

  An embodiment of the present invention aims at further increasing the burning rate of the neutron absorber in such a system in which the moderator and the neutron absorber are arranged.

  In order to achieve the above object, the fast neutron core according to the present embodiment includes a fuel section including a fissile substance and generating fast neutrons by reacting with neutrons, and a fuel section cladding tube including the fuel section. A plurality of core fuel assemblies that extend in the vertical direction and are arranged in parallel with each other, a neutron moderator that reduces the energy of the neutron flowing into a predetermined low energy region, and a neutron A neutron absorbing unit that absorbs, the neutron moderating unit and the neutron absorbing unit are arranged in a region where the core fuel assemblies are arranged, and the fuel unit and the neutron moderating unit include the neutron absorbing unit. And a thickness D1 of the neutron moderating portion is larger than a moderating distance Δτ.

  Further, the fast neutron reactor according to the present embodiment, a fast neutron core utilizing fast neutrons, and a reactor vessel containing the fast neutron core and holding a reactor coolant cooled by cooling means, A fast neutron reactor comprising: a fast neutron core, the fuel section including a fissile substance and generating fast neutrons by reaction with neutrons; and a plurality of fuels having a fuel section cladding tube including the fuel section. A plurality of core fuel assemblies that extend in the vertical direction and are arranged in parallel with each other, a neutron moderator that reduces the energy of the inflowing neutrons to a predetermined low energy region, and absorbs the inflowing neutrons A neutron absorbing section, wherein the neutron moderating section and the neutron absorbing section are arranged in a region where the core fuel assemblies are arranged, and the fuel section and the neutron moderating section Wherein arranged so as to sandwich the neutron absorbing portion, the thickness D1 of the neutron moderating portion is greater than the deceleration distance √Tau, characterized in that.

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

FIG. 1 is an elevational sectional view showing a configuration of a fast neutron reactor according to a first embodiment. FIG. 2 is a plan view showing a configuration of a fast neutron core according to the first embodiment. It is an elevation sectional view showing the composition of the burnable poisonous substance aggregate of the fast neutron core concerning a 1st embodiment. FIG. 4 is a horizontal sectional view taken along line IV-IV of 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 of FIG. 5. It is an elevation sectional view of a neutron absorber element of a fast reactor core concerning a 1st embodiment. FIG. 8 is a horizontal sectional view taken along line VIII-VIII of FIG. 7. It is explanatory drawing which showed notably the burnable poison aggregate. It is explanatory drawing of setting of the thickness of a neutron absorption part. 6 is a graph showing an example of a change in a neutron energy spectrum. FIG. 3 is a flowchart illustrating a procedure of a fast neutron core design method according to the first embodiment. 4 is a graph illustrating an example of an effective multiplication factor change between the fast neutron core according to the first embodiment and a conventional core. It is a horizontal sectional view showing the composition of the burnable poison aggregate for fast neutrons core concerning a 2nd embodiment. FIG. 11 is a horizontal sectional view showing a combination of a neutron moderator assembly for a fast neutron core and a plurality of first composite fuel assemblies according to a third embodiment. It is a top view showing the composition of the fast neutron core concerning a 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 compound fuel element of the 2nd compound fuel assembly of the fast neutron core concerning a 4th embodiment. It is an elevation sectional view showing a modification of the compound fuel element of the 2nd compound fuel assembly of the fast neutron core concerning a 5th embodiment. It is a top view showing the composition of the fast neutron core concerning a 6th embodiment. It is a top view which shows the structural example of the core of the conventional fast reactor. It is an elevation sectional view showing the composition of the conventional fuel assembly. FIG. 23 is a horizontal sectional view taken along line XXIII-XXIII of FIG. 22. It is an elevation sectional view of a fuel element of a conventional fuel assembly. FIG. 25 is a horizontal sectional view taken along line XXV-XXV of FIG. 24.

  Hereinafter, a fast neutron core and a fast neutron reactor according to embodiments of the present invention will be described with reference to the drawings. Here, the same or similar parts are denoted by the same reference numerals, and redundant description will be omitted. In the following description, each element constituting the core represents a posture such as a vertical relationship in a state where each element is disposed 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 including the core 3 and having a bottom and extending in a vertical direction, and an upper portion of the reactor vessel 2. It has a shielding plug 11 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 core internal structure 5 having a function of distributing the flow rate of coolant 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. I have. The core 3 is formed with a plurality of core fuel assemblies 16 described later and the like 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 a control rod guide tube 20 (FIG. 2) of the core 3. Further, a coolant inlet pipe 8 and a coolant outlet pipe 9 penetrate the reactor vessel 2 in a gas-tight manner, and a part thereof is similarly arranged in the reactor vessel 2. The coolant 10 flowing out of the coolant inlet pipe 8 into the lower space in the reactor vessel 2 turns upward, passes through the reactor internal structure 5 and the core support plate 4, and passes through the core 3, which is a heat generation source. Flows into. After being cooled in the reactor core 3, the coolant 10 flows out of the coolant outlet pipe 9 to exchange heat 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, if the reactor has a core having the characteristics described below, a primary cooling system including an intermediate heat exchanger (not shown) is a tank-type reactor housed in a reactor vessel housing the core. It may be.

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

  An outer core fuel assembly 18 is arranged in two layers in parallel with the inner core fuel assembly 17 so as to surround the inner core fuel assembly 17 radially outward of the inner core fuel assembly 17. Further, on the radially outer side, blanket fuel assemblies 19 are arranged in substantially 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 (Burnble Poison) assemblies 37 are not adjacent to each other, and a plurality of burnable poison assemblies 37 are arranged in parallel with the inner core fuel assemblies 17 in the core. It is arranged locally.

  Respective sides of the regular hexagonal column of the core fuel assembly 16, the blanket fuel assembly 19, the control rod guide tube 20, and the burnable poison assembly 37 are parallel to the sides of other assemblies to which each of these assemblies is adjacent. And adjacent to each other.

  The inner core fuel assembly 17 of the core fuel assembly 16 has a plurality of inner fuel elements (not shown) and a trumpet tube that houses these and has an outer shape of a hexagonal column. The inner fuel element has a lower blanket fuel, a core fuel, and an upper blanket fuel, which are disposed from the lower side in the vertical direction, and a fuel cladding for accommodating them. The core fuel, which is a portion that generates neutrons by fission (hereinafter, referred to as a fuel portion), 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 also has a plurality of outer fuel elements (not shown) and a trumpet tube that houses these and has a hexagonal column outer shape. 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 section cladding tube for accommodating them. The core fuel of the inner fuel element and the core fuel of the outer fuel element differ in the enrichment of Pu or the enrichment of U, that is, the concentration of fissile material.

  Although FIG. 2 shows the case of the two-region core, the present invention is not limited to this. As long as the burnable poison aggregates 37 are arranged in various places in the core region, for example, the case may be one region or three or more regions.

  FIG. 3 is an elevational sectional view showing the configuration of the burnable poisonous substance aggregate. FIG. 4 is a horizontal sectional view taken along line IV-IV of FIG. The burnable poison assembly 37 has a plurality of neutron moderator elements 38 and neutron absorber elements 39 arranged in parallel with each other and extending in the vertical direction, and the wrapper tube 24 that accommodates these elements and extends in the vertical direction. The lower portions of the neutron moderator element 38 and the neutron absorber element 39 are supported by the disk-shaped element support grid 23 attached to the inner wall of the flared tube 24. A plurality of flow holes are formed in the element support grid 23 so that the coolant can pass through.

  In the wrapper tube 24, a neutron moderator element 38 and a neutron absorber element 39 are housed side by side in a triangular arrangement. In the trumpet tube 24, a neutron moderator element 38 is disposed in the center in a substantially circular shape in a plane, 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. Are arranged. In addition, it is not limited to two layers.

  The trumpet tube 24 has a hexagonal cylindrical outer shape. The lower part of the entrance nozzle 22 at the lower part of the wrapper tube 24 is formed in a small diameter 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 wrapper tube 24 is opened to form a coolant outflow hole 26. In addition, a handling head 25 for handling with, for example, a refueling machine is formed from above on the wrapper tube 24. The coolant 10 flows from the coolant inlet 21 of the entrance nozzle 22, rises in the wrapper tube 24, passes through the neutron moderator element 38 and the neutron absorber element 39 in the radial direction, and flows through the coolant outlet 26. Spills upwards.

  FIG. 5 is a sectional elevation view of the neutron moderator element. FIG. 6 is a horizontal sectional view taken along line VI-VI in FIG. The neutron moderator element 38 has a neutron moderator 40 which is a portion for slowing down neutrons (hereinafter, referred to as a neutron moderator), and a neutron moderator covering tube 34a for accommodating the neutron moderator 40.

  The lower end of the neutron moderator 40 is supported by a disk-shaped support plate 31a attached to the inner wall of the neutron moderator coating tube 34a. The neutron moderating portion cladding tube 34a has a cylindrical shape with an open top and bottom, an opening at a lower end is closed by a lower end plug 35a, and an opening at an upper end is closed by an upper end plug 36a to form a closed space. In the enclosed space, a lower plenum 32a is formed below the neutron moderator 40, and an upper plenum 33a is formed above the neutron moderator 40. The neutron moderator may be packed in a neutron moderator element in liquid or powder form.

It is assumed that the material of the neutron moderator 40 includes any of compounds including hydrogen, lithium, beryllium, boron, and carbon (simple, oxide, carbide, and hydroxide), or a combination thereof. For example, zirconium hydride, lithium (concentrates Li ⁶ ), lithium oxide (concentrates Li ⁶ ), lithium hydroxide (concentrates Li ⁶ ), beryllium, beryllium oxide, graphite, boron carbide (concentrates B ¹¹ ), carbonized Silicon or 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 of FIG. The neutron absorber element 39 has a neutron absorber 41 that is a portion that absorbs neutrons (hereinafter, referred to as a neutron absorber), and a neutron absorber cladding tube 34b 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 absorbing portion covering tube 34b. The neutron absorbing portion coating tube 34b is a cylindrical shape having an open top and bottom, an opening at the lower end is closed by a lower end plug 35b, and an opening at the upper end is closed by an upper end plug 36b, thereby forming a closed space. In the enclosed space, a lower plenum 32b is formed below the neutron absorber 41, and an upper plenum 33b is formed above the neutron absorber 41. The neutron absorber may be packed in a neutron absorber element in a liquid or powder form.

The material of the neutron absorber 41 includes any of compounds including lithium, boron, indium, cadmium, samarium, gadolinium, hafnium, europium and erbium (simple, oxide, carbide, hydroxide), or a combination thereof. Shall be considered. 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 And erbium oxide.

  FIG. 9 is an explanatory view conceptually showing a burnable poisonous substance aggregate. In the burnable poisonous substance assembly 37 according to the present embodiment, as described with reference to FIG. 4, a neutron moderator element 38 serving as a neutron moderator is disposed in the center, and a neutron absorbing element serving as a neutron absorbing section is disposed radially outside. A body element 39 is provided. An inner core fuel assembly 17 (see FIG. 2) as a fuel portion is disposed radially outside the burnable poison assembly 37.

  As described above, in the basic configuration in which the fuel section and the neutron moderating section are arranged with the neutron absorbing section interposed therebetween, the thickness D1 of the neutron moderating section, the thickness D2 of the neutron absorbing section, and the material of the neutron absorbing section The method of selecting the above will be described below. Here, the thickness D1 of the neutron moderating portion and the thickness D2 of the neutron absorbing portion are each a thickness as viewed from the nearest fuel portion side. 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. Is an equivalent value based on the arrangement of.

  In other words, the equivalent thickness D1 in the radial direction of the neutron moderating portion and the equivalent thickness D2 of the neutron absorbing portion are determined under the conditions described below, and based on the determined thicknesses D1 and D2, FIG. The specific arrangement of the neutron moderating element 38 disposed at the center of the burnable poisonous substance assembly 37 and the neutron absorber element 39 disposed radially outward of the neutron absorbing element 38 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 reaction cross section such as neutron absorption increases as the energy of the neutrons decreases. Therefore, in order for neutrons to be effectively absorbed by the neutron absorber, the neutrons must 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 in which 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 unit, it is necessary to decelerate fast neutrons generated in the fuel unit to the low energy region in the neutron moderator. To this end, it is effective to secure the thickness D1 of the neutron moderating section in order to sufficiently decelerate the fast neutrons generated in the fuel section in the neutron moderating section.

From the above, the thickness D1 of the neutron moderating portion is set to be larger than the slowing-down length. That is, D1 is determined on condition of the following equation (1).
D1> √τ (1)
Here, τ is the Fermi age, and √τ is the deceleration distance.

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

The vertical axis represents the reaction rate of the neutron absorption reaction to the neutron absorption part. Assuming that the neutron absorption cross-sectional area of the material of the neutron absorption section is σ _a and the neutron flux at each position is Φ, the reaction rate of the neutron absorption reaction is σ _a Φ. In general, the reaction rate is given as a neutron absorption cross section at each energy and an energy integral of the neutron flux.

  Neutrons flow in from the surface facing the neutron moderator in the neutron absorber, that is, from the left side in FIG. The lower the energy of neutrons, the lower the energy of neutrons is absorbed by the neutron absorber, so that the reaction of low-energy neutrons decreases as moving to the right in FIG. 10 and only the reaction of high-energy neutrons remains. And gradually decreases like a solid line A curve, that is, a neutron absorption response curve. The distribution of neutrons tends to decrease with increasing distance from the surface. That is, as the neutron absorbing portion becomes thicker, the reaction amount of neutrons decreases, but the effect decreases even when the neutron absorbing portion becomes thicker. As for the size of the core, for example, as the diameter increases, the diameter of the reactor vessel and the diameter of the shielding structure outside the reactor vessel increase. As a result, this affects the layout of the entire reactor facility.

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

  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 as viewed from the fuel portion.

  FIG. 11 is a graph showing an example of a change in the energy spectrum of neutrons. In the fuel section, neutrons generated by fission have a fast spectrum with 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 fast components as shown by the curve M3.

  FIG. 12 is a flowchart illustrating 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, it is assumed that the neutron absorbing unit ends its role when the operation period T ends. Next, the thickness D1 of the neutron moderating section is determined by the equation (1) (step S02).

  Next, the material of the neutron absorbing section and the thickness D2 of the neutron absorbing section are set (step S03). Since the conditions of the system are determined in steps S01 to S03, the distribution of the 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 of the remaining amount of the neutron absorber to the initial amount 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 the remaining amount ratio obtained in step S05 is equal to or less than a specified value (step S06). For example, if the prescribed value is 10%, the following equation (3) is obtained, and as a result, the condition of equation (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 and is about 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 / ( ΦT).

  If the remaining amount of the material in the neutron absorbing unit is larger than the specified value (NO in step S06), the process returns to step S03, and the material of the neutron absorbing unit is selected and the thickness D2 of the neutron absorbing unit is set again. Step S03 and subsequent steps are repeated. If the remaining amount of the material in the neutron absorbing section is smaller than the specified value (YES in step S06), the thickness D1 of the neutron moderating section, the material of the neutron absorbing section, and the thickness D2 of the neutron absorbing section at that time. Is used to determine the system.

  FIG. 13 is a graph illustrating an example of a change in effective multiplication factor between 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 burning (days), which is obtained by converting the actual number of operating days into the number of days in the case of rated output. The vertical axis is the effective multiplication factor. When the burnable poison (BP) is included, the effective multiplication factor in this case includes the effect of BP.

  As shown in FIG. 13, in the case of a non-burnable BP assembly having no burnable poison, the effective multiplication factor monotonously decreases due to the consumption of fissile material in the fuel assembly and the generation of poison by fission, as indicated by the broken line. I do. As a result, as shown in the drawing, the difference between the effective multiplication factors 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, which is indicated by a solid line denoted as BP cored core, the decrease due to the consumption of nuclear fuel material and the decrease due to the consumption of toxic material are well matched, and before and after the combustion period. Is about 0.02.

  When converted into a combustion reactivity, the value is 5.8% Δρ for a BP assembly without a burnable poison and a 2.2% Δρ for a BP assembly with a burnable poison. In the present 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 as in the fast neutron core shown in FIG. 2, the number of control rod assemblies is reduced, or the reactivity value of one control rod is reduced. By doing so, the conditions of the control rod rear pull-out accident can be significantly reduced.

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

  In the burnable poisonous substance aggregate 37a according to the present embodiment, a neutron moderator element 38 is disposed at the center of the horizontal section, a neutron absorber element 39 is disposed radially outward, and the neutron moderator element 38 is further radially outward. The neutron moderator elements 38 and the neutron absorber elements 39 are alternately arranged in layers.

  In the present embodiment, one layer of each of the neutron moderator elements 38 does not satisfy the condition of D1, but a plurality of burnable poison aggregates 37a are viewed from the radially outer fuel portion. What is necessary is just to satisfy the condition of D1 equivalently as a whole layer. 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, it is possible to perform absorption in the neutron absorption unit even in an energy region where neutron energy is being reduced to a low energy region. This has an effect that the resonance absorption region can be used.

[Third Embodiment]
FIG. 15 is a horizontal sectional view showing a combination of a neutron moderator assembly for a fast neutron core and a plurality of first composite fuel assemblies according to the third embodiment. This embodiment is a modification of the first embodiment. In the reactor core 3, a neutron moderator is locally disposed, a neutron absorbing section is provided around each neutron moderator, and a fuel section is further provided therearound. This is another embodiment of the basic arrangement.

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

  Around the neutron moderator aggregate 43 in the radial direction, six first composite fuel assemblies 42 are arranged. Each first composite fuel assembly 42 has a plurality of basic fuel elements 27a and a plurality of neutron absorber elements 39. The neutron absorber elements 39 are arranged in layers on one side. Further, the six first composite fuel assemblies 42 are arranged in such a direction that the neutron absorber elements 39 of each of the first composite fuel assemblies 42 are adjacent to the neutron moderator aggregate 43.

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

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

[Fourth embodiment]
In the fourth embodiment, a basic arrangement in which a neutron absorbing section is arranged around a neutron moderating section and a fuel section is further arranged around the neutron absorbing section 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. A second combined fuel assembly 44 is provided instead of the inner core fuel assembly 17 in the central region. 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 has a plurality of composite fuel elements 45 and a plurality of basic fuel elements 27a arranged in parallel with each other in the wrapper tube 24. In a horizontal section, the composite fuel element 45 is arranged in the central region. A basic fuel element 27a is arranged so as to surround a radial outer side of the composite fuel element 45. Note that the composite fuel element 45 may be provided instead of all the basic fuel elements 27a.

  FIG. 18 is an elevational sectional view showing a composite fuel element of the second composite fuel assembly. The 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 neutron absorber 41 via an insulator 46. It has a fuel 28 and a composite part covering tube 45a for accommodating them. A lower blanket 29 is arranged below the core fuel 28 arranged below. An upper blanket 30 is provided above the core fuel 28 provided above. 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 absorber corresponds to the axial height of the neutron absorber 41 in the composite fuel element 45.

  As described above, according to the fourth embodiment, it is possible to realize the basic arrangement of disposing the neutron absorbing portion around the neutron moderating portion and further disposing the fuel portion around the neutron absorbing portion not only in the radial direction but also in the axial direction. By realizing this, it is possible to broaden the range of selection of combinations of components of the fast neutron core, and to improve design flexibility such as improvement of safety.

[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. As described above, the neutron moderator 40 and the neutron absorber 41 are alternately arranged in the axial direction.

  By arranging the neutron moderators 40 and the neutron absorbers 41 alternately in the axial direction in this manner, the neutron absorbing portions are also provided in the energy region where the neutron energy is decreasing to the low energy region, as in the second embodiment. And the effect of using the resonance absorption region can be obtained.

[Sixth Embodiment]
FIG. 20 is a plan view showing the configuration of the 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 provided instead 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 of dispersion is not limited to the region of the inner core fuel assembly 17. For example, they may be distributed in a range extending to the region of the outer core fuel assemblies 18.

  The present embodiment configured as described above provides versatility to the layout plan of the fast neutron core, and can increase the selection range and the design flexibility for ensuring safety.

[Other Embodiments]
Although some embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. For example, as a core utilizing fast neutrons, the case of a fast reactor is described as an example, but the present invention is not limited to the fast reactor. For example, a core of a fast reactor without blankets such as an axial blanket or a radial blanket, a reduced fast core in which the neutron spectrum is shifted to the fast side by limiting the amount of moderator, or an accelerator driven atom The core of the furnace may be used.

  Further, the features of each embodiment may be combined. Further, these embodiments can be implemented in other various forms, and various omissions, replacements, and changes can be made without departing from the spirit 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 equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... Fast neutron reactor, 2 ... Reactor vessel, 3 ... Fast neutron core, 4 ... Core support plate, 5 ... Reactor internal structure, 6 ... Core catcher, 7 ... Core upper mechanism, 8 ... Coolant inlet piping, 9: coolant outlet pipe, 10: coolant, 11: shielding plug, 15: control rod drive unit, 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 inlet hole, 22: entrance nozzle, 23: element support grid, 24: wrapper tube, 25: handling head, 26: coolant outlet 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 part Lenum, 34: cladding tube, 34a: neutron moderating portion cladding tube, 34b: neutron absorbing portion cladding tube, 35: lower end plug, 36: upper end plug, 37, 37a: burnable poison aggregate, 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 (7)

A fuel section including fissile material and generating fast neutrons by reaction with neutrons, and a plurality of fuel elements having a fuel section cladding tube enclosing the fuel section, are arranged in a vertical direction and arranged in parallel with each other. A plurality of core fuel assemblies;
A neutron moderator that reduces the energy of the incoming neutrons to a predetermined low energy region,
A neutron absorber that absorbs the incoming neutrons,
With
The neutron moderator and the neutron absorber are arranged in a region where the core fuel assemblies are arranged,
The fuel unit and the neutron moderating unit are arranged to sandwich the neutron absorbing unit,
The thickness D1 of the neutron moderator is larger than the moderation distance Δτ,
A fast neutron reactor core characterized in that: 2. The neutron absorber, wherein a material that satisfies a condition that a ratio of a remaining amount of the neutron absorber to an initial amount at the end of an operation period is equal to or less than a predetermined value is selected. Fast neutron core.
The condition is expressed by the following equation: σ _a ΦT> log _e 10
(However, σ _a indicates a neutron absorption cross section of the material, Φ indicates a neutron flux, T indicates an operation period, and log _e 10 indicates a value of natural logarithm of 10.)   The fast neutron core according to claim 1, wherein the predetermined low energy region is a neutron energy region in which an absorption cross-sectional area of the neutron absorbing unit is equal to or larger than a predetermined value.   The neutron absorption thickness of the neutron absorption section is determined by the neutron absorption reaction rate curve of the neutron absorption section, and the neutron absorption rate on the surface of the neutron absorption section facing the neutron reduction section, which gives the maximum slope of the neutron absorption reaction rate curve. The fast neutron core according to any one of claims 1 to 3, wherein the slope of the reaction rate curve is extended to be equal to or longer than a distance until the neutron absorption reaction rate becomes zero. A plurality of core fuel assemblies each including a plurality of fuel elements having a fuel portion and a fuel portion cladding tube including the fuel portion and arranged in parallel with each other and extending in a vertical direction,
A neutron moderator element having the neutron moderating section and a neutron moderating section cladding tube including the neutron moderating section, and a neutron absorber element including the neutron absorbing section and a neutron absorbing section cladding tube including the neutron absorbing section. A plurality of burnable poison assemblies extending in the vertical direction and arranged in parallel with each other and arranged in parallel with the core fuel assembly,
The fast neutron core according to any one of claims 1 to 4, further comprising: A basic fuel element having the fuel section and a basic fuel section cladding tube containing the fuel section;
The neutron moderator, the neutron absorber disposed above and below the neutron moderator, and the fuel section further disposed above and below the neutron moderator, the composite section including the neutron moderator, the neutron absorber, and the fuel section A composite fuel element having a cladding tube;
The fast neutron reactor core according to any one of claims 1 to 5, further comprising a plurality of core fuel assemblies extending in a vertical direction and arranged in parallel with each other. A fast neutron core utilizing fast neutrons,
A reactor vessel containing the fast neutron core and holding a reactor coolant cooled by cooling means,
A fast neutron reactor comprising:
The fast neutron core,
A fuel section including fissile material and generating fast neutrons by reaction with neutrons, and a plurality of fuel elements having a fuel section cladding tube enclosing the fuel section, are arranged in a vertical direction and are arranged in parallel with each other. A plurality of core fuel assemblies;
A neutron moderator that reduces the energy of the incoming neutrons to a predetermined low energy region,
A neutron absorber that absorbs the incoming neutrons,
With
The neutron moderator and the neutron absorber are arranged in a region where the core fuel assemblies are arranged,
The fuel unit and the neutron moderating unit are arranged to sandwich the neutron absorbing unit,
The thickness D1 of the neutron moderator is larger than the moderation distance Δτ,
A fast neutron reactor, characterized in that:

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