JPH0631795B2 - Spent fuel storage rack - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a spent fuel storage rack, and more particularly to a structure of a spent fuel storage rack suitable for high density storage.

[Background of the Invention]

Spent fuel generated from the reactor is stored in the spent fuel storage rack in the fuel pool inside the reactor building. However, in the current nuclear power generation situation, due to the delay in the construction of the reprocessing facility, which is the main process of the nuclear fuel cycle, the expansion and storage densification of spent fuel generated from nuclear reactors are important issues.

2 and 3 show a conventional spent fuel storage rack.

In the conventional spent fuel storage rack, a storage cell 3 is formed by a plurality of square stainless steel pipes 1 arranged at regular intervals 4 in order to maintain the subcriticality of the stored fuel. The square pipe 1 is connected from the upper portion to the lower portion of the spent fuel storage rack so as to cover the entire axial direction of the stored fuel. Further, the square pipe 1 is attached to the seat plate 2 by welding.

The fuel stored in the fuel pool in the nuclear reactor is not only used fuel but also new fuel as temporary storage during core fuel exchange and fuel in the process of burning in the event of an accident. From this, the design for the subcriticality of the spent fuel storage rack determines the storage interval 4 in consideration of the reactivity of the new fuel and the fuel in the process of combustion. There is a waste in the interval 4, which is one of the causes of hindering the high-density storage (making the storage interval 4 smaller and storing more fuel in the same space).

As a method of reducing the storage interval 4, a rack using a reactivity control material (neutron absorbing material such as boron) in the square pipe 1 has been devised, but the reactivity control material extends from the upper part to the lower part of the rack. If cooling water is supplied with the spent fuel assemblies existing in each pipe, the pressure loss increases.

Note that as a document showing the conventional technique, there are JP-A-57-24892 and JP-A-56-77797.

[Object of the Invention]

An object of the present invention is to provide a spent fuel storage rack capable of high-density storage and having a small pressure loss.

[Outline of Invention]

A feature of the present invention is that a plurality of square tubes which are made of a metal material containing boron and into which a spent fuel assembly is inserted are connected to each other, and the square tubes are located below the square tubes and are inserted into the square tubes, respectively. A base for holding a lower end portion of the spent fuel assembly, wherein the square tube is arranged so as to surround only an upper portion of the spent fuel assembly inserted in the square tube above the base. The outermost rectangular cylinder is held by a support member attached to the base.

Example of Invention

Conventionally, when storing a used fuel, in order to prevent the fuel from becoming critical during storage, the distance between the fuel assembly and the adjacent fuel assembly should be sufficiently wide, or natural boron etc. should be contained in the structural member of the storage rack. Reactivity control member (neutron absorber) is put in.

FIG. 4 shows a distribution 5 of neutron multiplication factors in the axial direction (height direction) of a spent fuel assembly of a typical BWR.

In BWRs, there is an axial void distribution in the reactor core, and in general, fuel burned at a high void fraction has a larger amount of plutonium production / accumulation, so fuel burned sufficiently in the core Then, in the upper part of the fuel, the amount of plutonium is larger and the neutron amplification is higher.

Therefore, as in the example of FIG. 4, the neutron multiplication factor in the axial center of the spent fuel is approximately 20
It will be reduced by about%.

Therefore, paying attention to the axial distribution (height distribution) of the neutron multiplication factor of such spent fuel, in order to make the fuel subcritical, it is necessary to lower the neutron multiplication factor above the fuel in particular. .

As a method of reducing the neutron multiplication factor of the stored fuel, a method of mixing a neutron absorber into the spent fuel rack is often used, but considering the characteristics of the spent fuel described above,
If a neutron absorber is mixed into the structural member of the spent fuel storage rack, which has a length of about ½ of the fuel having a high neutron multiplication factor, the storage interval between adjacent fuel assemblies can be shortened. It is possible to provide a dense and inexpensive spent fuel storage rack.

An embodiment of the present invention will be described below with reference to FIGS. 3 to 10. FIG. 1 shows the structure of the spent fuel storage rack invented from the distribution of the neutron multiplication factor of the spent fuel in the axial direction (FIG. 4).

FIG. 5 shows a view taken along the line V-V in FIG. 1, and a portion corresponding to about an upper half of the rack is a stainless steel square pipe 8 containing the neutron absorber 7 (see FIG. 6). ) Composed by welding together to form a storage cell. The lower half of the rack has a frame 9 structure only on the outside and is welded to the seat plate 10.

FIG. 7 shows a plan view of the seat plate 10 of the present rack. The seat plate 10 is provided with a support hole 11 for supporting the lower portion of the stored fuel material assembly 12 so that the fuel is always held vertically and the distance between the adjacent fuels is kept constant. There is.

The subcriticality of fuel in this rack will be described below. FIG. 9 shows an analytical model for calculating the neutron multiplication factor of the stored fuel.
Is surrounded by layers of water 13, aluminum 14 and aluminum 15 with natural boron.

The result of analysis under the present calculation conditions is shown by the dotted line 6 in FIG. The neutron multiplication factor of the stored fuel is 0.7-0.8,
The neutron multiplication factor can be reduced by 25 to 30% as compared with the case without natural boron.

For this reason, if a reactivity control member such as natural boron is inserted into the structural member of the spent fuel storage rack corresponding to the length of the upper half of the fuel with a high neutron multiplication factor, the neutron multiplication factor of the upper fuel will be Can be lower than the neutron multiplication factor of.

In FIG. 5, a square pipe 8 made of stainless steel is used instead of aluminum. However, since stainless steel has higher neutron absorption capacity and higher strength, there is no particular problem because the reliability is further improved.

Also, since the neutron multiplication factor is 1.0 or less (curve 5 in FIG. 4) in the lower portion of the spent fuel assembly, a stainless steel square pipe is used between the adjacent storage fuel assemblies as in the conventional case. Even if it is not provided, the subcriticality is sufficiently maintained only by the surrounding water. From this, the lower part of the rack can be made into a frame structure for connecting the upper square pipe 8 and the seat plate 10 to form the rack. This aram 9
Has a structure that can maintain a sufficient strength against the load acting on the rack during an earthquake.

Due to the structure of this rack, a storage capacity of about 1.2 times that of a rack using a conventional stainless steel square pipe can be secured, and the storage density can be increased. In addition, the amount of square pipe used is 1/2 that of the conventional type, welding of the square pipe and the sheet plate is eliminated, and the amount of expensive neutron absorbing material is 1 / thicker than that of the conventional rack with neutron absorbing material. Two
Can be reduced to

Next, the secondary effects obtained by the configuration of this rack will be described.

In the conventional spent fuel storage rack sheet plate 2, since the square pipe covers the length from the upper portion to the lower portion of the fuel assembly 12, in addition to the support hole 11 for supporting the lower portion of the stored fuel. It was necessary to make a number of holes such as a square pipe support hole for welding a square pipe and a seat plate, and a cooling water inflow hole for circulating cooling water between the stored fuel and the square pipe. However, in the seat plate 10 of this embodiment, as shown in FIG. 7, only the support hole 11 for supporting the lower portion of the stored fuel is required, and the manufacturing cost can be greatly reduced. 8 shows a cross section of the fuel assembly support hole 11 shown in FIG.
A chamfered portion 11a is provided to stably support the lower portion of the.

Regarding cooling of the stored fuel, the rack according to this embodiment is
The bottom of the fuel assembly can be stored naked, and since the length of the square pipe is about half that of the conventional rack, the cooling water flowing between the upper square pipe and the stored fuel assembly 16
Since the pressure loss of is about half, the cooling capacity is improved as compared with the conventional rack.

Further, the seat plate 10 is provided with a reinforcing rib 17 for preventing out-of-plane deformation with respect to the weight of the stored fuel.
The reinforcing rib 17 may be provided on the opposite side of the seat plate.

Next, an example of practical application of this rack as a fuel storage facility will be described.

FIG. 10 shows an arrangement example of the rack area when the rack is used in the fuel pool in the nuclear power plant. In the fuel pool in the reactor building, it is used not only for spent fuel but also for temporary storage of new fuel during core refueling and temporary storage of fuel during combustion in the event of an accident.
A storage area 18 only for spent fuel using this rack,
It is necessary to divide into the storage area 19 for the new fuel etc. using the conventional rack. The space of the storage area 19 may be 100% of the core storage for the above-mentioned reason.

However, in the independent fuel storage facility outside the nuclear power plant, since only the spent fuel is stored, it is not necessary to divide the storage area, and it is possible to deal only with this rack, so a greater effect can be obtained.

Also, with the increase in storage density due to the adoption of this rack, the pool can be downsized when compared to the conventional capacity, and there is also an effect of reducing the price of the fuel storage facility as a whole.

Furthermore, in the future, nuclear power plants are planned for long-cycle operation in order to improve the operating rate, and it is expected that studies on storage methods for high burnup fuel will be required. Since the high burnup fuel has a long neutron irradiation period in the core, the decay heat of the spent fuel increases, and it is necessary to efficiently cool the fuel in the fuel storage pool. Also in this regard, the present embodiment has no particular problem because of the above-described structure. Therefore, the present embodiment can provide a high-density and highly reliable storage method for storing spent fuel for nuclear power.

〔The invention's effect〕

According to the present invention, since the rectangular tube made of the boron-containing metal material is arranged so as to surround the large upper part of the neutron multiplication factor of the spent fuel assembly inserted in the rectangular tube, The spent fuel assembly can be maintained in a subcritical state, and the storage density of the spent fuel assembly can be increased.

Further, since the rectangular cylinder surrounds only the upper part of the spent fuel assembly, the lower part of the spent fuel assembly inserted in each adjacent rectangular cylinder is exposed outside the rectangular cylinder, and the cooling water is adjacent to it. It can flow freely between spent fuel assemblies. For this reason,
The cooling efficiency of the lower portion of the spent fuel assembly inserted in each rectangular cylinder is improved. The fact that the rectangular cylinder surrounds only the upper part of the spent fuel assembly leads to the reduction of the pressure loss in the rectangular cylinder, and the cooling efficiency of the upper part of the spent fuel assembly inserted in each rectangular cylinder also increases. .

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of a spent fuel storage rack of the present invention, FIG. 2 is a front view showing a conventional spent fuel storage rack, and FIG. 3 is a conventional spent fuel storage rack. FIG. 4 is a plan view, FIG. 4 is a diagram showing a neutron multiplication factor distribution in the axial direction of spent fuel of the BWR, FIG. 5 is a view taken along the line VV of FIG. 1, and FIG. 6 is a diagram of FIG. Details of part B, FIG. 7 shows VI of FIG.
Fig. 8 is a view taken along the line I-VII, Fig. 8 is a view taken along the line VIII-VIII of Fig. 7, Fig. 9 is a view showing an analytical model of the subcriticality of the stored fuel, and Fig. 10 is a rack according to the present invention. It is a figure which shows the layout drawing in a fuel pool when using in a reactor building. DESCRIPTION OF SYMBOLS 1 ... Stainless steel square pipe, 2 ... Conventional rack sheet plate, 3 ... Fuel assembly storage cell, 4 ... Fuel assembly storage interval, 5 ... Neutron multiplication factor distribution of spent fuel assembly, 6 ...
Neutron multiplication factor distribution of a spent fuel assembly when a neutron absorbing material is used, 7 ... Neutron absorbing material, 8 ... Neutron absorbing material-containing square pipe, 9 ... Frame, 10 ... Sheet plate in the present invention, 11 ... Fuel assembly Body support hole, 12 ... Fuel assembly, 13 ... Water, 14 ... Aluminum, 15 ... Boron-containing aluminum, 16 ... Cooling water, 17 ... Reinforcing rib, 18 ...
Spent fuel storage area, 19 ... New fuel temporary storage and 100
% Core area.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Matsumoto 3-1-1, Saiwaicho, Hitachi, Ibaraki Hitachi Ltd. Hitachi factory (72) Inventor Takaaki Mochida 3-chome, Saitama, Hitachi, Ibaraki No. 1-1 Hitachi Ltd., Hitachi Plant (72) Inventor Toshiharu Takahashi 3-2-1, Saiwaicho, Hitachi, Ibaraki Hitachi Engineering Co., Ltd. (56) Reference: JP-A-61-114191 JP, A)

Claims (1)

[Claims] 1. A plurality of mutually connected rectangular cylinders made of a metal material containing boron and having a spent fuel assembly inserted therein, and a plurality of rectangular cylinders located below the rectangular cylinders and inserted into the rectangular cylinders, respectively. A base for holding the lower end of the spent fuel assembly, and the square tube is arranged above the base so as to surround only an upper portion of the spent fuel assembly inserted into the square tube. The spent fuel storage rack, wherein the square cylinder located at the outermost periphery is held by a support member attached to the base.