JP3984088B2 - Disposal site structure - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a disposal site structure such as radioactive waste in which a bentonite-based material layer and a cement-based material layer are laid close to each other.
[0002]
[Prior art]
The radioactive waste discharged from the operation of nuclear power generation must be isolated from the human sphere until the radiation dose falls below the amount that the human body has. At present, such radioactive waste is buried in the ground several hundred meters below ground according to international treaties, etc., and is disposed of so that the radiation dose is below a certain value. When calculated from the quantity and half-life, it is necessary to dispose of it in the ground several hundred meters underground for a long period of hundreds to tens of thousands of years. Disposal laying installed in such an environment is configured using bentonite-based materials (clay-based materials) and cement-based materials (mortar or concrete with hydraulic cement as a hardener).
[0003]
Bentonite-based materials are used mainly to ensure sealing properties by swelling pressure due to water absorption of bentonite, and cement-based materials are used to ensure the strength of the structure. . In particular, in an environment where there are many cracks in the rock mass, such as in Japan, and the hydraulic conductivity of the entire rock mass is large, a waterproof layer is formed with a swellable soil material such as bentonite to prevent groundwater from entering the disposal site. It is indispensable to form. In addition, the use of cementitious materials is indispensable as a structural material for the construction of large cavities for disposal of waste at several hundred meters below the ground.
[0004]
Such a structure is not limited to radioactive waste, and there are many examples in which a waterproof layer is formed of a swelling soil material such as bentonite in the vicinity of a cement-based material layer in various industrial waste disposal sites.
[0005]
[Problems to be solved by the invention]
When the bentonite-based material layer and the cement-based material layer come into contact with each other and are left for a long period of several hundred to tens of thousands of years, mass transfer occurs between the two layers due to the difference in concentration of various components in the two layers. . The main ones are the movement of Ca ²⁺ from the cementitious material layer to the bentonite material layer and the movement of SO ₄ ²⁻ from the bentonite material layer to the cement material layer. These are performed using water as a medium. Further, with respect to pH, since the cement-based material layer is approximately 13 and the bentonite-based material layer is near 7, the alkali component also moves from the cement-based material layer to the bentonite-based material layer.
[0006]
The Ca ions that have moved to the bentonite-based material layer release Na and K at the same time that Ca is adsorbed in the bentonite-based material layer by an ion exchange reaction in the bentonite-based material layer. This phenomenon is referred to as “bentonite Caation”. When Ca of bentonite occurs, the water-blocking performance of the bentonite-based material layer is significantly reduced.
[0007]
On the other hand, SO ₄ ²⁻ transferred to the cementitious material layer reacts with cement hydrate to produce an expansive mineral, which causes expansion and destruction of the cementitious material layer.
[0008]
[Means for Solving the Problems]
According to the present invention, a water-impervious layer composed of a bentonite-based material layer and a structural material composed of a cement-based material layer of a mortar layer or a concrete layer using hydraulic cement as a binder are laid. In the disposal site structure, a buffer layer made of a silica-based material is interposed at a boundary portion where the water-impervious layer and the structural material are in contact, and Ca ions enter the bentonite-based material layer from the cement-based material layer . Provided is a repository structure characterized in that the buffer layer prevents the formation of calcium silicate . As the silica-based material constituting the buffer layer, at least one powder selected from silica fume, fly ash, iron refining slag, and non-ferrous refining slag is used. Buffer layer, Ru can be a precast product mainly composed of silica fume.
[0009]
Accordingly, an object of the present invention is to solve the problem of deterioration of the bentonite-based material layer and the cement-based material layer that occur due to the above-described causes in the disposal site structure.
[0010]
[Means for Solving the Problems]
According to the present invention, in a disposal site structure laid using a bentonite-based material layer and a cement-based material layer, a buffer layer made of a silica-based material at a boundary portion where the bentonite-based material layer and the cement-based material layer are in contact with each other. Disposal site structure characterized by having intervened. As the silica-based material constituting the buffer layer, at least one powder selected from silica fume, fly ash, iron refining slag, and non-ferrous refining slag is used.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
When a radioactive waste disposal facility is constructed in the ground using a bentonite-based material layer as a water-impervious layer, the water-impervious layer is usually laid so as to surround the cement-based material layer as a structural material. That is, the cementitious material layer is in contact with the outside of the bentonite material layer, and in this case, mass transfer occurs through water for a long period of time as described above. In order to prevent this, it is not practical to insert a non-porous industrial product such as a metal or a resin between the two layers. This is because there is virtually no artificial material that can be constructed between the two layers and can maintain its function for hundreds to tens of thousands of years. It may be.
[0012]
For this reason, it was also considered that sandy material used as a fine aggregate in the cementitious material layer was filled between the two layers. If the structure cannot be secured and the structure is filled with groundwater, the speed of the water moving through this fine aggregate layer will increase, and the function as a cushioning material will not be achieved. Can not.
[0013]
According to the present invention, the cause of deterioration of the structure mainly based on the above-mentioned bentonite Ca conversion phenomenon or the difference in pH value between the two layers can be eliminated by interposing silica-based powder between the two layers. Silica-based powder reacts with Ca in a high pH environment to form chemically stable CSH (calcium silicate hydrate) (this reaction is called pozzolanic reaction). Therefore, when such a layer of silica-based powder is interposed between both layers, it serves as a buffer layer that prevents Ca and high pH water from moving from the cement-based material layer to the bentonite-based material layer.
[0014]
Examples of the silica-based powder used in the present invention include silica fume, fly ash, iron refining slag such as blast furnace slag, non-ferrous refining slag such as copper slag and ferronickel slag, which are used alone or in combination. In particular, the fine particles have a large specific surface area, so that the process of moving the substance passing through the layer becomes long, and as a result, the reaction rate increases, so that Ca can be efficiently blocked from high pH. In addition, the packing properties and reactivity can be adjusted by mixing silica particles having different particle sizes. In use, it is preferable that these silica-based powders are made into a slurry form (paste form in the case of fine powders) using an aqueous medium and filled between the cement-based material layer and the bentonite-based material layer.
[0015]
It is also possible to spray such silica-based powder slurry or paste. The spraying may be performed on the surface of the existing bentonite material layer or the surface of the existing cementitious material layer, and after the spraying is completed, the cement material layer or the bentonite material layer is laid. Also, spraying and filling can be combined. In this way, it is practical to fill or spray the silica-based material layer at the boundary surface between the bentonite-based material layer and the cement-based material layer by field construction. A silica-based material layer can also be formed by pre-manufacturing as a precast product and placing it between both layers.
[0016]
FIG. 1 shows that when a bentonite-based material layer 2 is laid on the ground 1 in the ground and a cement-based material layer 3 is constructed on the inside thereof, the bentonite-based material layer 2 and the cement-based material layer 3 are A partial cross-section of the present invention example in which the silica-based material layer 4 is interposed as described above is shown schematically. According to this structure, the cement-based material layer 3 has been infiltrated for many years. In the case where high water and Ca ions are about to move from the cement-based material layer 3 side to the bentonite-based material layer 2 side as indicated by the arrows in the figure, the silica-based material layer 4, neutralization reaction proceeds with the acidic silica-based powder, and the aforementioned pozzolanic reaction occurs, so that the penetration of these into the bentonite-based material layer 2 side is prevented. In order for these reactions to proceed efficiently in the silica-based material layer 2, it is important that the silica-based material layer 2 is formed of a layer of silica-based powder that can penetrate water. According to the tests conducted by the present inventors, the blocking effect of high pH water and the blocking effect of Ca ions due to the presence of the silica-based material layer were clear. A representative example of the test will be described below.
[0017]
[Test example]
A schematic cross section of the water permeable cell used in the test is shown in FIG. This water permeable cell is formed with a stainless steel upper mold 5 and lower mold 6 to form a disk-shaped airtight cavity for loading a sample, and high-pressure water can be passed through the sample in the cavity. In this case, 7 is a water supply pipe, and 8 is a drain pipe. The water supply pipe 7 passes through the upper mold 5 and communicates with the cavity, and the drainage pipe 8 penetrates the lower mold 6 from the cavity and communicates outside the system. Both pipes are provided with a valve 9 and a valve 10, and actually a water pressure gauge (not shown) for detecting the water pressure in the cavity is attached to the upper mold 5. The inner surfaces of the upper mold 5 and the lower mold 6 facing the cavities are provided with radial grooves communicating with the pipes so that water is dispersed and spread over the entire surface of the sample.
[0018]
In the cavity of the water permeable cell A in FIG. 2, a disc-shaped cement-based material 11 (hardened mortar plate) having a thickness of 2 cm and a diameter of φ10 cm and a disc-shaped silica-based material 12 having a thickness of 1 cm and a diameter of φ10 cm are placed. The cement-based material 11 is placed upstream so that the water that has passed through the cement-based material 11 passes through the silica-based material 12, and the hydraulic pressure is maintained at 10 kgf / cm ² , while maintaining 0.1 to 1.0 ml / min. The water flow was continued for 2 weeks at a flow rate of 1, and the permeated water was collected periodically and its pH value and calcium concentration were measured. A filter paper was interposed between the inner surface of the mold cavity and the sample so that water could permeate the entire sample.
[0019]
Mortar plate 11 subjected to the test, water cement ratio = 100%, Portland cement = 529kg / m ^3, a mortar formulation of fine aggregate = 794 kg / m ^3, wood age 56 days (accelerated curing at 50 ° C. ) Later cured body. The silica-based material 12 used in the test was kneaded with 100 g of silica fume having an average particle size of about 50 μm, 100 g of water, 450 g of fine aggregate, 1.5 g of calcium hydroxide, and 45 g of ultrafine silica slurry. This is after forming into a disk of the above dimensions with a pressure of cm ² and after 7 days at 40 ° C. and RH 95%.
[0020]
For the sake of comparison, the water permeable cell B having the same structure is used and the silica material is not used, and only the same cement-based material 11 used in the above test (the test of the water permeable cell A) is used. The test was continued under the same conditions as the above-mentioned water permeable cell A by installing in the cavity of B, and the same water permeable water was collected periodically and its pH value and calcium concentration were measured. The results are shown in FIG. 4 and FIG.
[0021]
From the results of FIG. 4, in the water permeable cell B of the comparative example, the water that passed through the cement-based material layer maintained a pH value of approximately 13, whereas in the water-permeable cell A, the water passed through the cement-based material layer. It can be seen that the pH of water that has passed through the system material layer has dropped to about 10. Moreover, from the result of FIG. 5, in the water permeable cell A of the present invention example, the calcium concentration in the water permeable water is halved compared to the water permeable cell B of the comparative example.
[0022]
【The invention's effect】
As described above, according to the present invention, in a radioactive waste disposal facility where a bentonite-based material layer is laid in the ground adjacent to a cement-based material layer as a structural material, the concentration difference between the chemical components of both layers is determined. It is possible to reduce functional deterioration due to aging of each layer caused by water-mediated mass transfer due to water. In addition, the silica-based powder material used in the present invention is inexpensive and easy to construct, and hardly changes in properties and has water permeability, so that the effect can be maintained for many years. For this reason, it is suitable as a material that enhances the maintenance function of radioactive waste disposal sites that require storage for hundreds to tens of thousands of years.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view schematically showing a layer structure of a disposal site structure according to the present invention.
FIG. 2 is a schematic cross-sectional view of a water permeable cell used in a layer structure water permeability test according to the present invention.
FIG. 3 is a schematic cross-sectional view of a water permeable cell used in a water permeability test of a layer structure of a comparative example.
FIG. 4 is a diagram showing the behavior of pH value when water passing through a cement-based material layer has a silica-based material layer as compared to the case without a silica-based material layer.
FIG. 5 is a diagram showing a change in Ca concentration in the case of having a silica-based material layer for water permeation that has passed through a cement-based material layer, as compared with the case of not having a silica-based material layer.
[Explanation of symbols]
1 Ground 2 Bentonite Material Layer 3 Cement Material Layer 4 Silica Material Layer

Claims (4)

Disposal site structure laid using a water-impervious layer composed of bentonite-based material layers and a structural material composed of mortar layers or concrete-based cement-based material layers made of hydraulic cement as a binder In this case, a buffer layer made of a silica-based material is interposed at a boundary portion where the water-impervious layer and the structural material are in contact with each other , and Ca ions penetrate into the bentonite-based material layer from the cement-based material layer. Disposal site structure characterized by preventing by generating . The disposal site structure according to claim 1, wherein the buffer layer made of the silica-based material is configured using at least one powder selected from silica fume, fly ash, iron smelting slag, and non-ferrous smelting slag. The disposal site structure according to claim 1 or 2, wherein the disposal site is a radioactive waste disposal site constructed in the ground. The disposal site structure according to any one of claims 1 to 3, wherein the buffer layer is a precast product mainly composed of silica fume.

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