JP2015141086A - Fast reactor fuel assembly and fast reactor core - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the Doppler effect in the core of U-free metallic fuel reactor.SOLUTION: According to an embodiment, a fast reactor core fuel assembly 101 used in a fast reactor core for converting TRU comprises: a plurality of fuel elements 121 including a fuel containing TRU to be separated in reprocessing and a base material that is a substantial remainder, and cladding tubes containing the fuel; a plurality of neutron spectrum softening elements 122 including a neutron spectrum softening material for softening neutron energy spectrum, and cladding tubes containing the neutron spectrum softening material; and a wrapper tube 9 containing the fuel elements 121 and the neutron spectrum softening elements 122, having a cylindrical shape, and having openings on both ends thereof.

Description

  The present invention relates to a fast reactor core that converts a transuranium element (TRU) into an element other than TRU and a fast reactor fuel assembly used therefor.

  FIG. 11 is an elevational sectional view showing a configuration example of a conventional fast reactor. In a general fast reactor, for example, as shown in FIG. 11, the core 1 is supported by a core support plate 2. A core catcher 3 is disposed below the core support plate 2. The core 1, the core support plate 2, and the core catcher 3 are stored in a reactor vessel 4. Further, a part of the coolant inlet pipe 31 and the coolant outlet pipe 32 are similarly stored in the reactor vessel 4. The coolant 15 that has flowed out from the coolant inlet pipe 31 into the lower space in the reactor vessel 4 turns upward, passes through the core support plate 2, and flows into the core 1 that is a heat generation source. After the coolant 15 is heated in the core 1, it flows out of the coolant outlet pipe 32 in order to exchange heat outside the reactor vessel 4.

  FIG. 12 is a horizontal sectional view showing a configuration example of a core of a conventional fast reactor. As shown in FIG. 12, the core 1 includes a core fuel assembly 5 containing a lot of fissile material, a blanket fuel assembly 7 containing a large amount of a fissile parent material that is converted into a fissile material by neutron absorption, a neutron The control rod assembly 8 includes a large amount of absorbing material and controls the fission reaction. The core fuel assembly 5 includes an inner core fuel assembly 5a and an outer core fuel assembly 5b. Here, the inner core fuel assembly 5a is represented by a hexagon, the outer core fuel assembly 5b is a circle in the hexagon, the diameter blanket fuel assembly 7 is a double circle in the hexagon, and a control rod assembly. Reference numeral 8 denotes a hexagonal shape with C added.

  FIG. 13 is a longitudinal sectional view showing a configuration example of a core fuel assembly of a conventional fast reactor. FIG. 14 is a cross-sectional view showing a configuration example of a core fuel assembly of a conventional fast reactor. Each of the inner core fuel assembly 5a and the outer core fuel assembly 5b has a plurality of fuel elements 10 disposed in a cylindrical trumpet tube 9, as shown in FIGS.

  The fuel element 10 of the core fuel assembly 5 has a structure including an upper shaft blanket fuel region 11, a core fuel region 12, a lower blanket fuel region 13, and the like. A handling head 16 serving as a gripping part when the core fuel assembly 5 is loaded into the core 1 or taken out from the core 1 is provided on the upper portion of the trumpet tube 9. On the other hand, an entrance nozzle 17 for instructing fixation of the core fuel assembly 5 is provided below the trumpet tube 9. A coolant inlet 18 is formed in the side wall of the entrance nozzle 17. The coolant 15 flows into the trumpet tube 9 of the core fuel assembly 5 from the coolant inlet 18, passes through the coolant channel 14 in the trumpet tube 9, is heated by the fuel element 10, and then the handling head. It flows out of the core fuel assembly 5 from the 16 coolant flow paths 14.

  FIG. 15 is a cross-sectional view showing an example of a fuel element of a core fuel assembly of a conventional fast reactor. The fuel element 10 includes a fuel 22 that extends in a columnar shape and a cladding tube 20 that stores the fuel 22. The cladding tube 20 is made of a metal material such as stainless steel. A minute gap 23 exists between the fuel 22 and the cladding tube 20 in order to absorb swelling caused by the fuel 22 burning in the core.

  The core fuel and blanket fuel for fast reactors are produced from raw materials of plutonium (Pu) extracted from spent fuel reprocessing facilities in light water reactors (LWR) and depleted uranium produced as a by-product from uranium (U) enrichment facilities. Mixed oxide fuel is used.

  Spent fuel reprocessing waste contains minor actinides (MA) and long half-life fission products (LLFP) with half-lives on the order of tens of thousands of years, so they are deep as high-level radioactive waste (HLW). It is planned to be disposed of. However, since MA and LLFP have half-lives on the order of tens of thousands of years, disposal of HLW is accompanied by great difficulty. In the technical study for the sustainable use of nuclear energy, the study of burning uranium element (TRU) containing Pu by fission and making LLFP stable or short half-life nuclide by neutron transmutation is promoted. ing.

  Up to now, studies have been made to eliminate TRU and LLFP, that is, to convert them into other elements. In the main example, a fast reactor is used to burn TRU and LLFP, and the fuel is U. U-TRU fuel containing TRU is used (for example, refer to Patent Document 1). However, since TRU is newly generated from U simultaneously with the combustion of the loaded TRU, the net amount of TRU combustion is small. For example, when the conversion ratio is 1, U that is 1 ton of fissile parent substance is converted to TRU that is 1 ton of fissile substance. Accordingly, the amount of TRU taken out is 1 ton with respect to the amount of TRU loaded. In this case, the TRU from the LWR can be taken into the fuel only at the first loading.

  Therefore, in this method, TRUs from the LWR cannot be processed unless all the LWRs are replaced in the fast reactor. Therefore, in the era of LWR mainstream, in order to perform combustion processing without accumulating TRU generated from LWR, it is necessary to maximize the TRU combustion amount of the combustion facility.

  In order to maximize the amount of TRU combustion, a method of using U-free fuel can be considered so that no new TRU is generated in the combustion facility. That is, by making the combustion facility the core of a U fuel-free fast reactor, no new TRU is generated, and TRU containing Pu can be efficiently burned by nuclear fission.

On the other hand, in general, the safety design of nuclear reactors expects the Doppler effect as an immediate negative feedback reactivity that accompanies a rise in core temperature, resulting in a decrease in reactivity due to an increase in neutron absorption by the core fuel. . This Doppler effect is mainly due to an increase in the resonance absorption reaction due to ²³⁸ U in the fuel.

JP 7-294676 A

  Regarding the fuel form of the U fuel-free fast reactor, the combustion efficiency by fission is superior to the oxide fuel core such as the MOX core in the metal fuel core. For example, the ratio of TRU fission to capture reaction is higher for metal fuel than for mixed oxide fuel (MOX fuel). Further, by adopting a metal form having a high material density, there is a high possibility that more TRUs can be accepted in the fuel than in the case of using another oxide form or nitride form.

In the case of the above-described fuel that does not use U (no U fuel), it is necessary to reduce the fuel volume ratio, increase the coolant and the structural material, or add a diluent due to limitations from the critical amount of the core. is there. In addition, since the core of the U-free fuel fast reactor has no ²³⁸ U in the fuel, the Doppler effect is reduced, and the absolute value of the negative feedback reactivity is reduced.

In addition, there are many known examination evaluation examples of Pu-Th / TRU-Th fuel cores using thorium (Th) instead of U. Simultaneously with the combustion of Th, U isotopes such as ²³³ U and ²³² U are generated and accumulated. Here, “U-free” does not include those using Th or Th compounds.

  The neutron absorption reaction (absorption) which is a factor of the Doppler effect is composed of two kinds of reactions, a neutron capture reaction (capture) and a neutron fission reaction (fission). That is, the Doppler effect of the neutron absorption reaction is determined by the balance between a decrease in the number of neutrons accompanying an increase in the neutron capture reaction and an increase in the number of neutrons accompanying an increase in the neutron fission reaction. The number of neutrons generated per fission by the neutron fission reaction (ν) is about 2.9.

  The change in reactivity accompanying the increase in temperature of the fuel nuclide is mainly due to a change in the balance between the increase in neutron absorption and the increase in neutron generation due to neutron fission. Examples of the relationship between the neutron energy and the magnitude of the neutron absorption reaction cross section of the TRU fuel nuclide with temperature change are shown in FIGS. Here, the characteristics will be described using numerical examples using the standard 70 group nuclear constant set created for the fast reactor. Further, as a reference for temperature, attention is paid to a change from normal temperature (273K) to high temperature (1100K).

  The neutron absorption cross section at normal temperature (273K) for each nuclide is expressed as σa (273), the neutron fission cross section is expressed as σf (273), and the number of neutrons generated per fission is expressed as ν. If the change in neutron absorption cross section due to temperature rise from 273 K to 1100 K is Δσa (273 → 1100), and similarly, the change in neutron fission cross section due to temperature rise from 273 K to 1100 K is Δσf (273 → 1100). The neutron balance fluctuation (fluctuation) accompanying the fuel temperature rise can be expressed as follows.

(Change) = Δσa (273 → 1100) −νΔσf (273 → 1100)
Here, with the neutron absorption cross section σa (273) as a reference as a denominator, the following ratio value is defined as “Doppler effect potential”.

Doppler effect potential = (variation) / (reference value)
= [Δσa (273 → 1100) −νΔσf (273 → 1100)] / σa (273)
This Doppler effect potential is defined for each nuclide. The value obtained by integrating the Doppler effect potential in neutron energy and space corresponds to the Doppler reactivity coefficient. When the integral value of the Doppler effect potential is positive, the Doppler reactivity coefficient is negative. Looking at the Doppler effect potential for typical nuclides, we can say the following.

FIG. 16 is a graph showing the Doppler effect potential of ²³⁸ U. U238 in the figure shows a ²³⁸ U. Of the conventional U fuel core, Pu / U fuel core, and TRU / U fuel core, ²³⁸ U, which has a large abundance ratio, plays a central role in the Doppler effect. Figure 16, for the ^{238 U,} it shows the relationship between the Doppler effect potential and neutron energy.

Below the neutron energy in the non-separation resonance absorption reaction region where the Doppler effect is effective, the increase in neutron absorption due to the increase in fuel temperature is greater than the increase in fission, and in most cases the Doppler effect potential is a positive numerical value. You can see that it is in range. ²³⁸ U is a fission reaction that fissions in a relatively high energy range and has a threshold value of about 0.1 MeV. Since there is no substantial fission reaction in the resonance absorption reaction region of 0.1 MeV or less, absorption break-up due to a temperature rise. This is mainly due to the increase in area.

^Although the value of the ²³⁸ U Doppler effect has been demonstrated to be negative, the broken line in FIG. 17 and the vertical axis of the Doppler potential value of 0 (the horizontal direction in which the vertical axis value Y corresponds to 0) It can be seen that the characteristic weighted by the energy distribution of the neutron flux and the associated neutron flux to the area for each energy group between the axis and the Doppler effect. Therefore, the polygonal line in FIG. 16 is in the positive region, and the total area between the Y = 0 axis corresponds to a positive value here. That is, here, ²³⁸ U which gives a negative Doppler coefficient means that the integrated value of the neutron energy of the Doppler effect potential corresponds to a positive value.

FIG. 17 is a graph showing the Doppler effect potential of ²³⁹ Pu and ²⁴⁰ Pu. In the figure, Pu239 indicates ²³⁹ Pu and Pu240 indicates ²⁴⁰ Pu. The neutron energy dependence of the Doppler effect potential is shown for ²³⁹ Pu and ²⁴⁰ Pu which are the main components of Pu isotopes. ²³⁹ Pu, which has a large fission cross section in the entire energy region, has a large value in the region below the axis of Y = 0, and the relative value of the change in cross section tends to be small, and the value of the Doppler coefficient is negative. There is a possibility that the absolute value is small or positive.

On the other hand, ²⁴⁰ Pu may not have a fission reaction in the low energy region like the above-mentioned ²³⁸ U, and the value of the Doppler effect potential is positive in the resonance absorption region, and the Doppler coefficient becomes a negative feedback factor.

Other isotopes ²⁴¹ Pu have ²³⁹ Pu and ²⁴² Pu has a Doppler effect potential similar to ²⁴⁰ Pu. As the total of Pu, it is necessary to consider the weight of the Pu isotope composition ratio depending on the burnup of the spent fuel and the cooling period.

FIG. 18 is a graph showing the Doppler effect potential of ²³⁷ Np, ²⁴¹ Am, and ²⁴³ Am. In the figure, Np237 represents ²³⁷ Np, Am241 represents ²⁴¹ Am, and Am243 represents ²⁴³ Am. It shows the neutron energy dependence of the Doppler effect potential of ²³⁷ Np, ²⁴¹ Am, and ²⁴³ Am, which are the main components of MA in addition to Pu. These nuclides, like ²³⁸ U and ²⁴⁰ Pu, may not have a fission reaction in the low energy region, and the value of the Doppler effect potential is positive in the resonance absorption region, which becomes a negative feedback factor as the Doppler effect. I understand that.

Further, since the Doppler effect potential is shifted to a lower energy side than ²³⁸ U and ²⁴⁰ Pu, it can be understood that the arrangement of the moderator is effective for ensuring the Doppler effect of the U-free fast reactor.

  The above is summarized as follows.

Of TRU, attention needs to be paid to ²³⁹ Pu and ²⁴¹ Pu which are fissile Pu. The Doppler effect of ²³⁹ Pu and ²⁴¹ Pu has a small absolute value as a negative effect and can be positive depending on the composition / moderator ratio. The same applies to the thermal neutron spectrum reactor.

On the other hand, the main components of minor actinides (MA; Np, Am, Cm) obtained by removing Pu from TRU are ²³⁷ Np, ²⁴¹ Am, and ²⁴³ Am. These nuclides are characterized by fission by fast neutrons. In the energy region of the Doppler effect of the neutron absorption cross section (mainly 100 keV or less), the Doppler effect of the neutron capture cross section becomes the center, and the Doppler effect of the fission cross section becomes smaller. As a result, the Doppler effect of these Np and Am It becomes a negative feedback factor.

FIG. 19 is a graph showing examples of analysis results of Doppler reactivity of an oxide fuel core, a metal fuel core having uranium, and a uranium-free metal fuel core (U-free metal fuel core). Doppler of oxide fuel core using uranium and TRU, metal fuel core, and TRU metal fuel core using zirconium (Zr) as a base material (diluent) without using uranium in a fast spectrum core An example of the relative value of reactivity is shown. Here, the case of an oxide fuel core is used as a reference. For oxide fuels, ²³⁸ U, which accounts for 70% to 80% of the total fuel, is a factor in the negative Doppler effect. Similarly, for metal fuel, ²³⁸ U is the main factor of the negative Doppler effect, but the base material Zr (10 wt% = about 29 at%) also has a negative Doppler effect, but its absolute value is small.

  In a uranium-free core (U-less core), a negative Doppler effect is ensured by such overall optimization including the influence of the fuel material composition and the neutron spectrum. As for the fuel, the matrix element homogeneously mixed in the form of the fuel element and the alloy also immediately follows the increase in the temperature of the fuel element due to the increase in fission reaction, and the temperature of the matrix element rises. It becomes a factor.

As shown in FIG. 19, the core of a U metal-free fuel fast reactor has no ²³⁸ U, so the Doppler effect is reduced, and the influence of resonance absorption reaction in the middle and fast energy regions of Zr and minor actinides (Np, Am, etc.) Has a negative feedback reactivity, but there is a problem that the size is small.

  Embodiments of the present invention have been made in order to solve the above-described problems. In the core of a fast reactor, an immediate negative feedback reaction accompanying an increase in core temperature expected in the safety design of a nuclear reactor. The aim is to increase the Doppler effect, which is a degree.

  In order to achieve the above-described object, the present embodiment is a fast reactor fuel assembly used in a fast reactor core that converts TRU into an element other than TRU, and is substantially different from TRU separated in reprocessing. A plurality of fuel pipes that extend in a predetermined direction and have a tubular shape that contains the fuel and extends in the same direction as the predetermined direction and is closed at both ends. A plurality of neutron spectrum softenings comprising a fuel element, a neutron spectrum softening material for softening a neutron energy spectrum, and a cladding tube containing the neutron spectrum softening material and extending in the same direction as the predetermined direction and closed at both ends An element, the plurality of fuel elements and the plurality of neutron spectrum softening elements, and provided radially outside the plurality of fuel elements and the plurality of neutron spectrum softening elements A cylindrical shape extending along the longitudinal direction, characterized in that it comprises a wrapper tube having openings at both ends.

  Further, the present embodiment is a fast reactor core that includes a plurality of fast reactor fuel assemblies arranged in a grid, and converts TRU into an element other than TRU, each of the fast reactor fuel assemblies. Has a TRU that is separated in reprocessing and a base material that is substantially the remainder, a fuel extending in a predetermined direction, and a cylindrical shape containing the fuel and extending in the same direction as the predetermined direction A plurality of fuel elements each having a cladding tube closed at both ends, a neutron spectrum softening material for softening a neutron energy spectrum, and containing the neutron spectrum softening material and extending in the same direction as the predetermined direction, A plurality of neutron spectrum softening elements having closed cladding tubes, the plurality of fuel elements and the plurality of neutron spectrum softening elements, and the plurality of fuel elements and the plurality of neutron spectra. Provided radially outside the torr softening elements a cylindrical shape extending along the longitudinal direction, characterized in that it comprises a wrapper tube having openings at both ends.

  According to the embodiment of the present invention, in the core of a fast reactor, it is possible to increase the Doppler effect, which is an immediate negative feedback reactivity associated with the core temperature rise expected in the safety design of the reactor.

It is a horizontal sectional view showing typically the U metal free fuel core concerning a 1st embodiment. It is a cross-sectional view showing the core fuel assembly of the U metal-free fuel core according to the first embodiment. It is a transverse cross section showing composition of a fuel element of a core fuel assembly of a U metal free fuel core concerning a 1st embodiment. It is a cross-sectional view showing the configuration of the neutron spectrum softening element of the core fuel assembly of the U metal-free fuel core according to the first embodiment. It is a graph which shows the neutron spectrum of the U metal-free fuel core which concerns on 1st Embodiment, and the conventional core. It is a bar graph which shows the Doppler reactivity with respect to the neutron spectrum softening material in the U metal-free fuel core which concerns on 1st Embodiment. It is a graph which shows the relationship between the ratio of each neutron spectrum softening material in the core fuel assembly of the U metal-free fuel core which concerns on 1st Embodiment, and Doppler reactivity. It is a cross-sectional view which shows the structure of the core fuel assembly of the U metal-free fuel core which concerns on 2nd Embodiment. It is a cross-sectional view which shows the structure of the core fuel assembly of the U metal-free fuel core which concerns on 3rd Embodiment. It is a cross-sectional view showing a fuel element of a core fuel assembly of a U metal-free fuel core according to a third embodiment. It is an elevational sectional view showing a configuration example of a conventional fast reactor. It is a horizontal sectional view which shows typically the example of composition of the core of the conventional fast reactor. It is a longitudinal cross-sectional view which shows the structural example of the core fuel assembly of the conventional fast reactor. It is a cross-sectional view which shows the structural example of the core fuel assembly of the conventional fast reactor. It is a cross-sectional view which shows the example of the fuel element of the core fuel assembly of the conventional fast reactor. ²³⁸ is a graph showing the Doppler effect potential of U. It is a graph which shows the Doppler effect potential of ²³⁹ Pu and ²⁴⁰ Pu. It is a graph which shows the Doppler effect potential of ²³⁷ Np, ²⁴¹ Am, and ²⁴³ Am. It is a graph which shows the example of the analysis result of each Doppler reactivity of an oxide fuel core, a metal fuel core which has uranium, and a U metal-free fuel core.

  Hereinafter, a U-free fuel assembly and a U-free fuel core according to an embodiment of the present invention will be described with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

[First Embodiment]
FIG. 1 is a horizontal sectional view schematically showing a U-free metal fuel core according to the first embodiment. The U metal-free fuel core 100 includes a core fuel assembly 101 arranged in parallel with each other in the vertical direction and a diameter blanket fuel assembly 7 arranged on the outside in the radial direction. The core fuel assembly 101 includes an inner core fuel assembly 101a and an outer core fuel assembly 101b. The inner core fuel assemblies 101a are arranged in the central region, and the outer core fuel assemblies 101b are arranged outside in the radial direction. In the region where the core fuel assemblies 101 are arranged, control rod assemblies 8 are arranged so as to be scattered.

  FIG. 2 is a cross-sectional view showing a core fuel assembly of a U metal-free fuel core. The core fuel assembly 101 includes a fuel element 121 and a neutron spectrum softening element 122 arranged in a triangular arrangement in parallel with each other, and a wrapper tube 9 provided radially outside the fuel element 121 and the neutron spectrum softening element 122. Have. One neutron spectrum softening element 122 is arranged at the center of the array, and annular rows of the fuel element 121 and the neutron spectrum softening element 122 are alternately arranged in the radial direction around the neutron spectrum softening element 122. A hexagonal tube-shaped trumpet tube 9 is provided radially outward of the final annular row. A space between the fuel element 121 and the neutron spectrum softening element 122 in the wrapper tube 9 is a flow path for the coolant 15.

  Although FIG. 2 shows a case where four rows of fuel elements 121 and three rows of neutron spectrum softening elements 122 are alternately arranged around the central neutron spectrum softening element 122, the present invention is not limited to this. The number of columns may be smaller or larger. Further, the arrangement method of the fuel elements 121 and the neutron spectrum softening elements 122 is not a method of alternately repeating the annular rows as shown in FIG. 3, but for example, the fuel elements 121 and the neutron spectrum softening elements 122 in the rows parallel to each other. Alternatively, the methods may be arranged alternately. Moreover, although the case of the triangular arrangement as a whole has been shown, it may be arranged in a square lattice and the trumpet tube may be a square cylinder.

  FIG. 2 schematically shows the configuration of the core fuel assembly 101. Specifically, each of the inner core fuel assembly 101a and the outer core fuel assembly 101b has the configuration shown in FIG. have.

  FIG. 3 is a cross-sectional view showing the configuration of the fuel elements of the core fuel assembly of the U metal-free fuel core. The fuel element 121 includes a TRU fuel 121a extending in a columnar shape, and a cladding tube 121b provided around the radial direction of the TRU fuel 121a. The cladding tube 121b is cylindrical and has both ends closed. The TRU fuel 121a is a metal having, as a main component, a mixed TRU in which a part or all of elements of Pu, Np, Am, and Cm are collected in a mixed state in a nuclear fuel recycling process, and a base material that is substantially the remainder. It is a form of metal fuel. Here, examples of the base material include zirconium (Zr).

  The actual TRU fuel 121a also includes a small amount of U that has moved to the recovered mixed TRU, but the effect on the effect of this embodiment is small. That is, for the terms “uranium-free fuel”, “TRU fuel”, “U-free fuel”, “U-free” in each embodiment, the TRU after the uranium separation step in the recycling process is used. This refers to fuel as the main fuel material, and those containing uranium that could not be separated even after the separation process of uranium in the recycling process are also included in “uranium-free metal fuel”, “TRU fuel”, “U” It is included in “metal-free fuel” and “U-free fuel”.

FIG. 4 is a cross-sectional view showing the configuration of the neutron spectrum softening element of the core fuel assembly of the U metal-free fuel core. The neutron spectrum softening element 122 includes a neutron spectrum softening material 122a extending in a columnar shape and a cladding tube 122b provided around the radial direction of the neutron spectrum softening material 122a. The cladding tube 122b is cylindrical and has both ends closed. Examples of the neutron spectrum softening material 122a include LiH, B ₄ C, BeO, and ZrH ₂ that include an element having a small neutron capture cross section and a relatively small atomic number. Here, Li is ⁷ Li and B is ¹¹ B. Further, although the neutron moderating ability is reduced as compared with H or Be, carbon (C) may be used as the neutron spectrum softening material 122a in the form of graphite or SiC. Furthermore, a magnesium (Mg) alloy or the like may be used.

  The TRU fuel 121a and the neutron spectrum softening material 122a are cylindrical, and the cladding tube 121b and the cladding tube 122b are cylindrical. However, the present invention is not limited to this and may be a polygonal shape.

FIG. 5 is a graph showing neutron spectra of a U-metal-free fuel core and a conventional core. The horizontal axis represents neutron energy (eV), and the vertical axis represents the neutron spectrum (φi / φt). However, (phi) i is the neutron flux in each neutron energy, (phi) t is an integral value over all the energy of (phi) i. That is, the value on the vertical axis is normalized so that the integral value over the entire energy is 1. For example, 1.0E + 01 in the display of the values on the horizontal and vertical axes represents 1.0 × 10 ⁺⁰¹ . The same applies hereinafter.

  As shown in FIG. 5, the neutron energy spectrum of the U-metal-free fuel core according to the present embodiment has a lower peak at energy around 100 keV than the neutron energy spectrum of the conventional core. The distribution in the order of several eV to several keV is large. That is, neutrons in the energy region around 100 keV are decelerated, and the neutron spectrum is softened, that is, shifted in a direction of lower energy.

  FIG. 6 is a bar graph showing the Doppler reactivity with respect to the neutron spectrum softening material in the U metal-free fuel core. The horizontal axis roughly indicates a MOX fuel core and a U metalless fuel core as examples of the oxide fuel core. Moreover, about the U metal-free fuel core, the case where there is no neutron spectrum softening material and the case where it exists are shown.

Here, the U metal-free fuel without the neutron spectrum softening material is a material composed of TRU and Zr as a base material. In the case where there is a neutron spectrum softening material, the case where the neutron spectrum softening material is LiH, B ₄ C, BeO, or ZrH ₂ is shown. The vertical axis indicates the relative value of Doppler reactivity in each case, and the case of the MOX fuel core is 1.

As shown in FIG. 6, when there is no neutron spectrum softening material in the U metal-free fuel core, the Doppler reactivity decreases to about 1/3 that in the case of the MOX fuel core. When LiH is added to the U metal-free fuel core as a neutron spectrum softening material, the Doppler reactivity is about ½ that of the MOX fuel core. When B ₄ C or BeO is added as a neutron spectrum softening material to the U metal-free fuel core, the Doppler reactivity becomes more than twice that of the MOX fuel core. When ZrH ₂ is added as a neutron spectrum softening material to the U metal-free fuel core, the Doppler reactivity is more than three times that of the MOX fuel core.

FIG. 7 is a graph showing the relationship between the ratio of each neutron spectrum softening material and the Doppler reactivity in the core fuel assembly of the U metal-free fuel core. The horizontal axis represents the ratio (%) of the weight of the neutron spectrum softening material to the total weight of the fuel and the neutron spectrum softening material in the fuel assembly. Examples of the softening material include beryllium oxide BeO indicated by a broken line, lithium hydride LiH indicated by a two-dot chain line, boron carbide B ₄ C indicated by a one-dot chain line, and zirconium hydride ZrH ₂ indicated by a solid line. The vertical axis shows the Doppler reactivity as in FIG. 6, and is a relative value with 1 in the case of the MOX fuel core.

  As described above, in the case of the U metal-free fuel core using Zr as the base material shown in FIG. 6 and having no neutron spectrum softening material, the Doppler reactivity (relative value) is about 1/3 that of the MOX fuel core. It is. In order to significantly increase the Doppler reactivity (relative value) compared to this case, for example, to ½, when BeO is used as the neutron spectrum softening material, the ratio of the neutron spectrum softening material is about 20%. is there. Therefore, when BeO is used as the neutron spectrum softening material, the weight ratio (%) needs to be 20% or more in order to produce the effect of the present embodiment.

  On the other hand, the maximum value of the neutron spectrum softening material is restricted by the critical amount of the core. When BeO is used as the neutron spectrum softening material, the ratio of the neutron spectrum softening material at which the core becomes critical is about 80%, and when the ratio of the neutron spectrum softening material is larger than this, the core becomes subcritical. Therefore, when BeO is used as the neutron spectrum softening material, the ratio of the neutron spectrum softening material needs to be 80% or less.

  Therefore, in the present embodiment, the number ratio of the neutron spectrum softening material elements or the addition ratio of the neutron spectrum softening material is about 20% or more and about 80%, using BeO as the neutron spectrum softening material as an example. % Or less.

  Even when other neutron spectrum softening materials are used, the range of the ratio of the spectrum softening materials can be appropriately determined based on the same concept.

  As described above, the fast reactor fuel assembly according to the present embodiment, that is, the fast reactor core using the core fuel assembly 101 has a high energy region in which the Doppler effect of the fission reaction of the nuclide that performs fast fission is effective in the TRU. The negative Doppler effect can be increased by shifting the neutron energy spectrum from the side to the resonance absorption region side, which is a relatively low energy region where the Doppler effect of the neutron capture reaction is effective.

  That is, even in the core of a U metal-free fuel fast reactor, a large absolute value of negative Doppler reactivity can be secured as compared with the conventional MOX fuel core. As a result, the Doppler effect, which is an immediate negative feedback reactivity accompanying the temperature rise of the core, increases, so that the safety of the core is ensured.

[Second Embodiment]
FIG. 8 is a cross-sectional view showing the configuration of the core fuel assembly of the U-free metal fuel core according to the second embodiment. This embodiment is a modification of the first embodiment. The core fuel assembly 102 includes a fuel element 121, a neutron spectrum softening element 122, and an oxide fuel element 123. The difference from the first embodiment is that a part of the fuel element 121 is replaced with an oxide fuel element 123. A total of 18 oxide fuel elements 123 are arranged in place of one or every two fuel elements 121 instead of the outermost fuel element 121.

  In addition, although the case where arrangement | positioning of the oxide fuel element 123 is the outermost periphery and the number is 18 was shown, it is not limited to this. Furthermore, it may be arranged inside. Alternatively, the entire outermost periphery may be replaced with the oxide fuel element 123. The ratio of replacing the fuel element 121 with the oxide fuel element 123 is appropriately determined in consideration of the nuclear thermal characteristics of the metal fuel and the oxide fuel.

  The addition of the oxide fuel element 123 as an element of the core fuel assembly 102 has the effect of further softening the energy spectrum of neutrons and further increasing the negative Doppler effect.

[Third Embodiment]
FIG. 9 is a cross-sectional view showing the configuration of the core fuel assembly of the U-free metal fuel core according to the third embodiment. The core fuel assembly 103 includes a fuel element 124 and a trumpet tube 9. In the first embodiment, the core fuel assembly 101 has the fuel element 121 and the neutron spectrum softening element 122 in the trumpet tube 9, but the core fuel assembly 103 in this embodiment is instead The fuel element 124 is made of a mixed fuel 124a (see FIG. 10) described later.

  FIG. 10 is a cross-sectional view showing the fuel elements of the core fuel assembly of the U metal-free fuel core. The fuel element 124 includes a mixed fuel 124a extending in a columnar shape and a cladding tube 124b provided around the radial direction of the mixed fuel 124a. The cladding tube 124b is cylindrical and has both ends closed.

  The mixed fuel 124a is made of a material obtained by adding a neutron spectrum softening material to a metal fuel composed of a mixed TRU and a base material as main components.

  According to the present embodiment configured as described above, as in the first and second embodiments, neutrons in the energy region around 0.1 MeV are decelerated and the neutron spectrum is softened, that is, in the direction of lower energy. There is a shift effect. As a result, the Doppler effect, which is an immediate negative feedback reactivity accompanying the temperature rise of the core, increases, and the safety of the core is ensured.

[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, in the embodiment, the case of a so-called two-region core having two regions, that is, a region where the inner core fuel assembly is loaded and a region where the outer core fuel assembly is loaded is shown, but the present invention is not limited to this. . For example, the present invention can be applied even in the case of a one-region core or a core having three or more regions.

  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 ... Core, 2 ... Core support plate, 3 ... Core catcher, 4 ... Reactor vessel, 5 ... Core fuel assembly, 5a ... Inner core fuel assembly, 5b ... Outer core fuel assembly, 7 ... Diameter blanket fuel assembly 8 ... control rod assembly, 9 ... trumpet tube, 10 ... fuel element, 11 ... upper blanket fuel region, 12 ... core fuel region, 13 ... lower blanket fuel region, 14 ... coolant flow path, 15 ... coolant , 16 ... Handling head, 17 ... Entrance nozzle, 18 ... Coolant inlet, 19 ... Reflector, 20 ... Cladding tube, 22 ... Fuel, 23 ... Gap, 31 ... Coolant inlet piping, 32 ... Coolant outlet piping, DESCRIPTION OF SYMBOLS 100 ... U metalless fuel core, 101 ... Core fuel assembly, 101a ... Inner core fuel assembly, 101b ... Outer core fuel assembly, 102, 103 ... Core fuel assembly, 110 ... U metalless fuel assembly 121 ... Fuel element, 121a ... TRU fuel, 121b ... Cladding tube, 122 ... Neutron spectrum softening element, 122a ... Neutron spectrum softening material, 122b ... Cladding tube, 123 ... Oxide fuel element, 124 ... Fuel element, 124a ... Mixing Fuel, 124b ... cladding tube

Claims (7)

A fuel assembly for a fast reactor used in a fast reactor core that converts TRU into an element other than TRU,
The TRU separated in the reprocessing and the base material that is substantially the remaining part, the fuel extending in a predetermined direction, and a cylindrical shape containing the fuel and extending in the same direction as the predetermined direction A plurality of fuel elements having a cladding tube closed;
A plurality of neutron spectrum softening elements having a neutron spectrum softening material for softening a neutron energy spectrum, and a cladding tube containing the neutron spectrum softening material and extending in the same direction as the predetermined direction and closed at both ends;
A cylindrical shape that accommodates the plurality of fuel elements and the plurality of neutron spectrum softening elements, is provided radially outside the plurality of fuel elements and the plurality of neutron spectrum softening elements, and extends along a longitudinal direction; A trumpet tube having openings at both ends;
A fuel assembly for a fast reactor, comprising:   2. The fast reactor fuel assembly according to claim 1, wherein the TRU includes at least one of plutonium, neptunium, americium, and curium.   The fuel assembly for a fast reactor according to claim 1 or 2, wherein the fuel includes a metal.   The fuel assembly for a fast reactor according to any one of claims 1 to 3, wherein the fuel contains an oxide.   The fuel assembly for a fast reactor according to any one of claims 1 to 4, wherein the fuel and the neutron spectrum softening material are accommodated in the same cladding tube.   6. The fast reactor according to claim 1, wherein the neutron spectrum softening material includes at least one of beryllium oxide, lithium oxide, boron carbide, and zirconium hydride. Fuel assembly. A fast reactor core comprising a plurality of fast reactor fuel assemblies arranged in a grid and converting TRU to an element other than TRU,
Each of the fast reactor fuel assemblies is
The TRU separated in the reprocessing and the base material that is substantially the remaining part, the fuel extending in a predetermined direction, and a cylindrical shape containing the fuel and extending in the same direction as the predetermined direction A plurality of fuel elements having a cladding tube closed;
A plurality of neutron spectrum softening elements having a neutron spectrum softening material for softening a neutron energy spectrum, and a cladding tube containing the neutron spectrum softening material and extending in the same direction as the predetermined direction and closed at both ends;
A cylindrical shape that accommodates the plurality of fuel elements and the plurality of neutron spectrum softening elements, is provided radially outside the plurality of fuel elements and the plurality of neutron spectrum softening elements, and extends along a longitudinal direction; A trumpet tube having openings at both ends;
A fast reactor core comprising:

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