JPH02222866A - Spent fuel basket - Google Patents

Abstract

PURPOSE:To enable disposition of a neutron absorber by forming a vacancy inside a basket by disposing accommodation square tubes so that they come into internal contact with a cylindrical main body. CONSTITUTION:A spent fuel assembly is inserted into an accommodation square tube 2 which is accommodated inside a cylindrical main body 1 of a basket. Gaps (a), (b) and (c) formed between the main body 1 and the outermost square tubes 2 are narrowed, a space thus formed is transferred to the inside of the main body 1 of the basket, and neutron absorbers 3 are disposed in internal gaps including the gaps (a), (b) and (c). Each of these absorbers 3 is so constructed that it has a vacancy 4 inside and that water as a neutron moderator is put therein. By transferring the gaps to the inside of the main body 1 and by making these gaps be neutron trap regions, a neutron multiplication effect can be suppressed without changing the dimensions of the main body 1 and the number of spent fuel accommodation bodies, and thus a contribution can be made to improvement in a subcritical degree.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Field of Application) The present invention relates to a spent fuel basket used in a spent fuel cask.

(Prior Art) Spent fuel assemblies (hereinafter referred to as spent fuel) that have been used in a nuclear reactor are stored in spent fuel casks and transported to a reprocessing plant, where they are reprocessed.

Even in countries that do not carry out reprocessing or countries that do reprocess, if reprocessing is delayed, spent fuel must be removed from the pool of a nuclear power plant to a storage site.

The casks used in such cases are called spent fuel transport casks. If reprocessing is not performed, spent fuel may be stored in casks at a power plant pool, transported to an appropriate location, and used for storage as is. Such casks are called spent fuel transport and storage casks.

In these casks, a basket to store the spent fuel is placed in the center, and concentrically around the basket is a gamma ray shielding layer mainly made of lead, a neutron shielding layer made of hydrogen-containing substances, and an outermost layer around the basket. A heat dissipation and shock absorption layer is arranged. When storing spent fuel in casks, the casks are submerged in water and each spent fuel is stored in a basket inside the cask. Fissile nuclides remain in the spent fuel, making it a neutron multiplier system. If a large number of spent fuels are collected, the effective neutron multiplication factor K increases. However, 82. The value rr must be kept below the critical value (1,00) by a sufficient margin. In order to maintain sufficient margin, it was necessary to add sufficient neutron absorbing material to the rectangular tubes in the basket or to leave sufficient space between the rectangular tubes. As a result, there have been problems such as reducing the number of spent fuel containers in the basket, creating technical problems in adding sufficient neutron absorbing material, and complicating the structure.

(Problems to be Solved by the Invention) As mentioned above, spent fuel is stored in a basket placed in water to achieve sufficient subcriticality (l/K8r r-■
), it was necessary to widen the spacing between the spent fuel storage square tubes and to add sufficient neutron absorbing material to the square tubes or to the structural materials around the square tubes. As a result, the number of containers has decreased, the structural materials have become more complex, and the cost of transporting and storing spent fuel has increased.

The present invention has been made in view of these points, and its purpose is to focus on the slight gap between the cylindrical basket and the rectangular spent fuel storage tube, and to adjust the gap between the cylindrical basket and the square storage tube at the outermost periphery. An object of the present invention is to provide a spent fuel basket in which a rectangular storage cylinder is arranged so as to be formed not between the space between the fuel tank and the cylindrical body but inside the space to effectively ensure criticality safety.

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a spent fuel cask for use in a spent fuel cask in which a large number of rectangular storage tubes for storing spent fuel are arranged inside a metal cylindrical body. In the fuel basket, the storage rectangular tubes are arranged so as to be inscribed in the metal cylindrical body to form a void other than the storage rectangular tube inside, and a plurality of neutron absorbing elements are arranged in the void to form a gap. It is characterized by a trap-like neutron absorber.

(Function) As described above, according to the spent fuel basket of the present invention, the diameter of the metal cylindrical basket is hardly increased (the originally formed cylindrical basket and the storage rectangular tube group are The gap formed in a part between the fuel and the outer periphery of the fuel tank is moved to the inside of the basket, and neutrons are captured and absorbed in that part, so it is possible to suppress the neutron multiplication effect caused by the spent fuel inside the basket. It is possible to effectively ensure criticality safety.

(Example) Embodiments of the present invention will be described with reference to the drawings.

FIG. 1(A) is a cross-sectional view of one embodiment of the present invention. As shown in the figure, the basket has three parts inside the cylindrical body 1.
Three square storage tubes 2 are arranged, and a spent fuel assembly (not shown) is inserted into each square storage tube 2. In the conventional basket, as shown in FIG. 4, gaps a-, b-, and c- are formed between the cylindrical main body 1 and the outermost rectangular storage tube 2, as shown in the figure. In the basket, these gaps a
b″C″ have both been moved into the interior of the cylindrical body 1. In these internal gaps a, b, cl::, a columnar neutron absorbing material 3 having a rectangular cross section (not shown) is arranged. As shown in FIG. 1(B), this neutron absorbing material 3 has a void 4 on the inside, which is occupied by water, which is a neutron moderator, when the basket is submerged in water. These rectangular neutron absorbing material square tubes 3 enter from the outside as fast neutrons, are decelerated by the water inside the square tube 3, become thermal neutrons, and are absorbed by the inner surface of the square tube, so that they are absorbed by the fuel on the outer periphery. There isn't. In other words, since neutrons are trapped, they will be simply referred to as traps below. Therefore, the neutron multiplication properties of the basket can be reduced.

In particular, such a large number of traps, that is, rectangular neutron absorbing material square tubes 3, are arranged so as to roughly divide the spent fuel insertion area in the basket into an outer peripheral part and an inner part, and to suppress the neutron multiplication effect most effectively. Therefore, the effective neutron multiplication factor K can be effectively increased with a small gap. 2. can suppress the increase in

The outer diameter of the basket in this embodiment is the same as the outer diameter of the conventional basket shown in FIG. 4, and the neutron multiplication factor can be suppressed more than in the conventional case.

Note that a heat conductor such as aluminum may be placed between the outermost rectangular cylinder 2 and the cylinder 1, but this part is slightly curved in the present invention, which contributes to the weight reduction of the basket. do. Reducing the weight of the basket leads to a reduction in the weight of the cask, which also means that the cranes used to handle it do not have to become extremely large.

FIG. 2 is a cross-sectional view of a second embodiment of the invention. The same parts as in the previous embodiment are given the same reference numerals, and the explanation thereof will be omitted. This example is an example in which the present invention is applied to a basket for 39 bodies, and one part 5 of the neutron absorbing material rectangular tube is arranged so as to be orthogonal to the arrangement shown in FIG.

FIG. 3 is a cross-sectional view of a third embodiment of the present invention;
This is an example in which the present invention is applied to a body basket, and the same parts as those in the first embodiment are given the same reference numerals, and the explanation thereof will be omitted.

This embodiment differs from the first and second embodiments in that a trap 6 is also provided in the center, and a neutron monitor hole 7 is arranged inside the trap 6.

If a neutron detector (a nuclear fission counter is best) is inserted into this monitor hole 7, and spent fuel is sequentially inserted into the rectangular tube on the outer periphery, the neutrons released by the spent fuel will become a neutron source and increase the number of neutrons. Since the phenomenon of doubling can be closely monitored, the degree of subcriticality can be monitored more accurately. The method of monitoring was proposed by the inventor in Japanese Patent Application No. 82-32743.
1 and the neutron continuous multiplication method using spontaneous neutrons published at the 1983 Annual Meeting 7 of the Atomic Energy Society of Japan, patent application No. 6
The distribution shape of spontaneous neutron flux proposed in 3-22705 is K.

, it is possible to effectively use the "spontaneous neutron flux distribution penalty method" based on a novel idea that changes depending on f.

In addition, a heat transfer material portion 8 formed between the cylinder and the outermost rectangular cylinder
Make a hole (not shown) in the hole, insert a neutron detector, and insert californium 2 instead of the neutron detector into the central neutron monitor hole.
It is also possible to arrange a special chamber containing an external neutron source, such as 52 (Cr-252), and apply neutron noise methods (such as the method recently developed by Mlhalczo in the United States).

In addition, in this example, only an example in which all the side lines of all square cylinders are parallel or orthogonal is shown, but if possible, it is possible to give a rotation angle to the side lines of some square cylinders. Regardless of the arrangement, the object of the present invention can be achieved.

[Structure of the Invention] As explained above, according to the present invention, a small spacer is formed between the cylindrical outer cylinder of the spent fuel basket and the used fuel storage rectangular cylinder of the outer peripheral part stored inside the spent fuel basket. By moving the gap inside and using that gap as a neutron trap area, the neutron multiplication effect can be suppressed without changing the size of the spent fuel basket or the number of spent fuel storage units, and the degree of subcriticality can be improved. can contribute.

[Brief explanation of the drawing]

FIG. 1(A) is a cross-sectional view of a first embodiment of the present invention. FIG. 1(B) is a partially enlarged view of FIG. 1(A), FIGS. 2 and 3 are cross-sectional views of the second and third embodiments of the present invention, and FIG. 4 is a conventional spent fuel. FIG. 3 is a cross-sectional view of the basket. 1...Cylindrical body of the basket 2...Storage square tube 3.5.6...Neutron absorber square tube 4...Gap 7...Monitor hole 8...Heat transfer material part

Claims (1)

[Claims] In a spent fuel basket for a spent fuel cask, in which a large number of storage square tubes for storing spent fuel are arranged inside a metal cylindrical body, the storage square tubes are arranged so as to be inscribed in the metal cylindrical body. A spent fuel basket is characterized in that a void other than the storage rectangular cylinder is formed inside the basket by doing so, and a neutron trap-like neutron absorber in which a plurality of neutron absorbing elements are formed with gaps is disposed in the void.