JPS5812559B2 - Genshirosha Heikozo - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention suppresses the amount of activation in the reactor vessel to bring it below the accessible dose during inspection during service, and at the same time reduces the amount of activation in ordinary concrete, which serves as a bioshielding layer, to avoid problems during demolition. Reactor shielding structure whose main purpose is to reduce the radiation dose below the permissible level.

Furthermore, by installing an intermediate shielding layer between the primary shielding layer and the reactor vessel and suppressing the thermal neutron flux in this intermediate shielding layer, it is possible to approach in-service inspections.
It also relates to the reactor main shielding structure that enables safe disassembly and removal.

Recently, an in-service inspection (ISI) was conducted on the reactor vessel.
is required.

For this reason, it is often necessary for workers to approach the reactor vessel, and ensuring strict and highly reliable safety is an important issue.

The main source of radiation exposure for workers who approach the reactor vessel is gamma rays, which are the decay of long-half-life radioactive isotopes produced by thermal neutron irradiation of the outer surface of the reactor vessel or biological concrete liner, etc. The level is quite high.

Therefore, in order to reduce and suppress the exposure dose of workers to a permissible value or less, it is necessary to reduce the above-mentioned thermal neutron flux.

Normally, fast neutrons emitted from the nuclear reactor enter the thermal neutral element bundle into the raw concrete shielding layer, where they become thermal neutrons and are reflected, activating the reactor vessel and liner.

The present invention has focused on this point, and by installing an intermediate shielding layer between the reactor vessel and ordinary concrete, which is made of a shielding material that limits the water content or contains a thermal neutron absorbing substance, the shielding layer can be easily removed. The urgent task is to reduce the level of reflected thermal neutron flux.

Next, when a nuclear reactor plant reaches the end of its useful life and is to be dismantled, thermal neutron flux is high in a normal nuclear reactor at the front (inner surface) of the reactor side of the raw wood shielding layer made of ordinary concrete.

The cause is iron (Fe), cobalt (Co), and scantium (Sc) contained in ordinary concrete.
When an element such as eurobium (Eu) is activated,
Since it produces long-lived radioactive elements, the amount is an issue.

Therefore, the next feature of the present invention is Bi, FeCo, Sc, and Eu in the shielding material used as the above-mentioned intermediate shielding layer.
The purpose is to limit the content of such elements to below a permissible value to solve the above-mentioned problems.

A further feature of the present invention is that regarding the structure of the intermediate shielding layer, in consideration of the convenience of dismantling work after the life of the reactor, a block stacking method that can be easily dismantled and transported and removed, a gravel or fine sand filling method, etc. This is because it constitutes an intermediate shielding layer.

Next, the present invention will be explained using illustrated examples.

FIG. 1 represents an example of a nuclear reactor provided with an intermediate shielding layer according to the invention.

In the figure, 1 is the reactor core, 2 is the reflector, 3 is the neutron shield, 4 is the thermal shield, 5 is the reactor vessel, 6 is the intermediate shielding layer storage container, 7 is the intermediate shielding layer, and 8 is the biological shielding made of ordinary concrete. It shows the layers.

In the case of the present invention, it is important to have an intermediate shielding layer 7 between the reactor vessel 5 and the biological shielding layer 8, and as the shielding material, iron content is 103 ppm or less, cobalt is 10'' ppm or less, Scandium 5X10-2
It is also important to limit eurobium to 10-3 ppm or less, limit the water content, or use materials mixed with boron or cadmium as a thermal neutron absorbing substance.

The thickness of the intermediate shielding layer 7 will be determined under conditions that limit the amount of radioactivity due to activation of ordinary concrete, which is a bioshield, to a permissible amount or less.

Further, as a construction style in which the intermediate shielding layer 7 can be easily dismantled or removed, the following structure is appropriately adopted.

FIG. 2 shows a block stacking method in which the shielding material is formed into blocks and the blocks are stacked in the intermediate shielding layer storage container 6.

The block is loaded and unloaded from a filling/unloading port 9 opened at the top of the biological barrier layer 8.

FIG. 3 shows a gravel filling method in which a shielding material is granulated to the size of gravel and filled into a storage container 6 to form an intermediate shielding layer 7.

The granulated shielding material is supplied from the filling port 10 and discharged from the take-out port 11 during disassembly.

FIG. 4 shows a fine sand filling method in which the shielding material is atomized into fine sand and filled into the storage container 6.

The fine sand-like shielding material is supplied from the filling port 10 and discharged from the take-out port 11 during disassembly.

Thus, according to the above-described solid furnace construction structure of the present invention,
Neutrons emitted from the reactor core 1 pass through the walls of the reflector 2, the neutron shield 3, the heat shield 4, and the reactor vessel 5 in order, enter the intermediate shield layer 7, and also pass through that into the ordinary concrete. is incident on and is attenuated.

At that time, each time a thermal neutron passes through the neutron shield 3, the shield 4, and the reactor vessel 5 (gradually sharply decreases,
This state is also maintained in the intermediate shielding layer 7.

In this way, after the amount of radiation is reduced to the required level of thermal neutrons, which is not a problem, on a normal concrete surface, it is attenuated to the required radiation level by shielding with living trees.

Therefore, the intermediate shielding layer 7 does not require any special shielding, and it becomes possible for workers to approach and disassemble, or subsequently transport, remove, and dispose of in f minutes.

Furthermore, the dismantling of ordinary concrete shielding bodies can be carried out using conventional demolition methods without radioactive contamination of the surrounding environment.

In addition, FIG. 5 shows an example of the effects achieved by the present invention, that is, how much force the intermediate shielding layer 7 actually moves, as a graph.

The composition of the shielding material of the intermediate shielding layer used in this data was zero water content, 103 ppm Fe + 103 ppm Copper.
0-2ppm, Scfd5×10-2ppm, EuU
Density set to 10 ppm3. A heavy concrete aggregate of 4.5gr/cc was carried out with a wall thickness of 50c1rL.

Note that no thermal neutron absorbing substance was mixed. Figure 5 shows the attenuation of fast neutrons, thermal neutrons, and gamma rays. In particular, curve AH shows the thermal neutron flux of a conventional structure in which the intermediate shielding layer is also entirely made of ordinary concrete, and curve B shows the thermal neutron flux of the structure described above. The thermal neutron flux of the present invention is shown in which an intermediate shielding layer is constructed using a shielding material of .

As is clear from the graph in FIG. 5, according to the present invention, the radioactivity level around the reactor vessel and the radioactivity level in the intermediate shielding layer are significantly reduced compared to conventional ones, and at the time of UISI or It is also clearly understood that the dose is attenuated by the dose reduction required during dismantling and removal.

Furthermore, when a thermal neutron absorbing substance is mixed, the thermal neutron flux level can be further reduced.

[Brief explanation of drawings]

FIG. 1 is a simple vertical cross-sectional view of a nuclear reactor according to the present invention;
Figures 2 and 4 are partial sectional views showing the construction style of the intermediate shielding layer, and Figure 5 is a graph showing the attenuation levels of fast neutrons and thermal neutrons. 1...Reactor core, 2...Reflector, 3...Neutron shield, 4
...Thermal shield, 5...Reactor vessel, 6...Intermediate shielding layer storage container, 7...Intermediate shielding layer, 8...Biological shielding layer, 9...Filling/unloading port, 10...Filling port, 11 ...Exit.

Claims (1)

[Claims] 1. An intermediate shielding layer is provided between the reactor vessel and the biological shielding layer made of ordinary concrete, and the components of the concrete forming the intermediate shielding layer are FelO3ppm or less, Co
102 ppm or less, Sc5×10-2 ppm or less, Eu
1. A nuclear reactor shielding structure characterized by adding a thermal neutron absorbing substance to 1O-3 ppm or less and reducing the water content to almost zero.