JPH0562960B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application The present invention relates to a method for burying a sealed container containing radioactive waste.

(b) Conventional technology Conventionally, the method of burying airtight containers (hereinafter referred to as canisters) containing radioactive waste has been to bury tunnels (underground cavities) at a depth of 500 to 1000 m in rock such as granite, rock salt, shale, etc. After excavation, a well (small hole) for burying the canister with a depth of 7,500 mm and a diameter of about 1,500 mm is dug below this tunnel.

Then, the inside of this well is surrounded with clay mineral such as compressed bentonite, and the above-mentioned cylindrical canister with a length of 4,500 mm and a diameter of about 800 mm is inserted into the well, compressed bentonite is placed on top of the canister, and then Generally, the tunnel is covered with ordinary bentonite as a shielding material to completely eliminate contact with the outside, and then the entire tunnel is finally filled with powdered bentonite.

However, in this conventional method, a canister containing radioactive waste (apparent specific gravity
8.18 to 9.73) and the filler bentonite (apparent specific gravity approximately 2), the compressed bentonite surrounding the canister may flow.
This raised the possibility that the canister would sink through the bentonite and come into direct contact with the bedrock, potentially spreading radioactive contamination through the water in the bedrock.

(c) Disclosure of the invention The present invention prevents the canister from sinking and moving through bentonite, which is a shielding material, and coming into direct contact with bedrock when radioactive waste is stored in a canister and buried in a geological formation. This provides a construction method that can be used.

That is, the present invention involves attaching a container filled with an inert gas (hereinafter referred to as a float) that acts as a float in the soil (in bentonite) to a canister containing radioactive waste and having a large apparent specific gravity. The hanging specific gravity is set to a value equal to or close to that of the surrounding filler bentonite, thereby preventing the canister from sinking and moving in the bentonite.

Although it is generally difficult to accept that solid substances exhibit fluid-like properties in geological formations, it is a geologically proven phenomenon.

In other words, in Japan, salt is extracted from seawater, but in the United States, the Soviet Union, and European countries, rock salt is mined and supplied. This rock salt has a dome or anticline shape,
This is a rock salt layer located several kilometers underground that has risen through the overlying sedimentary rock (this is called the diamond peel structure).

There have been various theories as to the cause of this phenomenon, but it is currently interpreted as follows. In other words, if the density of the sediment covering solid rock salt is greater than the density of rock salt, that alone will cause a rock salt dome to develop, and the rock salt dome will float due to its buoyancy due to its lightness. No other compression force is required.

A phenomenon similar to this can be seen in many forms of gypsum in Japan's black ore deposits. The Kuroko deposit is a submarine fumarole deposit produced by acidic volcanic activity during the Tertiary Miocene. From the lower to the upper part, it consists of white rhyolite, Shimowaite andesite, and silica (the surrounding area of the ore body). It shows a formation structure of gypsum), yellow ore, black ore, mudstone, or tuff.

In this case, the lower part of the gypsum is parallel to the strata, but the upper part of the black ore deposit is raised in a dome shape and can be seen cutting through the bedding of the host rock.

It is not clear whether the cause of this distinctive form is the same as that of the salt dome or a different factor, but it is a fact that at least the gypsum is pushing up and penetrating the surrounding rock.

Similarly, another interesting phenomenon is observed in black ore deposits. In other words, black ore deposited in layers (specific gravity
After 4.5) is consolidated and solidified, it undergoes deformation due to wide-area sliding action or intrusion of intrusive rocks, causing the hard black ore to undergo a budden phenomenon, forming blocks in the shape of vertical sausages. It has been confirmed that the surrounding tuff and clay are flowing and filling the torn parts.

Moreover, in this case, the tuff and clay are mainly composed of sericite (or a mixed layer of sericite and montmorinite) and chlorite in the center of the ore deposit, but in the periphery they are mainly composed of montmorinite (partially zeolite). This montmorinite has almost the same composition as bentonite (i.e., a general term for a clay-like substance whose main component is montmorinite and generally includes quartz, cristoballast, zeolite, feldspar, etc.), which is used as a filler in the present invention. It is.

In other words, this indicates that solid substances flow in the geological formations as described above, and it also means that compressed bentonite, which is used as a filler when processing radioactive waste, may also flow. .

Therefore, according to the conventional construction method, the canister eventually sinks into the bentonite due to the difference in specific gravity between the canister containing radioactive waste (apparent specific gravity 8.18 to 9.73) and the compressed bentonite filler (apparent specific gravity approximately 2). There is a strong possibility that radioactivity may leak out of the canister due to the water in the rock that comes into contact with the canister.

Moreover, if the well into which the canister is inserted is shaken due to an earthquake or the like, it is highly conceivable that this flow will be further promoted.

Therefore, if the apparent specific gravity values of the canister and the clay mineral (bentonite) filled around it are made close to each other, the canister will not sink or move even if the clay mineral flows, so in the present invention, an inert gas is sealed. This problem was solved by attaching a container float to the canister.

Embodiments of the present invention will be described below with reference to the drawings.

(d) Example In a mine, the width from the main shaft (not shown) to the side.
The required number of horizontal tunnels 3 of 3.3 m and height of 4.5 m are closed in advance, and a canister burial well 4 of 1.5 m in diameter and 12.5 m in depth is drilled in the deepest part of the horizontal shaft 3 as shown in the attached diagram. did.

Therefore, first, powdered bentonite 5 is applied to the inner peripheral wall of the well 4 to a thickness of 50 mm, and compressed bentonite 6 is laid to a thickness of 0.5 m at the inner bottom of the well 4. Then, block-shaped compressed bentonite 6 is sequentially piled up along the inner peripheral wall of the well 4 so that the canister 1 having a diameter of 0.8 m and a length of 4.5 m can be inserted into the 4-axis center of the well. 1 m diameter x 6.494 m length, with 1 atm helium gum sealed in the upper end of the canister 1.
A stainless steel float 2 with a wall thickness of 6 mm was attached and fixed.

For connecting the canister 1 and the float 2, an automatic coupler 7 similar to that used for hydraulic hose connections can be used, for example.

In addition, since the canister 1 and the float 2 are longer than the back of the horizontal tunnel 3, it is impossible to embed them at right angles from the horizontal tunnel 3. Therefore, the excavation must be made diagonally from the direction of the main tunnel toward the middle of the well 4. After inserting the canister 1 and float 2 into the well 4,
Once re-formed into a cylindrical shape, canister 1
A compressed vent nant 6 is buried between the canister 1 and the inner peripheral wall of the well 4, and a compressed vent nant 6 is buried between the float 2 and the bottom of the horizontal tunnel 3.
At the same time, the canister 1, which has a total weight of 22 tons and an apparent specific gravity of 9.7 g/cm ^{3 ,} is adjusted to have an apparent specific gravity of 2.0, which is equivalent to the bentonite filler.

In this way, the canisters 1 and the floats 2 are sequentially buried from the back of the horizontal tunnel 3 toward the main shaft.

In this example, He gas was used as the inert gas sealed into the float 2, but this was replaced with Ne,
Inert gases such as Ar, Kr, Xe, Rn or other gases that are not affected by radioactivity may also be used.

Further, in the embodiment, He gas was sealed at 1 atm, but the pressure can be adjusted at will, and the size of the float 2 is also determined according to the weight (apparent specific gravity) of the canister 1.

Furthermore, although stainless steel is used as the material for the canister 1 and the float 2, it is naturally possible to use general radioactive shielding materials such as lead and lead alloys.

In addition, as a combination of the canister 1 and the float 2, in the above embodiment, the float 2 filled with inert gas is connected to the upper part of the canister 1, but the float 2 may be connected to the lower part of the canister 1, or the float 2 may be connected to the lower part of the canister 1, or A configuration in which the canister 1 is incorporated may also be used. Furthermore, when burying canisters into a well, the canister is inserted vertically into the well, but in horizontal or inclined shafts, it is natural to bury the canister horizontally.

(e) Effects of the invention As mentioned above, according to the construction method of the present invention, the apparent specific gravity values of bentonite, which is the surrounding filler of the canister, and the canister can be made almost equal by using the float, so that the filler There is no possibility that the canister will sink or move due to bentonite flowing in the well, and there is no risk that the canister containing radioactive waste will come into direct contact with the bedrock, and it will last for an extremely long period of time (more than 1,000 years). It can prevent radioactive contamination and has great safety effects.

[Brief explanation of the drawing]

The figure is an explanatory cross-sectional view of a state in which the construction method of the present invention has been applied. Description of symbols: 1... Canister (radioactive waste storage sealed container) 2... Inert gas filled float 3... Horizontal tunnel 4... Well 5... Powdered bentonite 6... Compressed bentonite 7... Connector.

Claims (1)

[Claims] 1 In a burial method in which a sealed container containing radioactive waste is inserted into a well excavated in an underground cavity and filler is filled around the container, a float is attached to the sealed container to reduce the apparent specific gravity of the container compared to the surrounding filler. A method for burying radioactive waste storage containers, characterized by burying them at the same level or close to each other.