JP5041239B2 - Buffering material and disposal method of waste - Google Patents

Description

  The present invention relates to a buffer material and a method for burying and disposing of a waste body, and in particular, a buffer material laid between a waste body enclosing radioactive waste and a disposal hole for burying the waste body, and the The present invention relates to a disposal method of a waste body in which a buffer material is laid between the waste body and a disposal hole.

  Radioactive waste generated from the use of nuclear power at nuclear power plants is finally disposed of after being treated in an appropriate manner so that it can be safely managed so as not to have a significant impact on the human environment. It has come to be. For example, high-level radioactive waste (high-level radioactive waste) remaining after separating useful materials such as uranium and plutonium from spent fuel by reprocessing is solidified with glass and then cooled for 30 to 50 years. For storage, and then buried deep underground.

  An embedding disposal facility for embedding radioactive waste solidified with glass (glass solidified body) has a disposal tunnel and a disposal hole. The disposal mine is an underground mine that is deep underground, and has a hood-type cross section in hard rock and a tricentric cross section in soft rock. In addition, the slope of the disposal tunnel is the slope necessary for the discharge of spring water. The disposal hole is a hole formed by excavating the floor surface of the disposal tunnel, and is provided at regular intervals. This disposal tunnel can be buried after the radioactive waste is buried, and then the entire underground tunnel is closed.

  The glass solid is encapsulated in an overpack, then placed in a disposal hole, and a buffer material is laid around it. The overpack is a metal container intended to physically contain groundwater so that it does not come into contact with the vitrified body until the radioactivity and calorific value of the vitrified body are attenuated to some extent. The cushioning material is composed of compacted bentonite-based soil material mixed with bentonite and sand to physically protect the overpack (waste material) and radioactive even after the overpack confinement function is lost. It is expected to suppress nuclide migration (see, for example, Patent Document 1 and Non-Patent Document 1).

  By the way, it is requested | required that a buffer material swells with the approach of groundwater, and can fill the clearance gap between the surrounding rock mass at the time of overpack placement, and the clearance gap produced in the buffer material. For this reason, after the overpack (waste body) is placed in the disposal hole and the buffer material is laid around it, if the groundwater seeps out from the rock surface of the disposal hole, the buffer material swells, as shown in FIG. In addition, the cushioning material 15 bulges from the floor surface of the disposal mine shaft 12. In such a situation, the cushioning material 15 cannot maintain a predetermined density, and there is a concern that the water shielding performance is deteriorated. Moreover, when it swells extremely, there is a concern that the waste body 16 cannot be supported at the correct position.

  In order to prevent such a situation, as shown in FIG. 6, a water blocking film 17 is laid between the disposal hole (saddle hole) 13 and the buffer material 15, or as shown in FIG. It is conceivable to install a reaction force column 18 between the disposal tunnel 12 or to fix a lid 19 covering the buffer 15 with an anchor 20 as shown in FIG.

JP 2007-319732 A Japan Nuclear Power Environment Organization “Technology and Safety of High-Level Radioactive Waste Geological Disposal” [online] May 2004 publication: Reference Room / Technology / Technical Explanation [Search April 10, 2008] Internet < URL: 1210658787861_0.html>

  However, in these proposals, it is necessary to lay or install something that is not directly related to the burial of the waste body, such as laying the water blocking film 17, installing the reaction force column 18, and installing the anchor 20. , Will be more burdensome. In particular, when a reaction force column is installed, the use of the disposal tunnel is restricted, which is not preferable.

  The present invention has been made in view of the above, and is a buffer material capable of suppressing bulging at least until the disposal tunnel is backfilled, and a burial of waste body capable of suppressing bulging of the buffer material at least until the disposal tunnel is backfilled It aims to provide a method.

In order to solve the above-described problems and achieve the object, the cushioning material according to the present invention is a cushioning material laid between the waste body and the disposal hole in a manner surrounding the waste body placed in the disposal hole. The electrolyte is added to the clay-based soil material that swells by absorbing water so that the concentration of the electrolyte that suppresses the swelling of the clay-based soil material is 0.3 mol / l or more in a predetermined period. And

  In the above invention, the buffer material according to the present invention is characterized in that the electrolyte is sodium chloride.

  Moreover, the buffer material according to the present invention is characterized in that, in the above invention, the electrolyte is calcium chloride.

The method for burying and disposing of a waste body according to the present invention is a method for burying and disposing of a waste body in which a waste body is placed in a disposal hole and a buffer material is laid between the waste body and the disposal hole in a manner surrounding the waste body. In addition, a buffer material can be obtained by adding an electrolyte to the clay-based soil material that swells by absorbing water so that the concentration of the electrolyte that suppresses the swelling of the clay-based soil material is 0.3 mol / l or more in a predetermined period. Then, it is laid between the disposal hole and the waste body.

  Further, in the above invention, the waste disposal method according to the present invention is the method of adding the electrolyte, in which an electrolyte solution in which the electrolyte is dissolved in water as a solvent is prepared in advance, and the powder or It is characterized by being added to a granular clay-based soil material, kneaded, and then compacted to a predetermined density to obtain a buffer material.

In addition, the method for burying and disposing of a waste body according to the present invention embeds the waste body by placing a waste body in the disposal hole and laying a buffer material between the waste body and the disposal hole in a manner surrounding the waste body. In the disposal method, after adding the electrolyte that suppresses the swelling of the clay-based soil material to the clay-based soil material that swells by absorbing water, the buffer material is laid between the disposal hole and the waste body, As a method for adding the electrolyte, the granular electrolyte and the powder or granular clay-based soil material are previously kneaded and homogenized, and then added with predetermined moisture and kneaded. It is characterized by using as a buffer material by compacting to a density of.

The buffer material according to the present invention has an electrolyte so that the concentration of the electrolyte that suppresses swelling of the clay-based soil material in the clay-based soil material that swells by absorbing water becomes 0.3 mol / l or more in a predetermined period. Since added, the swelling of the clay-based soil material can be suppressed until the electrolyte diffuses. For this reason, the expansion of the buffer material can be suppressed at least until the disposal tunnel is backfilled.

In the waste disposal method according to the present invention, the concentration of the electrolyte that suppresses the swelling of the clay-based soil material in the clay-based soil material that swells by absorbing water is 0.3 mol / l or more in a predetermined period. Since the buffer material is formed by adding an electrolyte and then laid between the disposal hole and the waste body, the swelling of the clay-based soil material can be suppressed until the electrolyte diffuses. For this reason, the bulging of the buffer material can be suppressed until the disposal tunnel is backfilled after the disposal of the waste body.

  Embodiments of a buffer material and a waste disposal method according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

  The cushioning material described in this embodiment is laid between a waste body enclosing radioactive waste and a disposal hole in which the waste body is embedded. Radioactive waste is generated with the use of nuclear power at nuclear power plants, and after being treated in an appropriate manner so that it does not have a significant impact on the human environment, Will be disposed of. For example, high-level radioactive waste (high-level radioactive waste) remaining after separating useful materials such as uranium and plutonium from spent fuel by reprocessing is solidified with glass and then cooled for 30 to 50 years. For storage and then buried deep underground.

  FIG. 1 is a conceptual diagram showing an embedding disposal facility for embedding radioactive waste. As shown in FIG. 1, the buried disposal facility is a facility for embedding radioactive waste solidified with glass (hereinafter referred to as “glass solidified body 1”), and has a disposal tunnel 2 and a disposal hole 3. . The disposal mine 2 is an underground mine provided deep underground. The hard rock base has a hood-type cross section and the soft rock base has a three-centric cross section. Moreover, the gradient of the disposal tunnel 2 is a gradient necessary for the drainage of spring water. The disposal holes 3 are dredging holes excavated from the floor surface of the disposal tunnel 2 and are provided at regular intervals.

  After the glass solid body 1 is sealed in the overpack 4, it is placed in the disposal hole 3, and the cushioning material 5 is laid around it. The overpack 4 is a metal container for the purpose of physically containing groundwater so that it does not come into contact with the glass solidified body 1 until it decays to the extent that the radioactivity and heat generation amount of the glass solidified body 1 exists. In consideration of the radioactivity and heat generation characteristics of the vitrified body 1, a service life of 1000 years is set. The overpack 4 is assumed to have a cylindrical shape from the viewpoint of the vitrified body 1 to be accommodated in a cylindrical shape and pressure resistance. In addition, what sealed the glass solidified body 1 in the overpack 4 is called "waste body 6" in the following description.

  The buffer material 5 is intended to physically protect the waste body 6 and to suppress the migration of the radionuclide even after the confinement function of the overpack 4 is lost. Functions such as sorption of dissolved nuclides and prevention of colloid migration are required. The buffer material 5 is composed of bentonite mixed with silica sand as an aggregate.

  FIG. 2 is a graph showing the swelling rate of bentonite (reference documents: Nakashima, H., Ishi, T. Nakahata, A., Tanabe, H., Tamura, S .: swelling deformation of compacted bentonite / sand mixture, Clay Science for Engineering, Adachi & Fukue (eds), Balkema, Rotterdam ISBN 90 5809 1759). This figure shows the swelling rate (expansion amount (change in height) as a ratio to the height of the test specimen) when fresh water is absorbed into bentonite compressed to a dry density of 1600 kg / m3. The horizontal axis indicates time, and the vertical axis indicates the swelling rate. Moreover, the numerical value described in the figure is the value of the overload, for example, it shows that the pressure expands to about 1.9 times even when receiving the pressure due to the overload of 78.5 kPa. ing.

  FIG. 3 is a diagram showing the swelling characteristics of bentonite (reference: Maeda et al., Basic properties of calcium-type and calcium-type bentonite, PNC TN841098-021, Power Reactor / Nuclear Fuel Development Corporation, 1998). In this figure, the horizontal axis represents the dry density and the vertical axis represents the swelling pressure. According to this figure, bentonite exhibits a higher swelling pressure as the density increases.

  Such swelling characteristics of bentonite are due to the water-absorbing and expanding action of a clay mineral called montmorillonite contained in bentonite. When the dry density of bentonite is high, the montmorillonite contained in the material has a large swelling pressure. This is because there is little space that can expand around the particles.

  It is understood that the mechanism of water absorption expansion of bentonite is the osmotic pressure of water existing between crystal layers. That is, if the electrolyte concentration of the water of montmorillonite particles (pore water of the soil material) is low, the osmotic pressure of the water taken in between the crystal layers is large, and as a result, the swelling pressure increases, whereas the electrolyte concentration is high The osmotic pressure of water taken in between the crystal layers is small, and as a result, the swelling pressure is small.

  Figure 4 shows the relationship between the distance between the crystal layers of montmorillonite and the electrolyte concentration (reference: Philip F. LOW, Interparticle Forces in Clay Suspensions: Flocculation, Viscous Flow and Swelling, p. 163 cms workshop lectures, Volume 4 Clay-Water Interface and its Rheological Implications, 1992, N. Guven, RMPollastro Editors, published by CLAY MINERALS SOCIETY. In this figure, the horizontal axis represents NaCl concentration (electrolyte concentration), and the vertical axis represents crystal interlayer distance. According to this figure, in the crystal structure of the montmorillonite mineral, it is shown that the interlayer distance of the crystal having a multilayer structure varies depending on the NaCl concentration (electrolyte concentration) of water. In addition, the interlayer distance of the montmorillonite crystal when there is no water around is assumed to be about 1.8 nm.

  According to this figure, when water whose NaCl concentration (electrolyte concentration) is smaller than 0.3 mol / l is present around the montmorillonite mineral, the interlayer distance is expanded to about three times that when water is not present in the surroundings. Will do. On the other hand, when water having a NaCl concentration (electrolyte concentration) greater than 0.3 mol / l is present around the montmorillonite mineral, the interlayer distance hardly increases.

  Considering the characteristics of bentonite described above, the buffer material 5 according to the present embodiment has a NaCl concentration (electrolyte concentration) of water around the montmorillonite mineral contained in the bentonite of 0.3 mol to 1.4 mol. It is assumed that NaCl (electrolyte) is added.

The specifications of the cushioning material are shown below.
-Bentonite = Na type bentonite-Aggregate: Silica sand = Silica sand No. 1 and Silica sand No. 5 are mixed at a ratio of 1: 1-Bentonite and Keisa mixing ratio = 70%: 30%
・ Dry density = 1600kg / m3
・ Vacuum volume ratio of buffer material = about 40%
Electrolyte content = 400 mol / m3 = 400 mol / 1600 kg
(In the case of NaCl, 58 g / mol × 400 mol / 1600 kg = 23.2 kg / 1600 kg = 1.45%)

  When fresh water soaks into the buffer material of this specification, the amount of water present in the gap volume of the buffer material 1 m3 corresponds to 0.4 m3, the weight is 400 kg, and the volume is 400 l. Since the amount of NaCl is 23.2 kg, the concentration of pore water is 1 mol / l.

  As shown in FIG. 4, when the NaCl concentration (electrolyte concentration) of the surrounding water is 1 mol / l, the interlayer distance of the montmorillonite mineral is 2.0 nm, which is almost the same as 1.8 nm, so that bentonite does not swell. It will be.

  The buffer material 5 according to the present embodiment described above is compressed and shaped to a predetermined density and then laid between the waste body 6 and the disposal hole 3 (block method). And, for a predetermined period (for example, a period until the waste is buried in all the disposal holes provided in the disposal tunnel, the waste is buried in all the disposal holes provided in the disposal tunnel constituting one section. And the period until the safety of the buried disposal facility is confirmed), the disposal tunnel 2 is monitored without being backfilled.

  According to the buffer material 5 which is this Embodiment mentioned above, swelling is suppressed until NaCl concentration (electrolyte concentration) becomes smaller than 0.3 mol from laying. Therefore, if the period until the NaCl concentration (electrolyte concentration) becomes lower than 0.3 mol is longer than the predetermined period described above, the buffer material swells from the floor surface of the disposal tunnel 2 during the predetermined period. There is no. In the state where the swelling of the cushioning material 5 is suppressed, the water shielding performance of the cushioning material 5 is not sufficiently exhibited, but the overpack 4 has sufficient water shielding properties in a short period (1000 years) after the disposal of the waste body 6. Safety is ensured because of its performance.

  Then, after a predetermined period of time, the disposal tunnel 2 is backfilled. The NaCl (electrolyte) in the buffer material gradually diffuses and disappears immediately after laying toward the surrounding groundwater. Here, the permeability coefficient of the compressed bentonite is about 1E-13 m / s, which is significantly smaller than a general soil material, so that the water shielding performance is excellent, but the diffusion coefficient is about 1E-10 m2 / s. Since it is about 1/10 of the diffusion coefficient in free water, it diffuses into the ground water on the natural ground side at a fairly high speed. Eventually, the NaCl (electrolyte) concentration in the buffer material becomes smaller than 0.3 mol / l, so that the intermonte distance of montmorillonite becomes prominent and the swelling characteristics increase as a result. If it becomes such a state, although the buffer material 5 will bulge from the disposal hole 3, since the disposal tunnel 2 has already been refilled, the buffer material 5 cannot expand and the swelling pressure is exerted. Maintained at. Therefore, the buffer material 5 can exhibit the original water absorption expansion performance and water shielding performance.

  Although the buffer material 5 which is embodiment mentioned above shall add sodium chloride (NaCl) as an electrolyte, it may replace with sodium chloride and may add calcium chloride (CaCl).

  In addition, although the electrolyte is directly added to the buffer material 5, an electrolyte solution in which the electrolyte is dissolved in water as a solvent is prepared in advance and added to the powdery or granular clay-based soil material. The cushioning material 5 may be obtained by kneading and then compacting to a predetermined density.

  Furthermore, the granular electrolyte and the powdery or granular clay-based soil material are previously kneaded and homogenized, and then kneaded after adding a predetermined amount of water and then compacting to a predetermined density. The cushioning material 5 may be used.

  The buffer material 5 according to the embodiment described above is compression-molded to a predetermined density and then laid between the waste body 6 and the disposal hole 3, but the buffer material 5 is filled around the waste body 6. It is good also as what lays down compactly.

It is a conceptual diagram which shows a burying disposal facility. It is a figure which shows the swelling rate of a bentonite. It is a figure which shows the swelling characteristic of a bentonite. It is the figure which showed the relationship between the crystal interlayer distance of montmorillonite, and electrolyte concentration. It is sectional drawing which shows the burying disposal facility which shows the state which the buffer material swelled from the floor surface of the disposal mine shaft. It is sectional drawing which shows the burying disposal facility which laid the water stop film between the disposal hole and the buffer material. It is sectional drawing which shows the burying disposal facility which installed the reaction force column between the buffer material and the disposal tunnel. It is sectional drawing which shows the burying disposal facility which fixed the cover body which covers a buffer material with an anchor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Vitrified body 2 Disposal tunnel 3 Disposal hole 4 Overpack 5 Buffer material 6 Waste body

Claims (6)

A cushioning material laid between the waste body and the disposal hole in a manner surrounding the waste body placed in the disposal hole, and swelling the clay-based soil material to the clay-based soil material that swells by absorbing water A buffer material , wherein an electrolyte is added so that a concentration of the electrolyte to be suppressed is 0.3 mol / l or more in a predetermined period .   The buffer material according to claim 1, wherein the electrolyte is sodium chloride.   The buffer material according to claim 1, wherein the electrolyte is calcium chloride. In the method of embedding disposal of a waste body in which a waste body is placed in the disposal hole and a buffer material is laid between the waste body and the disposal hole in a manner surrounding the waste body.
After the electrolyte is added to the clay-based soil material that swells by absorbing water so that the concentration of the electrolyte that suppresses the swelling of the clay-based soil material is 0.3 mol / l or more in a predetermined period, the buffer material is obtained. A method for burying and disposing of waste, characterized in that it is laid between the disposal hole and the waste.   As a method of adding the electrolyte, an electrolyte solution in which the electrolyte is dissolved in water as a solvent is prepared in advance, and it is added to a powdery or granular clay-based soil material and kneaded, and then has a predetermined density. The waste disposal method according to claim 4, wherein the cushioning material is formed by compaction. In the method of embedding disposal of a waste body in which a waste body is placed in the disposal hole and a buffer material is laid between the waste body and the disposal hole in a manner surrounding the waste body.
In the method of laying between the disposal hole and the waste body after adding the electrolyte that suppresses the swelling of the clay-based soil material to the clay-based soil material that swells by absorbing water,
As a method for adding the electrolyte, the granular electrolyte and the powder or granular clay-based soil material are previously kneaded and homogenized, and then added with predetermined moisture and kneaded. A method for burying and disposing of a waste body characterized in that it is made into a cushioning material by compacting to a density of.

Images