JP2008096338A - Basket, method for designing the same, method for manufacturing the same, and basket design program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly economical fuel container basket that ensures subcriticality. <P>SOLUTION: A method for designing a fuel container basket, containing a fuel assembly, which keeps cells arrayed so as to be surrounded by first basket materials and second basket materials having macroscopic neutron cross-sections larger than the first basket materials, includes the step of: determining a first model bundle in consideration of a reduction in reactivity due to flammable poison contained in the fuel assembly and a second model bundle in consideration of limited burnup (Step S1); calculating an effective multiplication factor on a normal time by using the first model bundle (Step S3); calculating an effective multiplication factor at the time of an accidental fall at which the distance between fuel rods of the second model bundle is enlarged (Step S4); and changing a cell wall-material array repeatedly until the effective multiplication factors become smaller than a critical limit value and the amount of second basket materials becomes unable to be made smaller (Step S6). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fuel container basket including a plurality of rectangular tube-shaped cells for containing a fuel assembly, a design method and a manufacturing method thereof, and a basket design program.

  The fuel assemblies whose reactivity has decreased due to combustion are removed from the reactor and loaded into a spent fuel storage rack of a nuclear power plant. A fuel assembly that is thus removed from the reactor and is not subsequently loaded into the reactor is called a spent fuel assembly.

  The spent fuel assembly is cooled in a spent fuel storage rack for a certain period of time, then stored in a transport container, transported to a reprocessing plant, and reprocessed. The transport container is sometimes referred to as a transport cask.

  In addition, the spent fuel assembly may be stored in an intermediate storage transport / storage container and transported to an intermediate storage facility, where it may be cooled for a long time. A container for transportation and storage is sometimes called a dual-purpose cask. The spent fuel assembly accommodated in the dual-purpose cask may be transported to a reprocessing plant for long-term cooling and then reprocessed or may be finally disposed of.

  Since transportation casks and dual-use casks are expensive, it is desirable to reduce the number of necessary casks by allowing a large number of spent fuel to be stored in each of the casks. Further, when the number of spent fuel storage bodies per cask is small, the number of necessary caskes increases, and the number of transportation increases. This increases the likelihood of an accident. Furthermore, since the number of times the cask is handled increases, the radiation exposure dose may increase.

  For fuel containers such as transport casks and dual-use casks, heat removal and radiation shielding are particularly important during the short cooling period after removal from the reactor. However, it is known that as the cooling period becomes longer, critical safety, that is, the performance to guarantee that it never becomes critical becomes important.

  The fuel container is provided with a basket in a cylindrical main body, for example. When the period from removal of the fuel assembly from the reactor to reprocessing is long, a fuel container that can store more spent fuel in a limited space while ensuring sufficient critical safety is desirable. . Therefore, a fuel container is used in which boron (B) having excellent neutron absorption capability is added to the structural material of the basket of the fuel container to increase the neutron absorption rate and to ensure critical safety. As such a structural material, for example, materials (B-SUS material and B-Al material) in which boron (B) is added to stainless steel (SUS) or aluminum alloy (Al) and which has good structural strength are used. .

  In the B-SUS material, when the B addition rate is about 0.5 to 0.7%, there is no significant difference in mechanical properties and workability from SUS not containing B. However, if the reactivity of the spent fuel increases due to an increase in the initial enrichment accompanying an increase in the burnup of the fuel assembly, the addition rate of B needs to be increased. For example, a boron addition amount of about 1% may be required.

  As a typical fuel container basket, there is one in which a rectangular tube is passed through a spacer to form a lattice arrangement. The spacers are formed by cutting through the insertion holes in a grid pattern on the plate, and are arranged at intervals in the longitudinal direction of the rectangular tube. Patent Document 1 discloses a basket in which square cylindrical cells are arranged in a lattice pattern.

However, when the boron addition rate is increased, the B-SUS material becomes hard and brittle, the strength against impact force becomes insufficient, and the processing becomes difficult. In addition, when concentrated boron obtained by concentrating boron 10 ( ¹⁰ B) having a large neutron absorption cross section is used, although such a problem is greatly alleviated, it becomes very expensive.

Therefore, there is a demand for a fuel assembly storage device that can store a large amount of spent fuel in a limited space as described above, while being inexpensive and capable of sufficiently securing mechanical strength and ensuring sufficient critical safety. . Patent Documents 2 and 3 disclose examples of fuel containers that can store more spent fuel in a limited space while ensuring critical safety.
JP 2001-281392 A JP 2001-311794 A JP 2002-40192 A Tsukasa Kikuchi, et al., "Application of Gadolinea Credit to Cask Transport of BWR-STEP3 SFAs", Proceedings of the Seventh International Conference on Nuclear Criticality Safety (ICNC2003), Tokai Mura, Japan, October 2003, II, pp. 711- 716 Yoshihira Ando, et al., "BURNUP CREDIT APPLICATION TO TRANSPORTATION AND STORAGE OF SPENT BWR FUEL ASSEMBLES (1) -AXIAL ZONING MODEL-", ICNC '99, Versailles (France), 1999 Akihiro Terada, et al., "BURNUP CREDIT APPLICATION TO TRANSPORTATION AND STORAGE OF SPENT BWR FUEL ASSEMBLES (2) -METHODS AND RESULTS OF CALCULATION-", ICNC '99, Versailles (France), 1999 "Topical Report on Actinide-Only Burnup Credit for PWR Spent Nuclear Fuel Packages", DOE / RW-0472 Rev.2, September 1998 ORNL-RSIC, "SCALE 4: A Modular Code System for Performing Standardized Computer Analysis for Licensing Evaluation", CCC-545, 1990

  When the fuel assembly burns in the reactor, the reactivity decreases. Moreover, combustible neutron poisons such as gadolinia and erbia are contained in all fuel assemblies of the boiling water reactor (BWR) and about half of the fuel assemblies of the pressurized water reactor (PWR). The reactivity is also reduced by the inclusion of combustible neutron poisons. However, such a decrease in reactivity is not considered in the critical safety evaluation of a system in which spent fuel is stored in a fuel container (see Non-Patent Document 1).

  Accordingly, an object of the present invention is to provide a fuel container basket that is highly economical while ensuring subcriticality.

  In order to solve the above-described problems, the present invention provides a method for designing a basket that is accommodated in a fuel container, and includes a plurality of rectangular tube-shaped cells each containing a fuel assembly having a burnup level equal to or higher than a limit burnup rate Determining a first model bundle that gives a reactivity greater than a maximum value at a burnup less than the limit burnup of the reactivity considering the decrease of the fuel bundle due to a flammable poison; and A step of obtaining a second model bundle that gives a degree of reactivity greater than the degree of reactivity at the limited burn-up, and the material of the four cell walls that surround the cell are obtained from the first basket material and the first basket material. An initial material arrangement setting step for setting a cell wall material arrangement to be one of the second basket materials having a large macroscopic neutron cross-sectional area, and the first model bundle in the cell A normal effective multiplication factor of the fuel container calculated based on the cell wall material arrangement as being contained, and the second model bundle is contained in the cell and included in the second model bundle Under the restriction that any of the effective multiplication factors at the time of the fuel container dropping accident calculated based on the cell wall material arrangement is smaller than a predetermined critical limit value, assuming that the interval between the fuel rods is expanded by dropping. Cell wall material arrangement change in which at least part of the cell wall material arrangement is changed to a new cell wall material arrangement until there is no room for reducing the amount of the second basket material contained in the cell wall material. And the cell wall material arrangement in which the effective multiplication factor is calculated in the cell wall material arrangement changing step, the effective multiplication factor at the normal time and the fall accident is not the critical limit value. In, and characterized by having a the steps of the cell walls is least said cell wall material arranged to use said second basket material and last cell wall material arranged.

  The present invention is also directed to a method of manufacturing a basket accommodated in a fuel container, comprising a plurality of rectangular cylindrical cells each containing a fuel assembly having a burnup greater than or equal to the limit burnup. Determining a first model bundle that gives a reactivity greater than a maximum value of the degree of burnup less than the limit burnup considering the decrease due to the flammable poison; and a reaction of the fuel assembly at the limit burnup And obtaining a second model bundle that gives a degree of reactivity greater than that of the first basket material, and the four cell wall materials surrounding the cell are separated from the first basket material and the first basket material by a macroscopic neutron break. An initial material arrangement setting step for setting a cell wall material arrangement to be one of the second basket materials having a large area, and the first model bundle is stored in the cell The effective multiplication factor at the normal time of the fuel container calculated based on the layout of the wall material, and the second model bundle is accommodated in the cell, and the distance between the fuel rods included in the second model bundle is Included in the cell wall material under the restriction that any of the effective multiplication factors at the time of the fuel container dropping accident calculated based on the cell wall material arrangement as expanded by dropping is smaller than a predetermined critical limit value A cell wall material arrangement changing step in which at least a part of the cell wall material arrangement is changed to a new cell wall material arrangement until there is no room for reducing the amount of the second basket material to be reduced, and the cell Among the cell wall material arrangements in which the effective multiplication factor is calculated in the wall material arrangement changing step, the effective multiplication factors at the normal time and the fall accident are both less than the critical limit value, and the second The cell wall material arrangement with the least number of cell walls using a basket material is a final cell wall material arrangement, and the first and second baskets are such that the cell wall material is the final cell wall material arrangement. And a cell forming step of arranging the material.

  Further, according to the present invention, a neutron that is more macroscopic than the first basket material and the first basket material, in a basket that is housed in a fuel container and accommodates fuel assemblies each having a burnup level that is greater than or equal to the limit burnup degree. A plurality of rectangular tubular cells each surrounded by four cell walls formed of one of the second basket materials having a large cross-sectional area, each of which accommodates the fuel assembly; Each material of the wall has a first model bundle in the cell that gives a reactivity greater than a maximum at a burnup less than the limit burnup of the reactivity considering the degradation of the fuel assembly by a flammable poison. There is a second model bundle that gives a normal effective multiplication factor calculated as contained and a reactivity greater than the reactivity of the fuel assembly at the limited burn-up. Each effective multiplication factor at the time of a fall accident calculated as the interval between the fuel rods contained in the cell and included in the second model bundle is expanded by dropping is less than a predetermined critical limit value. It is determined to be small and there is no room for reducing the amount of the second basket material.

  The present invention also relates to a basket design program for designing a basket accommodated in a fuel container in which a plurality of rectangular tube-like cells each accommodating a fuel assembly having a burnup level equal to or higher than a limit burnup degree are arranged. A function of storing a first model bundle that gives a reactivity greater than a maximum value in a burnup less than the limit burnup of the reactivity considering a decrease due to a flammable poison of the fuel bundle; A function of storing a second model bundle that gives a reactivity greater than the reactivity at the limited burnup of the body, and four cell wall materials surrounding the cell, the first basket material and the first A function of storing a cell wall material arrangement as one of the second basket materials having a macroscopic neutron cross-sectional area larger than that of the basket material; A normal effective multiplication factor of the fuel container calculated based on the cell wall material arrangement as a dollar is received in the cell, and the second model bundle is received in the cell, and the second It is said that any of the effective multiplication factors at the time of the fuel container dropping accident calculated based on the cell wall material arrangement is smaller than a predetermined critical limit value, assuming that the interval between the fuel rods included in the model bundle is expanded by dropping. Until there is no room for reducing the amount of the second basket material contained in the cell wall material under the restriction, it is repeated that at least a part of the cell wall material arrangement is changed to a new cell wall material arrangement. And the cell wall material arrangement in which the effective multiplication factor is calculated, both the normal multiplication factor and the effective multiplication factor at the time of the fall accident are less than the critical limit value, and the second Characterized in that to achieve a function of outputting the cell wall using the basket material the smallest said cell wall material placed as the last cell wall material disposed, the.

  ADVANTAGE OF THE INVENTION According to this invention, the basket of a fuel container with high economical efficiency can be provided, ensuring subcriticality.

  Embodiments of a fuel container basket according to the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected to the same or similar structure, and the overlapping description is abbreviate | omitted. Moreover, the basket of the fuel container is not limited to the vertical and horizontal directions in the following description and drawings.

[First Embodiment]
FIG. 2 is a top view of the basket according to the first embodiment of the present invention. 3 and 4 are side views of the first lattice plate 21 of the present embodiment, and FIG. 5 is a side view showing an example of the second lattice plate 22 of the present embodiment.

  The fuel container basket 20 is formed by arranging a plurality of rectangular tube-like cells 23 in a grid pattern. The basket 20 has a substantially cylindrical shape as a whole and is accommodated in a fuel container (not shown). The basket of the present embodiment can accommodate 69 fuel assemblies 7, but may be appropriately increased or decreased according to the shape of the fuel container. Further, any shape is acceptable as long as it can be accommodated inside the fuel container.

  The cell 23 is a rectangular tube region slightly larger than the fuel assembly 7 and is surrounded by first and second lattice plates 21 and 22 that are orthogonal to each other. One fuel assembly 7 is accommodated in each cell 23. The length in the axial direction of each cell 23, that is, the length in the axial direction of the basket 20 is approximately the same as the length in the axial direction of the fuel assembly 7, and the fuel assembly 7 can be appropriately stored and stored. Can be held. In FIG. 2, only one fuel assembly 7 is shown, but the fuel assemblies 7 can be accommodated in all the cells 23.

  Each cell 23 of the basket 20 is formed by a first lattice plate 21 and a second lattice plate 22. The first lattice plates 21 are plates that are slightly wider than the width of the fuel assemblies 7 and are arranged in parallel to each other at intervals slightly wider than the width of the fuel assemblies 7. The second lattice plates 22 are plates arranged in parallel with each other at a slightly larger interval than the width of the fuel assembly 7. The first grid plate 21 and the second grid plate 22 are arranged perpendicular to each other.

  The first and second lattice plates 21 and 22 are formed of a first basket material 91 and a second basket material 92 obtained by adding a neutron absorbing material to the first basket material. Here, the first basket material is, for example, stainless steel or aluminum alloy. An example of the neutron absorber is boron.

  The first lattice plate 21 is a substantially rectangular plate, and has a protrusion 24 on one of its long sides. In addition, a dent 25 that fits with the protrusion 24 is formed on the side opposite to the side having the protrusion 24. There are two types of the first lattice plate 21, one formed of the first basket material 91 and one formed of the second basket material 92.

  The second grid plate 22 is a substantially rectangular plate. The second grid plate 22 is formed with slits 26 that fit into the protrusions 24. The slits 26 are arranged at a width interval slightly wider than the width of the fuel assembly 7. The slits 26 are arranged in the vertical direction at the same intervals as the protrusions 24 of the first grid plate 21. The second grid plate 22 is formed by combining the first basket material 91 and the second basket material 92 for each region corresponding to the width of the cell 23.

  FIG. 5 shows an example of a second lattice plate in which a region corresponding to the two cells 23 from the outer periphery of the basket 20 is a first basket material 91 and a second basket material 92 is formed between the first basket material 91. The portion of the first basket material 91 and the portion of the second basket material 92 are joined separately by, for example, welding after being molded separately.

  The first lattice plate 21 and the second lattice plate 22 are fitted by inserting the protrusions 24 into the slits 26 and fixed to each other by welding or the like to form the basket 20. The basket 20 is attached and held at the bottom of the fuel container, for example.

  The fuel container holds the fuel assembly 7 having a burning degree equal to or higher than a predetermined limit burning degree in each cell of the basket 20 and is used for transportation and intermediate storage.

  6 to 23 are top views schematically showing examples of the arrangement of the cell wall materials in the present embodiment.

  The first and second lattice plates 21 and 22 are formed of a first basket material and a second basket material having a macroscopic neutron cross-sectional area larger than that of the first basket material. Here, the first basket material 91 is, for example, stainless steel or aluminum alloy. The second basket material is obtained, for example, by adding a rare earth neutron absorbing material such as boron or gadolinium to the first basket material.

  6 to 23, the portion using the first basket material 91 is indicated by a broken line, and the portion using the second basket material 92 is indicated by a solid line.

  The first basket material 91 does not need to contain any neutron absorber, and the neutron absorption performance, that is, the macroscopic neutron absorption cross section is smaller than that of the second basket material 92. Any material can be used as long as it can be manufactured at a lower cost than the basket material 92.

  Next, a method for designing such a basket 20 under the condition of ensuring critical safety will be described.

  The criticality safety of the fuel container containing the fuel assembly is evaluated by determining the effective multiplication factor during normal and drop accidents. The effective multiplication factor in the normal state is determined by considering the reactivity effect of the flammable poison and assuming the fuel assembly having the maximum reactivity over the entire combustion period of the fuel type to be stored.

  In addition, in the spent fuel transportation in the transportation cask and the transportation storage / cask, it is necessary to consider the 9m drop accident of the fuel container (during the fall accident) in addition to the normal transportation (normal time). Since only the fuel assembly having the burnup exceeding the limit burnup is stored in the fuel container, the effective multiplication factor at the time of the fall accident is evaluated based on the fuel composition of the limit burnup. At the time of a fall accident, it is assumed that the distance between the fuel rods of the fuel assembly (fuel rod pitch) is enlarged in the basket in which it is stored, and the criticality safety is evaluated.

  Normally, the BWR and PWR fuel assemblies are in a state where the amount of water as a moderator is small relative to the nuclear fuel held and the neutrons are insufficiently decelerated. However, the expansion of the fuel rod pitch assuming a fall accident promotes neutron deceleration and increases the reactivity. The subcriticality of the fuel container needs to be ensured even in this case.

  There is a possibility that the fuel assembly 7 that is contained in the cell 23 but has a different shape is accommodated in the fuel container. Further, there is a possibility that the fuel assemblies 7 having the same shape but different enrichment and combustible poison concentration may be accommodated. Therefore, the basket 20 is designed so that critical safety can be ensured even if each such fuel assembly 7 is accommodated.

  FIG. 1 is a flowchart showing a method for designing a basket of a fuel container according to the present embodiment.

  The design method of the basket 20 includes a criticality evaluation preparation step (step S1) for creating a model bundle used for subcriticality evaluation.

  In this criticality evaluation preparatory step (step S1), for each fuel assembly to be accommodated in the fuel container, fuel conditions that are conservative in criticality safety evaluation even if each fuel design and combustion history are considered A model bundle is created in consideration of combustion history conditions.

  The first model bundle for evaluating the effective multiplication factor in the normal state is a fuel assembly having the maximum reactivity over the entire combustion period of each fuel assembly. This maximum reactivity is determined in consideration of the effect of reducing the reactivity caused by the flammable poison. The first model bundle may be an unirradiated fuel assembly having this maximum reactivity.

  The second model bundle for evaluating the effective multiplication factor at the time of a drop accident is all kinds of fuel assemblies to be accommodated both when the fuel rod pitch is normal and when the fuel rod pitch is expanded due to the drop accident. It is assumed that the fuel assembly gives a reactivity higher than the maximum value of the reactivity at the limited burnup. When obtaining the maximum value of the reactivity, the fuel assembly design and the combustion history condition that are conservative in the criticality safety evaluation are used even if each fuel design and combustion history are taken into consideration. Based on the fuel assembly design and the combustion history condition, a model bundle is created and burned to the limit burnup is the second model bundle. In creating the second model bundle, the effect of reducing the reactivity due to the flammable poison may be taken into consideration.

  When creating the first and second model bundles, the calculation code error is also taken into consideration.

  FIG. 24 is a graph showing an example of a change accompanying combustion in the reactivity of a fuel assembly containing a flammable poison.

  Since combustible poisons decrease with combustion, the reactivity of the fuel assembly once increases with combustion. Thereafter, the reactivity reaches a maximum value of 81 due to the disappearance of the flammable poison, and then decreases as the fissile material decreases. The first model bundle is an unirradiated fuel assembly having a reactivity greater than the maximum reactivity value R0. The second model bundle is a fuel assembly having a reactivity higher than the reactivity R1 at the limited burnup B1.

  In FIG. 24, only the reactivity of one type of fuel assembly is shown. However, when a plurality of types of fuel assemblies are stored in the fuel container, the first reactivity is considered in consideration of all the reactivity. And a second model bundle. When the fuel composition is different in the axial direction of the fuel assembly, the first and second model bundles are set in consideration of the reactivity for each cross section.

  As a method for taking into account a decrease in reactivity due to combustion of the fuel assembly, for example, the methods described in Non-Patent Document 2 and Non-Patent Document 3 can be used for the BWR fuel assembly. Regarding the fuel assembly for PWR, the method described in Non-Patent Document 4 can be used.

  In addition, in the basket design method, the initial material arrangement for setting the initial state of the cell wall material arrangement in which the cell walls forming the cells of the basket 20 are either the first basket material 91 or the second basket material 92 is set. A setting step (step S2) is included. In addition, you may perform process S2 after process S1.

  In the present embodiment, as shown in FIG. 6, as the initial state of the cell wall material arrangement, all cell wall materials are, for example, a second basket having a large neutron absorption cross-section containing a neutron absorber such as boron. It is assumed that the material 92 is used.

  After performing Step S1 and Step S2, the material arrangement is changed while increasing the first basket material 91 having a smaller neutron absorption cross-sectional area than the second basket material 92 until subcriticality can be secured. In order to determine the subcriticality, the effective multiplication factor at the normal time is calculated (step S3), and the effective multiplication factor at the time of the fall accident is calculated (step S4).

  In step S3, the effective multiplication factor (k1) at the normal time is calculated for the state in which the first model bundle is accommodated in all the cells 23 of the basket 20. In step S4, the effective multiplication factor is calculated for the state in which the second model bundle is accommodated in all the cells 23 of the basket 20. In step S4, assuming a drop accident, an effective multiplication factor (k2) at the time of the drop accident in a state where the interval (fuel rod pitch) of the fuel rods constituting the second model bundle is enlarged is obtained. Note that it is calculated that water is present around the fuel assembly at both the normal time and the fall accident.

  In step S3, the effective multiplication factor at the normal time may be obtained for a state where the fuel rod pitch of the model bundle is enlarged.

  The subcriticality of the fuel container in which the model bundle after combustion is stored in the basket 20 is evaluated using, for example, a critical analysis code described in Non-Patent Document 5 and a nuclear constant library.

  Next, it is determined whether there is room for reducing the second cell wall material under the condition that subcriticality is ensured (step S5).

  Here, the effective multiplication factor calculated | required in process S3 and process S4 is first compared with a critical limit value. In other words, it is determined that subcriticality is ensured when the effective multiplication factors k1 and k2 at the normal time and the falling accident evaluated using the critical analysis code and the nuclear constant library are both below the critical limit value. . The critical limit value is, for example, 0.95.

  Next, it is examined whether or not an effective multiplication factor is obtained for the first cell wall material arrangement in which subcriticality has been ensured up to that point and the second cell wall material arrangement in which subcriticality has not been ensured. . When the effective multiplication factor is calculated only for one of the first and second cell wall material arrangements, the second cell is provided under the condition that subcriticality is ensured. There is room to reduce wall materials.

  If the effective multiplication factor has already been calculated for both the first and second cell wall material arrangements, the second basket in the first cell wall material arrangement and the second cell wall material arrangement When an integer exists between the number of cell walls using the material 92 (an integer of 0 or more), there is room for reducing the second cell wall material under the condition that subcriticality is ensured.

  If there is room to reduce the second cell wall material under the condition that subcriticality is ensured, of the cell walls that are supposed to use the first basket material 91 in the cell wall material arrangement At least one is changed to using the second basket material 92 (step S6), and the process returns to step S3 and step S4 to calculate the effective multiplication factor again.

  If there is no room to reduce the second cell wall material under the condition that subcriticality is ensured, the subcriticality of the cell wall material arrangements for which the effective multiplication factor has been obtained so far is The arrangement in which the second basket material 92 is secured and has the smallest number is the final cell wall material arrangement (step S7), and the design is completed.

  For example, the cell wall material located on the outer periphery is sequentially changed to the first basket material 91 with mirror symmetry, and the cell wall material arrangement is changed to FIGS. 7, 8, 9,. Cell walls using the basket material 91 are increased. In this way, the cell wall material arrangement is changed until both the normal multiplication factor and the effective multiplication factor during the fall accident are smaller than the critical limit value, and the number of cell walls using the second basket material 92 cannot be reduced. Thus, the final cell wall material arrangement is obtained.

  Further, the cell wall material using the first basket material 91 is increased while changing the second basket material 92 from the cell wall material close to the center to the first basket material 91, and the final cell wall material is obtained. An arrangement may be sought.

  In this way, when the final cell wall material arrangement is obtained while increasing the cell walls using the first basket material 91, either the normal multiplication factor or the effective multiplication factor at the time of the drop accident is determined in step S5. When becomes smaller than the critical limit value, the cell walls using the second basket material 92 cannot be reduced. That is, the cell wall material arrangement at that time becomes the final cell wall material arrangement.

  Assume that the initial state of the cell wall material arrangement set in step S2 is that all cell wall materials are the first basket materials 91, and the cell wall material arrangements are as shown in FIGS. 23, 22, 21,. While changing the basket material 91 from the center to the second basket material 92 in order of mirror symmetry, the number of cell walls using the first basket material 91 may be reduced to obtain the final cell wall material arrangement. .

  As described above, in the case where the final cell wall material arrangement is obtained while reducing the cell walls using the first basket material 91, either the normal multiplication factor or the effective multiplication factor at the time of the fall accident is determined in step S5. When becomes larger than the critical limit value, the cell walls using the second basket material 92 cannot be reduced. That is, the final cell wall material arrangement is the arrangement before changing in step S6 to the cell wall material arrangement in which either the normal multiplication factor or the effective multiplication factor at the time of the drop accident is larger than the critical limit value.

  Further, the design of the basket 20 including the steps S1 to S6 can be executed by a computer.

  In step S1, a computer creates a model bundle for subcriticality evaluation. In this case, the design of the fuel assembly accommodated in the fuel container and the combustion history may be input to cause the computer to create a model bundle, or a model bundle separately created by the designer may be input to the computer.

  In step S2, the computer is caused to set an initial state of cell wall material arrangement. For example, a simple initial state in which all the cell wall materials are the first basket material 91 or the second basket material 92 is set. Further, the designer may input the initial state into the computer.

  Since the cell wall material arrangement is finite, the computer repeats steps S3 to S6 until there is no room to reduce the second cell wall material under the condition that subcriticality is ensured in a finite time. By calculating the effective multiplication factor while changing the arrangement, the final cell wall material arrangement can be obtained.

  The basket 20 is manufactured by manufacturing and combining the first and second grid plates 21 and 22 so that the cell wall material arrangement obtained finally is obtained. The basket 20 is attached to the fuel container to manufacture the fuel container.

  In this way, a basket material containing an expensive neutron absorber is secured while ensuring critical safety by making a part of the basket material a material such as inexpensive stainless steel or aluminum alloy that does not contain a neutron absorber. The amount of use can be reduced. This improves the economics of the fuel container basket.

[Second Embodiment]
25 to 33 are top views schematically showing examples of the cell wall material arrangement in the second exemplary embodiment of the present invention.

  A part of the first grid plate 21 of the basket 20 of the present embodiment is entirely formed of only the first basket material 91 or the second basket material 92 and is axisymmetric. Under such restrictions on the cell wall material arrangement, the final cell wall material arrangement is calculated sequentially from FIG. 25 to FIG. 33 or from FIG. 33 to FIG. 25 in the same manner as in the first embodiment. The basket 20 is manufactured.

  Such a first grid plate 21 does not need to be joined by welding or the like between the portion formed of the first basket material 91 and the portion formed of the second basket material 92. Such costs can be reduced. Therefore, the economical efficiency of the basket 20 is improved.

  Moreover, when the basket 20 is designed under such restrictions on the cell wall material arrangement, the time required to find an appropriate arrangement among the many cell wall material arrangements is shortened.

[Third Embodiment]
34 to 37 are top views schematically showing examples of the cell wall material arrangement in the third exemplary embodiment of the present invention.

  The basket 20 of the present embodiment has, in part, four cells 23 that are surrounded by a second basket material 92 and partitioned inside by a first basket material 91 and arranged in a square. It is formed with mirror symmetry. The fuel assemblies 7 housed in such four cells 23 face each other with the second basket material 92 sandwiched between two fuel assemblies 7 among the four adjacent fuel assemblies 7. For this reason, the effective multiplication factor at the normal time and at the time of a fall accident can be effectively reduced.

  Under such restrictions on cell wall material arrangement, the final cell wall material arrangement is calculated by sequentially calculating from FIG. 34 to FIG. 37 or from FIG. 37 to FIG. 34 in the same manner as in the first embodiment. In other words, when the basket 20 is designed, the time required to find an appropriate arrangement among many cell wall material arrangements is shortened.

[Fourth Embodiment]
FIG. 38 is a top view of a part of the basket of the fuel container according to the fourth embodiment of the present invention.

  In the fuel container basket of the present embodiment, the cells 23 are formed by the rectangular tube bodies 41 and 42 and the flat plates 43 and 44. The rectangular cylinders 41 and 42 are connected via the flat bar 11, and the pitch of the cells 23 can be changed by changing the thickness of the flat bar 11.

  The rectangular cylinders 41 and 42 are formed of a first rectangular cylinder 41 formed of the first basket material and a second basket material having a macroscopic neutron cross-sectional area larger than that of the first basket material. In addition, a second rectangular tube 42 is included. The flat plates 43 and 44 include a first flat plate 43 formed of a first basket material and a second flat plate 44 formed of a second basket material.

  The arrangement of the rectangular cylinders 41 and 42 and the flat plates 43 and 44 in such a fuel container basket is determined by the same method as in the first embodiment.

  For example, the cell wall material arrangement of the basket formed of the second rectangular tube body 42 and the second flat plate 44 formed of the second basket material is first set as the initial material arrangement. Thereafter, while increasing the number of places where the first rectangular tube body 41 and the first flat plate 43 formed of the first basket material are used, both of the normal multiplication and the effective multiplication factor during the fall accident are critically limited. The change of the cell wall material arrangement is repeated until it is smaller than the value and the portion using the second basket material cannot be reduced.

  In this way, a final cell wall material arrangement is obtained, and a basket of fuel containers is manufactured by combining the square cylinders 41 and 42 and the flat plates 43 and 44 so as to obtain the cell wall material arrangement.

[Fifth Embodiment]
FIG. 39 is a perspective view showing a method of assembling the basket of the fuel container according to the fifth embodiment of the present invention. FIG. 40 is a side view of the fuel container basket of the present embodiment.

  The fuel container basket 20 according to the present embodiment includes square tubes 2 arranged in a lattice pattern and spacers 3 disposed at a plurality of positions in the axial direction of the square tubes 2. The rectangular tube body 2 is inserted into the insertion hole 31 of the spacer 3.

  41 and 42 are top views schematically showing examples of cell wall material arrangement of the basket 20 in the present embodiment.

  Also in the present embodiment, the cell wall material arrangement of the basket 20 is determined by the same method as in the first embodiment. For example, from the initial material arrangement in which the cell walls surrounding all the rectangular tube bodies 2 are all the second basket material 92 indicated by the solid line in the drawing, the neutron absorption cross-sectional area smaller than that of the second basket material 92 is indicated by the broken line in the drawing. The final cell wall material arrangement is obtained while reducing the portion where the second basket material 92 is used instead of the first basket material 91. The material arrangement of the fuel container basket is such that both the normal multiplication factor and the effective multiplication factor during a fall accident are smaller than the critical limit value, and the cell wall using the second basket material 92 cannot be reduced. is there. The cell wall material arrangement shown in FIGS. 41 and 42 is an example obtained by such a method.

  The above description is merely an example, and the present invention is not limited to the above-described embodiments, and can be implemented in various forms. Moreover, it can also implement combining the characteristic of each embodiment.

It is a flowchart which shows the design method of the basket of the fuel container of 1st Embodiment which concerns on this invention. It is a top view of the basket of the fuel container of the first embodiment according to the present invention. It is a side view of the 1st lattice board of a 1st embodiment concerning the present invention. It is a side view of the 1st lattice board of a 1st embodiment concerning the present invention. It is a side view showing an example of the 2nd lattice board of a 1st embodiment concerning the present invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 1st Embodiment based on this invention. It is a graph which shows the example of the change accompanying the combustion of the reactivity of the fuel assembly containing a combustible poison. It is a top view which shows typically an example of cell wall material arrangement | positioning in 2nd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 2nd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 2nd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 2nd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 2nd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 2nd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 2nd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 2nd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 2nd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 3rd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 3rd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 3rd Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 3rd Embodiment which concerns on this invention. It is a top view of a part of a basket of a fuel container according to a fourth embodiment of the present invention. It is a perspective view which shows the assembly method of the basket of the fuel container of 5th Embodiment which concerns on this invention. It is a side view of the basket of the fuel container of 5th Embodiment which concerns on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 5th Embodiment based on this invention. It is a top view which shows typically an example of cell wall material arrangement | positioning in 5th Embodiment based on this invention.

Explanation of symbols

2 ... Square tube body, 3 ... Spacer, 11 ... Flat bar, 20 ... Basket, 21 ... First lattice plate, 22 ... Second lattice plate, 23 ... Cell, 24 ... Projection, 25 ... Recessed portion, 26 ... Slit, 31 ... Insertion hole, 41 ... first square tube, 42 ... second square tube, 43 ... first flat plate, 44 ... second flat plate, 91 ... first basket material, 92 ... Second basket material

Claims (13)

In a design method of a basket that is housed inside a fuel container, and includes a plurality of rectangular tube-shaped cells that each contain a fuel assembly having a burnup greater than or equal to the limit burnup,
Obtaining a first model bundle that gives a reactivity greater than a maximum value at a burnup less than the limit burnup of the reactivity considering the degradation of the fuel assembly due to a flammable poison;
Obtaining a second model bundle that provides a reactivity greater than the reactivity at the limited burnup of the fuel assembly;
The cell wall material arrangement is set such that the material of the four cell walls surrounding the cell is one of the first basket material and the second basket material having a larger macroscopic neutron cross-sectional area than the first basket material. An initial material placement setting process,
A normal effective multiplication factor of the fuel container calculated based on the cell wall material arrangement as the first model bundle is accommodated in the cell, and the second model bundle is accommodated in the cell. Any of the effective multiplication factors at the time of the fuel container dropping accident calculated based on the cell wall material arrangement assuming that the interval between the fuel rods included in the second model bundle is expanded by dropping is a predetermined critical limit. New cell wall material arrangement by changing at least a part of the cell wall material arrangement until there is no room to reduce the amount of the second basket material contained in the cell wall material under the restriction that the cell wall material is smaller than the value. And the cell wall material arrangement changing step that repeats,
Among the cell wall material arrangements in which the effective multiplication factor is calculated in the cell wall material arrangement changing step, the effective multiplication factors at the normal time and the fall accident are both less than the critical limit value, and the second A step of setting the cell wall material arrangement with the least cell walls using a basket material as a final cell wall material arrangement;
A basket design method characterized by comprising:   In the initial material arrangement setting step, all cell wall materials are used as the second basket material, and in the new cell wall material arrangement, a part of the second basket material is used as the second basket material. The basket design method according to claim 1, wherein the basket material is changed to one basket material.   In the initial material arrangement step, all cell wall materials are used as the first basket material, and in the new cell wall material arrangement, a part of the portion used as the first basket material is the second basket material. The basket design method according to claim 1, wherein the basket material is changed to a basket material. In a method for manufacturing a basket that is housed inside a fuel container, comprising a plurality of rectangular tube-like cells each containing a fuel assembly having a burnup greater than or equal to the limit burnup,
Obtaining a first model bundle that gives a reactivity greater than a maximum value at a burnup less than the limit burnup of the reactivity considering the degradation of the fuel assembly due to a flammable poison;
Obtaining a second model bundle that provides a reactivity greater than the reactivity at the limited burnup of the fuel assembly;
The cell wall material arrangement is set such that the material of the four cell walls surrounding the cell is one of the first basket material and the second basket material having a larger macroscopic neutron cross-sectional area than the first basket material. An initial material placement setting process,
A normal effective multiplication factor of the fuel container calculated based on the cell wall material arrangement as the first model bundle is accommodated in the cell, and the second model bundle is accommodated in the cell. Any of the effective multiplication factors at the time of the fuel container dropping accident calculated based on the cell wall material arrangement assuming that the interval between the fuel rods included in the second model bundle is expanded by dropping is a predetermined critical limit. New cell wall material arrangement by changing at least a part of the cell wall material arrangement until there is no room to reduce the amount of the second basket material contained in the cell wall material under the restriction that the cell wall material is smaller than the value. And the cell wall material arrangement changing step that repeats,
Among the cell wall material arrangements in which the effective multiplication factor is calculated in the cell wall material arrangement changing step, the effective multiplication factors at the normal time and the fall accident are both less than the critical limit value, and the second A step of setting the cell wall material arrangement with the least cell walls using a basket material as a final cell wall material arrangement;
A cell forming step of disposing the first and second basket materials such that the material of the cell wall is the final cell wall material arrangement;
A method for manufacturing a basket, comprising: In each of the baskets that are contained in the fuel container and contain fuel assemblies having a burnup level equal to or higher than the limit burnup rate,
Surrounded by four cell walls formed of any one of the first basket material and the second basket material having a macroscopic neutron cross section larger than that of the first basket material, each of the fuel assemblies Having a plurality of prismatic cells in which the body is housed,
Each material of the cell wall has a first model bundle that provides a reactivity greater than a maximum value at a burnup less than the limit burnup of the reactivity considering the degradation of the fuel assembly by a flammable poison. A second model bundle that provides an effective multiplication factor in a normal time calculated as being accommodated in each cell and a reactivity higher than the reactivity at the limited burnup of the fuel assembly is accommodated in each cell. The effective multiplication factor at the time of the fall accident calculated as the interval between the fuel rods included in the second model bundle is expanded due to the drop is smaller than a predetermined critical limit value, and The basket is determined so that there is no room for reducing the amount of the second basket material.   The first grid plate having a plurality of cells and having protrusions arranged in parallel to each other, and slits fitted to the protrusions are formed and are parallel to each other. The basket according to claim 5, wherein the basket is formed by a second lattice plate arranged perpendicularly to each other.   6. The basket according to claim 5, wherein a plurality of the cells are formed by independent rectangular tubes.   The basket according to any one of claims 5 to 7, wherein the first basket material is stainless steel.   The basket according to any one of claims 5 to 7, wherein the first basket material is an aluminum alloy.   The basket according to any one of claims 5 to 9, wherein the neutron absorber is boron.   The basket according to any one of claims 5 to 10, wherein the neutron absorber is a rare earth.   12. The neutron absorbing material according to claim 5, wherein the neutron absorption cross-sectional area of the nuclide having a larger neutron absorption cross section is larger than that of natural isotopes, as compared with that of a natural material. basket. In a basket design program for designing a basket accommodated in a fuel container, in which a plurality of rectangular cylindrical cells each containing a fuel assembly having a burnup greater than or equal to the limit burnup are arranged,
A function of storing a first model bundle that gives a reactivity greater than a maximum value at a burnup less than the limit burnup of the reactivity considering a decrease due to a flammable poison of the fuel assembly;
A function of storing a second model bundle that provides a degree of reactivity greater than the degree of reactivity at the limited burnup of the fuel assembly;
The cell wall material arrangement in which the material of the four cell walls surrounding the cell is one of the first basket material and the second basket material having a macroscopic neutron cross-sectional area larger than that of the first basket material is stored. Function
A normal effective multiplication factor of the fuel container calculated based on the cell wall material arrangement as the first model bundle is accommodated in the cell, and the second model bundle is accommodated in the cell. Any of the effective multiplication factors at the time of the fuel container dropping accident calculated based on the cell wall material arrangement assuming that the interval between the fuel rods included in the second model bundle is expanded by dropping is a predetermined critical limit. New cell wall material arrangement by changing at least a part of the cell wall material arrangement until there is no room to reduce the amount of the second basket material contained in the cell wall material under the restriction that the cell wall material is smaller than the value. And the function to repeat
Among the cell wall material arrangements for which the effective multiplication factor has been calculated, the effective multiplication factor at the normal time and the fall accident is less than the critical limit value, and the cell wall using the second basket material is A function of outputting the least cell wall material arrangement as the final cell wall material arrangement;
A basket design program for realizing this.

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