JP2012112841A - Fuel assembly and fuel rod - Google Patents

Abstract

A fuel assembly structure having a high neutron shielding and heat removal effect when storing a cask is provided so that MOX spent fuel can be transported and stored in a conventionally used cask.
A fuel assembly includes a plurality of MOX fuel rods and an upper tie plate and a lower tie plate that bundle them. Each MOX fuel rod is enclosed in a cladding tube, a lower end plug that seals the lower end of the cladding tube, an upper end plug that seals the upper end of the cladding tube, and a lower end plug and an upper end plug. And a plurality of fuel pellets arranged in the vertical direction and made up of a plurality of uranium fuel pellets and a plurality of MOX fuel pellets. A uranium fuel pellet enclosure 71 is formed at least at the lower end and in the vicinity of the lower end of the MOX fuel rod, and an MOX fuel pellet enclosure 72 is formed at other positions.
[Selection] Figure 1

Description

  The present invention relates to a fuel assembly and a fuel rod using MOX fuel.

  The fuel used in the power generation light water reactor for a certain period is taken out from the core and stored in a spent fuel pool or the like for a certain period, and then transported to a reprocessing plant. The plutonium recovered by reprocessing is taken out as a recycled resource and reused as MOX fuel (mixed fuel of plutonium dioxide and uranium dioxide). Currently, the amount of spent fuel that is generated is increasing along with the demand for power generation, so even if the reprocessing plant is in operation, the spent fuel that is generated in the country will exceed the processing capacity of the reprocessing plant and will be reprocessed. Need to be properly managed and stored. The required storage capacity is expected to be 6,000 tU scale in 2010 and 15,000 tU scale in 2020.

  There are dry storage systems such as dry cask storage and concrete cask storage and water pool wet storage systems as methods for managing and storing spent fuel inside and outside the nuclear power plant site. Even when long-term stable storage is considered, dry storage is attracting attention. Among the dry storage systems, the cask storage system currently in practical use in Japan is a method of storing spent fuel in a dry cask and storing it in a fuel assembly state. In the cask storage facility, it may be possible to store spent fuel in various states depending on the fuel specifications at the power plant and the storage period in the site spent fuel storage pool. There was a need to do.

  By the way, in the MOX spent fuel obtained as a result of burning MOX fuel in a light water reactor, the amount of actinide nuclides increases compared to uranium fuel of the same burnup. Therefore, according to Non-Patent Document 1, in one example of boiling water reactor (BWR) fuel, MOX spent fuel has a neutron emission rate due to Cm-242, Cm-244, etc., compared to uranium spent fuel. The calorific value tends to increase by 40% due to 7 times, Pu-238 and the like. Therefore, in transport and storage of MOX spent fuel, measures against neutron shielding and heat removal are considered issues.

  In order to improve the neutron shielding ability and the heat removal ability, countermeasures have been examined not only for MOX spent fuel but also for the past. For example, from the viewpoint of ensuring the shielding performance, as shown in Patent Document 1, a shielding block is inserted between the cask body and the outside of the basket cell, and as shown in Patent Document 2, the weight of the cask is reduced and heat transfer efficiency is improved. For this purpose, a method of inserting an aluminum alloy dummy pipe between the trunk body and the basket has been devised.

  Regarding the method for storing the MOX spent fuel assembly in the cask, Patent Document 3 discloses an arrangement example of uranium and the MOX spent fuel assembly for reducing the neutron multiplication factor from the viewpoint of critical safety in the transport / storage cask. It is shown.

  Patent Document 4 and Patent Document 5 show a method for efficiently transporting and storing a basket of a transport storage cask by increasing the shielding effect by arranging a shield having substantially the same weight as the aggregate. ing.

Japanese Utility Model Publication No. 61-140999 JP 2001-74884 A Japanese Patent Laid-Open No. 7-260991 Japanese Patent Laid-Open No. 2-176498 JP-A-2005-9960

"Latest Nuclear Fuel Engineering-Current Situation and Prospects for Advanced Technology", "Advanced Fuel Technology" Research Special Committee, Japan Atomic Energy Society, June 2001

  As can be seen in the conventional examples of Patent Documents 1 and 2, for different fuel specifications, the design of the cask is changed according to the specifications and storage conditions. The specific arrangement method of spent fuel is not shown.

  Patent Document 3 shows an example of the arrangement of spent MOX fuel for criticality safety measures, but the outer surface dose rate in the radial direction of the cask is reduced because the spent MOX fuel is also placed around the basket. In addition, because some of the MOX spent fuel is arranged adjacent to each other, the dose rate locally increases on the outer surface of the lid and bottom of the cask, and from the viewpoint of neutron shielding, It does not reduce the neutron dose.

  In Patent Documents 4 and 5, the shielding effect is increased when a stainless steel square tubular shield is placed around the basket in contact with the inner wall of the cask. On the other hand, the square tube itself is neutron reflective. Since it also behaves as a body, the neutron multiplication in the central region where the fuel assembly is stored increases and the number of multiplied neutrons also increases. As a result, the intensity of the neutron source increases and the increase in shielding effect may be offset by that amount.

  In the future, considering the use of MOX fuel, suppressing the increase in manufacturing costs by operating the conventional spent fuel transportation / storage cask without changing the design of the cask will be one of the economic benefits. .

  An object of the present invention is to make the structure of the MOX fuel itself a structure having a high neutron shielding and heat removal effect when storing the cask so that the MOX spent fuel can be transported and stored in a conventionally used cask.

  To achieve the above object, each of the fuel assemblies according to the present invention comprises a cladding tube, a lower end plug that seals the lower end of the cladding tube, and an upper end plug that seals the upper end of the cladding tube, A plurality of fuel pellets, each of which is enclosed in the cladding tube between the lower end plug and the upper end plug and is arranged in the vertical direction and includes a plurality of uranium fuel pellets and a plurality of MOX fuel pellets, A plurality of MOX fuel rods arranged in a grid in parallel; an upper tie plate for bundling upper portions of the plurality of MOX fuel rods; a lower tie plate for bundling lower portions of the plurality of MOX fuel rods; and the upper tie plate; A fuel assembly comprising: a plurality of fuel spacers that are spaced apart from each other in the vertical direction between the lower tie plates and support the plurality of MOX fuel rods in a horizontal direction; Charges and uranium fuel pellets sealed in the vicinity of at least the lower end and the lower end of the rod, the uranium fuel pellets are MOX fuel pellets are enclosed in a position that is not sealed, characterized by.

  The fuel rod according to the present invention includes a cladding tube, a lower end plug that seals a lower end of the cladding tube, an upper end plug that seals an upper end of the cladding tube, and the lower end plug and the upper end plug. In a MOX fuel rod having a plurality of fuel pellets sealed in the cladding tube and arranged in the vertical direction, uranium fuel pellets are sealed at the lower end and near the lower end, and the uranium fuel pellets are not sealed MOX fuel pellets are enclosed at the position.

  According to the present invention, the structure of the MOX fuel itself having a high neutron shielding effect and heat removal effect at the time of storing the cask can be used so that the MOX spent fuel can be transported and stored in a conventionally used cask.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional elevation showing an axial arrangement of MOX fuel and uranium fuel inside each fuel rod in an embodiment of a fuel assembly according to the present invention, wherein (a) is a long MOX fuel rod; b) shows a uranium fuel rod with a long gadolinia, and (c) shows a short MOX fuel rod. 1 is an elevational sectional view of an embodiment of a fuel assembly according to the present invention. FIG. 3 is a cross-sectional plan view taken along the line III-III in FIG. 2. FIG. 4 is a horizontal cross-sectional view taken along line IV-IV in FIG. 2. FIG. 3 is a partially cutaway elevation view showing one long fuel rod in the fuel assembly of FIG. 2 taken out. FIG. 5 is a plan sectional view showing the arrangement of various fuel rods and the like in the fuel assembly of FIG. It is a partial notch perspective view which shows an example of the cask which accommodates the multiple body of the used fuel assembly which concerns on one Embodiment of this invention. FIG. 8 is a cross-sectional view showing the trunk main body of the cask and its interior when the spent fuel assembly is stored in the cask of FIG. 7.

  Embodiments of a fuel assembly according to the present invention will be described below with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

  FIG. 1 is a schematic vertical sectional view showing the axial arrangement of MOX fuel and uranium fuel inside each fuel rod in an embodiment of a fuel assembly according to the present invention, wherein (a) is a long MOX fuel. Fuel rod, (b) is a uranium fuel rod with long gadolinia, and (c) is a short MOX fuel rod. FIG. 2 is an elevational sectional view of one embodiment of the fuel assembly according to the present invention. 3 is a cross-sectional view taken along the line III-III in FIG. 4 is a cross-sectional view taken along the line IV-IV in FIG. FIG. 5 is a partially cutaway elevation view showing one long fuel rod in the fuel assembly of FIG. 6 is a plan sectional view showing the arrangement of various fuel rods and the like in the fuel assembly of FIG.

  The fuel assembly 50 according to this embodiment is a fuel assembly for a boiling water reactor, and the external configuration shown in FIGS. 2 to 5 is common to a conventional typical boiling water reactor fuel assembly. It is. That is, a large number of fuel rods 51, 52, 53 are arranged in a square lattice pattern, the upper ends of which are bound by the upper tie plate 54, and the lower ends thereof are bound by the lower tie plate 55. The fuel rods 51, 52, 53 have a plurality of long fuel rods 51, 52 extending from the lower tie plate 55 to the upper tie plate 54, and a plurality of fuel rods 51, 52 extending from the lower tie plate 55 to below the upper tie plate 54. It consists of a short fuel rod 53. As will be described later, the long fuel rods 51 and 52 include a plurality of long MOX fuel rods 51 and a plurality of long gadolinia-containing uranium fuel rods 52.

  Between the upper tie plate 54 and the lower tie plate 55, a plurality of fuel spacers 56 are arranged at intervals in the height direction, whereby the horizontal intervals between the fuel rods 51, 52, 53 are maintained. Yes. Near the center in the horizontal direction, for example, two water rods 57 are arranged extending in the vertical direction. A square tubular channel box 58 is attached to the outside of the fuel assembly.

  The long fuel rods 51 and 52 and the water rod 57 are pressed toward the lower tie plate 55 by an external spring 80 disposed in contact with the lower surface of the upper tie plate 54.

  As shown in FIG. 5, each of the long fuel rods 51 and 52 has a large number of columnar fuel pellets 61 stacked in a vertical direction and enclosed in a metal cylindrical cladding tube 60. The lower end of the cladding tube 60 is sealed by a lower end plug 62, and the upper end of the cladding tube 60 is sealed by an upper end plug 63. A plenum 64, which is a gas space, is formed between the upper end of the uppermost fuel pellet 61 and the lower end of the upper end plug 63, and an internal spring 65 is disposed in the plenum 64 so that the fuel pellet 61 is pressed downward. ing. The short fuel rod 53 is shorter than the long fuel rods 51 and 52, but the structure is substantially the same as the long fuel rods 51 and 52.

  The fuel pellet 61 is obtained by baking and solidifying either uranium dioxide, a mixture of uranium dioxide and gadolinia, or MOX (a mixture of uranium dioxide and plutonium dioxide).

  The long MOX fuel rod 51, the long gadolinia-containing uranium fuel rod 52, and the short MOX fuel rod 53 are arranged, for example, as shown in FIG. In FIG. 6, the long gadolinia-filled uranium fuel rod 52 is shown with a symbol G in the circle, and the short MOX fuel rod 53 is shown with a symbol S in the circle. Further, the long MOX fuel rod 51 is indicated by numbers 1 to 5 in a circle, and the numbers 1 to 5 indicate the different plutonium contents.

  As shown in FIG. 1 (a), in the long MOX fuel rod 51, the uranium fuel pellet enclosing portion 71 is formed over the length of at least 1/24 of the total length at the upper end portion and the lower end portion of the fuel effective portion. The MOX fuel pellet enclosing part 72 is formed in other parts. Further, as shown in FIG. 1C, in the short MOX fuel rod 53, a uranium fuel pellet enclosing portion 71 having a length of at least 1/24 of the total length is formed at the lower end portion of the fuel effective portion, A MOX fuel pellet enclosure 72 is formed in the portion. The uranium used here is natural uranium, degraded uranium, or low enriched uranium. As shown in FIG. 1B, the entire gadolinia-filled uranium fuel rod 52 is a mixture of uranium dioxide and gadolinia in the entire effective fuel portion.

  When this fuel assembly 50 is burned in a light water reactor and then stored in a cask for storage or transportation, the distance between the MOX spent nuclear fuel with a high neutron generation rate and the wall of the cask bottom and lid increases. Moreover, since the emitted neutrons are also shielded by uranium at the upper and lower ends, the neutron dose radiated from the outer surface of the cask bottom and lid is higher than when uranium is not used at the upper and lower ends of the MOX fuel rods. Will be reduced.

  Next, with reference to FIG. 7 and FIG. 8, an example in which a plurality of the above-described spent fuel assemblies are accommodated in a cask for storage and transportation will be described.

  FIG. 7 is a partially cutaway perspective view showing an example of a cask that houses a plurality of spent fuel assemblies according to an embodiment of the present invention. FIG. 8 is a cross-sectional view showing the trunk main body of the cask and its interior when the spent fuel assembly is stored in the cask of FIG. However, illustration of the basket 30 is omitted in FIG.

  The cask 100 of this embodiment contains, for example, a MOX spent fuel assembly 40 obtained after using the above-described fuel assembly 50 in a boiling water reactor, and a plurality of uranium spent fuel assemblies 41. Used for transportation and storage. The uranium spent fuel assembly 41 is different from the MOX spent fuel assembly 40 in that uranium oxide fuel is used without using the MOX fuel, but the appearance configuration is different from that of the MOX spent fuel assembly 40. It is the same.

  The cask 100 has a cask body 16 in which a cavity 22 is formed. The cask main body 16 includes a trunk main body 21 having a cylindrical outer periphery. A basket 30 is accommodated in the cavity 22. The basket 30 divides the cavity 22 into a square lattice to form a plurality of storage spaces (cells) 31, and the fuel assemblies 40, 41 are inserted into the cells 31 by inserting one fuel assembly 40, 41. Are arranged in a square lattice and spaced apart from each other. The cavity 22 is machined according to the outer peripheral shape of the basket 30.

  A disk-shaped bottom plate 24 is welded and sealed to the bottom of the trunk body 21, and a disk-shaped primary lid 10 is attached to the upper opening of the trunk body 21. The trunk body 21, the bottom plate 24, and the primary lid 10 have gamma ray shielding ability, and are made of, for example, carbon steel or stainless steel. Further, in order to ensure the sealing performance as a pressure vessel, a metal gasket (not shown) is disposed between the primary lid 10 and the trunk body 21 and is attached to and detached from the trunk body 21 with bolts (not shown). It is attached as possible.

  The outer side of the trunk body 21 is covered with an outer cylinder 25 through a gap, and a plurality of internal fins (not shown) made of a material having high thermal conductivity (for example, copper) are interposed between the trunk body 21 and the outer cylinder 25. It is welded radially. A resin 26 is filled in a gap between the trunk body 21, the outer cylinder 25, and the internal fins. The resin 26 is a polymer material containing a large amount of hydrogen and has a neutron shielding ability. The resin 26 is injected in a state of being melted at a high temperature into a gap sandwiched between the trunk main body 21, the outer cylinder 25, and the internal fins, and then cooled and solidified.

  A disc-shaped secondary lid 11 made of carbon steel or stainless steel is disposed above the primary lid 10, and a metal gasket for ensuring hermeticity between the secondary lid 11 and the trunk body 21. (Not shown) is disposed, and is detachably attached to the trunk body 21 by bolts (not shown). A resin 12 is sealed on the upper surface of the secondary lid 11 as a neutron shield.

  A lid 19 is formed by the primary lid 10 and the secondary lid 11, and an auxiliary shield 15 enclosing the resin 14 is provided around the lid 19. A trunnion 17 is provided so as to protrude laterally from the upper part of the trunk body 21, and the entire cask 100 can be suspended by the trunnion 17.

  In this embodiment, the fuel assembly accommodated in the cask 100 includes the MOX spent fuel assembly 40 and the uranium spent fuel assembly 41. As shown in FIG. 8, one uranium spent fuel assembly 41 is provided in each outermost storage space facing the outer periphery of the cavity 22 among the plurality of storage spaces 31 formed in the cavity 22 by the basket 30. Stored. One of the MOX spent fuel assembly 40 and the uranium spent fuel assembly 41 is stored in each of the storage spaces 31 other than the outermost peripheral storage space. The MOX spent fuel assemblies 40 are not adjacent to each other and are uniformly distributed.

  According to this embodiment, the distance between the MOX spent fuel assembly 40 having a high neutron generation rate and the inner wall of the trunk body 21 is large, and the neutron beams emitted from these are the uranium spent fuel assemblies on the outer periphery. 41 is also shielded. Therefore, the neutron dose emitted from the surface of the outer cylinder 25 is significantly attenuated as compared with the case where the MOX spent fuel assemblies 40 are randomly arranged in the basket 30.

  Further, in the MOX spent fuel assembly 40, as shown in FIG. 1, the distance between the MOX spent nuclear fuel having a high neutron generation rate and the bottom wall of the cask and the wall of the lid portion increases, and the neutron beam emitted from them. Is also shielded by uranium at the upper and lower ends, so the neutron dose emitted from the outer surface of the cask bottom and lid is reduced compared to the case where uranium is not used at the upper and lower ends of the MOX fuel rod.

[Other Embodiments]
The embodiments described above are merely examples, and the present invention is not limited to these. For example, in the above embodiment, the BWR fuel assembly has been described as an example, but the present invention can also be applied to a pressurized water reactor (PWR) fuel assembly.

DESCRIPTION OF SYMBOLS 10 ... Primary lid 11 ... Secondary lid 12, 14 ... Resin 15 ... Auxiliary shield 16 ... Cask main body 17 ... Trunnion 19 ... Lid part 21 ... Trunk main body,
22 ... Cavity 24 ... Bottom plate 25 ... Outer cylinder 26 ... Resin 30 ... Basket 31 ... Storage space (cell)
40 ... MOX spent fuel assembly 41 ... Uranium spent fuel assembly 50 ... fuel assembly 51 ... long MOX fuel rod (long fuel rod)
52 ... Long gadolinia containing uranium fuel rod (long fuel rod)
53 ... Short MOX fuel rod (short fuel rod)
54 ... upper tie plate 55 ... lower tie plate 56 ... fuel spacer 57 ... water rod 58 ... channel box 60 ... cladding tube 61 ... fuel pellet 62 ... lower end plug 63 ... upper end plug 64 ... plenum 65 ... internal spring 71 ... uranium Fuel pellet enclosure 72 ... MOX fuel pellet enclosure 80 ... External spring 100 ... Cask

Claims (8)

Each is enclosed in the cladding tube between the cladding tube, the lower end plug that seals the lower end of the cladding tube, the upper end plug that seals the upper end of the cladding tube, and the lower end plug and the upper end plug A plurality of fuel pellets arranged in a vertical direction and having a plurality of fuel pellets each including a plurality of uranium fuel pellets and a plurality of MOX fuel pellets,
An upper tie plate that bundles the upper portions of the plurality of MOX fuel rods;
A lower tie plate that bundles the lower portions of the plurality of MOX fuel rods;
A plurality of fuel spacers arranged in the vertical direction between the upper tie plate and the lower tie plate so as to support the plurality of MOX fuel rods in a horizontal direction;
In a fuel assembly having
Uranium fuel pellets are sealed at least at the lower end and in the vicinity of the lower end of the MOX fuel rod, and the MOX fuel pellets are sealed at a position where the uranium fuel pellets are not sealed,
A fuel assembly characterized by The plurality of MOX fuel rods include a plurality of long MOX fuel rods extending between the upper tie plate and the lower tie plate, and at least one extending between a position below the upper tie plate and the lower tie plate. Consisting of a short MOX fuel rod,
In the long MOX fuel rod, uranium fuel pellets are sealed at positions near the lower end and the lower end of the long MOX fuel rod, and near the upper end and the upper end.
In the short MOX fuel rod, uranium fuel pellets are enclosed at the lower end and the vicinity of the lower end of the short MOX fuel rod,
2. The fuel assembly according to claim 1, wherein:   3. The fuel assembly according to claim 1, wherein uranium fuel pellets are sealed from a lower end of the MOX fuel rod at a position that is 1/24 or more of an effective length of the MOX fuel rod.   A cladding tube, a lower end plug that seals the lower end of the cladding tube, an upper end plug that seals the upper end of the cladding tube, and the upper and lower ends sealed between the lower end plug and the upper end plug. A plurality of gadolinia-filled uranium fuel pellets arranged in a direction, and having at least one gadolinia-filled uranium fuel rod bundled together with the MOX fuel rods by the upper tie plate and the lower tie plate. The fuel assembly according to any one of claims 1 to 3, characterized in that:   The fuel assembly according to any one of claims 1 to 4, further comprising at least one water rod bundled together with the MOX fuel rod by the upper tie plate and the lower tie plate. A cladding tube;
A lower end plug for sealing the lower end of the cladding tube;
An upper end plug for sealing the upper end of the cladding tube;
A plurality of fuel pellets enclosed in the cladding tube between the lower end plug and the upper end plug and arranged in the vertical direction;
In a fuel rod having
Uranium fuel pellets are enclosed at the lower end and near the lower end, and MOX fuel pellets are enclosed at positions where uranium fuel pellets are not enclosed,
A fuel rod characterized by   The fuel rod according to claim 6, wherein uranium fuel pellets are enclosed in the lower end and the vicinity of the lower end, and in the vicinity of the upper end and the upper end.   The fuel rod according to claim 6 or 7, wherein uranium fuel pellets are sealed at a position of 1/24 or more of the effective fuel length from the lower end.

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