JP4691526B2 - Radioactive waste disposal method - Google Patents

Description

  The present invention relates to a method for treating radioactive waste in which radioactive waste generated from a nuclear power plant or nuclear facility is enclosed in a container, and more particularly, radioactive material having a relatively high radioactivity level exceeding the concentration upper limit value of current laws and regulations. The present invention relates to a waste disposal method.

  Radioactive waste generated from nuclear power plants and facilities related to nuclear power is stored in containers at the power plants and facilities, and then transported and buried in a radioactive waste burial site.

  At that time, it is considered that radioactive waste having a relatively high radioactivity level exceeding the upper limit of concentration of current laws and regulations is sealed and sealed in a container in order to prevent the radioactive material from scattering. Such radioactive waste having a relatively high radioactivity level exceeding the upper limit of concentration of current laws includes radioactive metals and used resins stored in pools.

  For example, the activated metal is cut into a size that fits in the container and enclosed in the container. At that time, since the activation metal is stored in the pool, the surface is wet with water, and if not subjected to a drying treatment, the water is enclosed inside the container together with the activation metal.

  In addition, even if the activation metal is dried and stored in a container, in the processing method in which the gap is filled with sand, the sand absorbs moisture in the air, so that the sand is dried and the moisture is contained in the air. If not blocked, water is enclosed inside the container along with the activated metal.

  Furthermore, it is also conceivable that the filler in the gap between the activated metal and the container is filled with cement paste or mortar. Cement paste or mortar is obtained by kneading cement or aggregate and water, and has fluidity for a while after kneading, so that it can be filled in the gap. The cement paste or mortar after hardening has a structure composed of cement hydrate or aggregate and pores when viewed microscopically, and usually contains water in the pores. This is because in order to ensure the initial fluidity, a cement paste or mortar is produced using a larger amount of water than the amount of water used as cement hydrate. Therefore, also in this case, water is enclosed inside the container together with the activated metal.

  The used resin is stored in a tank containing water, and various methods for treating the resin itself are conceivable. Even if moisture is removed during the treatment process, water is sealed inside the container together with the used resin or the treated product, as in the case of the activated metal.

  The water sealed in the container is decomposed into hydrogen and oxygen by the radiation emitted from the radioactive waste. The generated hydrogen accumulates inside the container and increases the hydrogen concentration. The hydrogen concentration inside the container is ultimately proportional to the amount of water sealed inside the container. If the amount of water is small and the space inside the container is large, almost all of the water can be converted into hydrogen and oxygen.

  Assuming that the allowable hydrogen concentration is 4 vol%, which is the explosion limit, for example, when a 200 L drum can is used as a container, the hydrogen generation amount must be 200 [L] × 4 [%] = 8 L or less. In the radiolysis of water, 1 mol of hydrogen is generated from 1 mol of water. Therefore, when the amount of water generating 8 L of hydrogen is determined, 8 [L] /22.4 [L / mol] × 18 [g / mol] = 6.4g.

  Considering the case where the activated metal is sealed in a container, for example, the amount of water can be about 300 kg per container, so that the moisture content per weight of the activated metal is about 20 ppm or less. Without treatment, the hydrogen concentration inside the container will exceed the explosion limit. Actually, because the volume of the activated metal causes the space volume inside the container to be less than 200 L, the moisture content required for the drying process is less than 20 ppm.

  In addition, even if a drying process is performed to completely remove moisture from the activated metal, considering the case where the gap is filled with sand in order to fix the activated metal to the container, the filling sand requires about 50 kg to 100 kg. Therefore, if the moisture content per weight of the sand is not 130 ppm or less in the case of 50 kg of sand and 60 ppm or less in the case of 100 kg of sand, the hydrogen concentration inside the container becomes the explosion limit or more.

  Actually, since the space volume inside the container is less than 200 L due to the volume of the activated metal and sand, the moisture content required for the drying treatment is smaller than the above-mentioned value. Since actual sand absorbs moisture in the air, sand having such a low water content cannot be obtained unless it is treated in dry gas after being dried.

  Further, consider the case where the gap is filled with cement paste or mortar in order to fix the activated metal to the container. Water contained in the hardened cement paste and mortar pores undergoes radiolysis to generate hydrogen. However, the generated hydrogen permeates through the pores and flows out from the surface layer to the upper space, increasing the hydrogen concentration in the upper space. In general, since the pores have a size of about 10 nm to 100 nm, gas permeation is unlikely to occur, and hydrogen flowing out into the upper space is limited to hydrogen generated in the pores near the surface layer. Accordingly, the hydrogen concentration in the upper space is known to be high although the hydrogen concentration in the upper space cannot be easily estimated as in the case where no filler is used or when the sand is filled.

  In this way, if the radioactive waste is sealed in the container, the hydrogen concentration inside the container is increased by the hydrogen generated from the water attached to the radioactive waste and the filler, so it is strictly inspected that the container is completely sealed. However, if a large facility for removing water adhering to the radioactive waste or the filler is not used to suppress the hydrogen concentration, there is a risk of hydrogen explosion.

  On the other hand, a method of reducing the hydrogen concentration by reacting and removing the generated hydrogen is also conceivable. As a method of removing hydrogen by reacting, when radioactive waste is sealed in a container using cement, a catalyst is installed between the hardened cement and the container lid, and the generated hydrogen reacts with oxygen. Thus, a method has been proposed in which water is converted to water, the hydrogen concentration inside the container is kept low, and the risk of explosion caused by the increase in hydrogen concentration is avoided (see, for example, Patent Document 1).

  The catalyst used is housed in a brown SUS wire mesh. In order to reduce the hydrogen concentration, the catalyst must be in contact with hydrogen. Since the reaction is suppressed when a water film is formed around the catalyst, the size of the wire mesh that does not form the water film is preferably 1.5 mm. From this, the size of the catalyst is also estimated to be larger than the size of the wire mesh 1.5 mm.

  The above prior art catalyst has a function of generating a reaction for generating water from hydrogen and oxygen. However, oxygen used in the reaction is oxygen present in the atmosphere inside the container, and it is estimated that the catalyst has no function of supplying oxygen.

Japanese Patent Laid-Open No. 2002-286893 (pages 2 to 3 and FIGS. 1 to 5)

  When hydrogen is reacted with oxygen to form water, it adheres to the surface of the catalyst, and if it is reacted in a large amount, the water film covers the catalyst itself and suppresses the reaction.

  Also, even if oxygen inside the container reacts with a large amount of hydrogen, if the oxygen inside the container disappears, the reaction of hydrogen to oxygen and water cannot be continued, and the hydrogen concentration inside the container must be kept low. I could not.

  In the radiolysis of water, hydrogen and hydrogen peroxide are mainly generated from water, and oxygen is hardly generated. Therefore, even if hydrogen is reacted with oxygen by a catalyst to make water, if the oxygen inside the container is consumed, the hydrogen concentration inside the container cannot be kept low.

An object of the present invention suppresses the hydrogen produced by radiolysis of the container inside the water, is to provide a method of processing radioactive waste to seal the radioactive waste in the container.

  In order to solve the above-mentioned problems, the present invention proposes a method for treating radioactive waste in which a powdered catalyst that promotes the reaction of forming water from hydrogen and oxygen is put together with the radioactive waste into a container.

  When a powdered catalyst is used, the surface area per unit weight of the catalyst is increased, and even if a large amount of reaction occurs and a large amount of water is generated, the surface area of the catalyst is large, so that the entire surface of the catalyst is covered with a water film. Disappear. The size of such a powdery catalyst is desirably 0.1 mm or less.

  The present invention also provides a method for treating radioactive waste, which is a non-catalyst filler, for example, a mixture of a chemically stable sand, a hardened cement paste, and a catalyst and a filler as a filler and is put into the container together with the radioactive waste. suggest.

  Compared to the case where only the catalyst is used as the filler, the amount of the catalyst required can be reduced.

  At that time, when a mixture in which a filler that is not a catalyst and a catalyst is uniformly mixed is used, even if a space is generated inside the filler, hydrogen can be prevented from staying there.

  On the other hand, light hydrogen flows from the bottom to the top inside the container if there is no space inside the filler and there is no space, so it is necessary to increase the catalyst concentration in the mixture at the surface layer of the filler. The amount of catalyst can be reduced.

  More specifically, the present invention proposes a method for treating radioactive waste using a catalyst obtained by loading an active ingredient that promotes the reaction of forming water from hydrogen and oxygen with a powdery stable compound.

  The size of the stable compound is desirably 0.1 mm or less. Since the active ingredient is significantly smaller than the size of the stable compound, the size of the catalyst can be reduced to 0.1 mm or less.

  The powdery stable compound that carries the active ingredient that promotes the reaction of forming water from hydrogen and oxygen is a stable compound that absorbs and releases oxygen depending on the oxygen concentration in the atmosphere. A stable compound that absorbs or releases oxygen releases oxygen even if the oxygen inside the container is consumed, so hydrogen and this oxygen can be used to form water, reducing the hydrogen concentration inside the container. Can hold. An oxide containing at least a rare earth element is used as such a stable compound.

  In the present invention, since radioactive waste is sealed in a container and treated with a catalyst or a mixture of a catalyst and a non-catalyst as a filler, waste with a relatively high radioactivity level is generated. Even in this case, the hydrogen concentration inside the container can be kept low.

  When a stable compound that absorbs or releases oxygen according to the oxygen concentration in the atmosphere is used as a powdery stable compound that supports an active ingredient that promotes a reaction to form water from hydrogen and oxygen, oxygen inside the container Even when the gas is consumed, oxygen is released, so that the hydrogen concentration inside the container can be kept low.

  Next, with reference to FIGS. 1-3, the Example of the processing method of the radioactive waste by this invention is described.

  FIG. 1 is a cross-sectional view of a container showing Example 1 of a radioactive waste processing method according to the present invention.

  The radioactive waste 3 cut into a size and length that fits in the container is placed in the container 1. Next, the powdered catalyst 4 that promotes the reaction of forming water from hydrogen and oxygen is charged. Finally, the lid 2 is closed and the seam between the container 1 and the lid 2 is welded and sealed.

  The powdered catalyst 4 that promotes the reaction of forming water from hydrogen and oxygen was impregnated with a solution containing platinum equivalent to 0.5% of the cerium weight in a cerium oxide having a size of 0.1 mm or less and dried. Thereafter, the catalyst was obtained by holding at a temperature of 500 degrees for 1 hour.

  A non-radioactive waste was placed in a container 1 and filled with a catalyst 4 that promoted the reaction of forming water from hydrogen and oxygen, the lid 2 was closed, and the container 1 was sealed. At that time, the catalyst 4 contained about 5 g of water. Originally, gamma rays of cobalt 60 of 300 Gy per unit time were irradiated from the outside in order to simulate the radiation dose emitted by the radioactive waste 3. The hydrogen concentration in the container after 24 hours was 0.015 vol%.

  As a reference sample, when a cerium oxide having a size of 0.1 mm or less is not impregnated with platinum and filled with a powder obtained by holding at a temperature of 500 ° C. for 1 hour, the hydrogen concentration in the container after 24 hours is It became 0.11 vol%.

  The hydrogen concentration inside the container is 0.015 / 0.11 = about 1/7 when filled with the catalyst 4 that promotes the reaction of forming water from hydrogen and oxygen, as compared to the reference sample not filled with this catalyst. Met.

  In the case where the volume of the container is 200 L, if the allowable value of the hydrogen concentration is 4 vol%, the allowable water content is 6.4 g. On the other hand, from the above test results, the allowable moisture amount when using the catalyst of the present invention that promotes the reaction of forming water from hydrogen and oxygen is about 45 g, which is 7 times.

  Therefore, according to the radioactive waste processing method of the first embodiment, the drying treatment of the radioactive waste 3 can be simplified. If the equipment has the same drying capacity per unit time, the drying processing time can be shortened to 1/7. Alternatively, simple equipment having a somewhat low drying capacity per unit time can be used.

  In Example 1, a solution containing platinum corresponding to 0.5% of cerium weight in cerium oxide having a size of 0.1 mm or less is used as the powdery catalyst 4 for promoting the reaction of forming water from hydrogen and oxygen. After impregnation and drying treatment, a catalyst obtained by holding at 500 ° C. for 1 hour is used.

  The catalyst that can be used in the present invention is not limited to this. That is, instead of cerium oxide, a rare earth element oxide or a complex oxide of a rare earth element and another metal may be used as a compound that absorbs or releases oxygen.

  In addition, as an active ingredient for supporting a powdery stable compound, palladium, an alloy of platinum and palladium, an alloy of platinum and rhenium, etc., which promote water-forming reaction from hydrogen and oxygen, instead of platinum Anything can be used.

  Further, the amount of the active ingredient to be supported is also in the range of 0.01% to 30%, and is not limited to 0.5%.

  The holding temperature and time after the drying treatment are not limited to 500 degrees or 1 hour. It is sufficient that the active ingredient is not easily separated from the powdery stable compound.

  The powdery stable compound may be a compound that does not absorb or release oxygen as long as there is sufficient oxygen inside the container consumed by the hydrogen to be reacted. Aluminum oxide can be used as such a compound.

  Thus, if the radioactive waste 3 is sealed in a container using the catalyst 4 according to the first embodiment as a filler, even when processing a waste having a relatively high radioactivity level with a large amount of hydrogen generation, the inside of the container is further improved. Even after the oxygen is consumed, the hydrogen concentration inside the container can be kept low.

  FIG. 2 is a cross-sectional view of a container showing a second embodiment of the method for treating radioactive waste according to the present invention.

  In this second embodiment, instead of the powdery catalyst 4 of the first embodiment, a mixed material obtained by uniformly mixing a powdery catalyst 4 that promotes a reaction for forming water from hydrogen and oxygen and sand 5 is used as a filler. . The powdery catalyst 4 and the sand 5 are mixed in advance uniformly, filled in the container 1 containing the radioactive waste 3, the lid 2 is closed, and the container 1 and the lid 2 are sealed.

  The mixing ratio of the powdered catalyst 4 and the sand 5 may be arbitrary. However, the smaller the ratio of the catalyst that promotes the reaction for forming water from hydrogen and oxygen, the lower the hydrogen removal capability, but the amount of catalyst used can be reduced compared to Example 1.

  In Example 2, sand 5 was used as a compound having no catalytic function. The present invention is not limited to this sand. Instead of the sand 5, any material that is chemically stable and fine enough to fill the gaps in the radioactive waste 3 may be used.

  Further, instead of the sand 5, a substance that forms a cured product may be used. As such a substance, cement paste or mortar can be used.

  Thus, if the mixture which mixed the catalyst 4 of this Example 2 and the substance 5 which is not a catalyst uniformly is used as a filler, and the radioactive waste 3 is sealed and processed in a container, the hydrogen concentration inside the container Can be kept low.

  FIG. 3 is a cross-sectional view of a container showing a third embodiment of the method for treating radioactive waste according to the present invention.

  In this third embodiment, instead of the powdery catalyst 4 of the first embodiment, a mixed material in which the powdery catalyst 4 that promotes the reaction of forming water from hydrogen and oxygen and the sand 5 are mixed unevenly is used as a filler. Use. The container 1 containing the radioactive waste 3 is filled with sand 5, and the powdered catalyst 4 that promotes the reaction of forming water from hydrogen and oxygen is filled so as to cover the surface. Thereafter, the lid 2 is closed, and the container 1 and the lid 2 are sealed.

  Naturally, the radiolysis of water occurs where water is present and generates hydrogen. When water is unevenly distributed, the catalyst present at a position away from the location does not contribute to the function of reacting hydrogen with oxygen. Since hydrogen is light, it moves upward from the bottom of the container 1, passes through the surface layer of the filler, and reaches the upper space. Therefore, if the catalyst 4 that promotes the reaction of forming water from hydrogen and oxygen is arranged on the surface as in Example 3, the proportion of the catalyst 4 that contributes to the function of reacting hydrogen with oxygen can be increased. it can.

  In Example 3, sand 5 was used as a compound having no catalytic function. As in the second embodiment, instead of the sand 5, a chemically stable substance or a substance that forms a cured product that is fine enough to fill the gaps in the radioactive waste 3 may be used.

  In Example 3, the catalyst 4 that promotes the reaction of forming water from hydrogen and oxygen is disposed only on the surface layer. However, the catalyst 4 may be disposed such that the ratio of the catalyst 4 increases from the inside toward the surface layer.

  According to the third embodiment, if the radioactive waste 3 is sealed in a container and treated using a mixture in which the catalyst 4 and the non-catalyst substance 5 are mixed inhomogeneously as the filler, the hydrogen inside the container is treated. The concentration can be kept low.

It is sectional drawing of the container which shows Example 1 of the processing method of the radioactive waste by this invention. It is sectional drawing of the container which shows Example 2 of the processing method of the radioactive waste by this invention. It is sectional drawing of the container which shows Example 3 of the processing method of the radioactive waste by this invention.

Explanation of symbols

1 Container 2 Lid 3 Radioactive waste 4 Powdered catalyst 5 Sand (non-catalyst substance)

Claims (8)

In a radioactive waste processing method of enclosing a radioactive waste and a filler filled in a gap between the radioactive waste in a container,
A method for treating radioactive waste, wherein the filler is a powdered catalyst that promotes a reaction of forming water from hydrogen and oxygen. In a radioactive waste processing method of enclosing a radioactive waste and a filler filled in a gap between the radioactive waste in a container,
A method for treating radioactive waste, wherein the filler is a mixture of a powdery catalyst that promotes a reaction of forming water from hydrogen and oxygen and a chemically stable substance. In the radioactive waste processing method of Claim 2,
The method for treating radioactive waste, wherein the filler is a heterogeneous mixture in which the catalyst ratio in the surface layer is larger than the catalyst ratio in the non-surface layer. In a radioactive waste processing method of enclosing a radioactive waste and a filler filled in a gap between the radioactive waste in a container,
The method for treating radioactive waste, wherein the filler is a mixture of a powdery catalyst that promotes a reaction of forming water from hydrogen and oxygen and a substance that forms a cured product. In the radioactive waste processing method of Claim 4,
The method for treating radioactive waste, wherein the filler is a heterogeneous mixture in which the catalyst ratio in the surface layer is larger than the catalyst ratio in the non-surface layer. In the processing method of the radioactive waste as described in any one of Claims 1 thru | or 5,
A method for treating radioactive waste, wherein the catalyst is a catalyst in which an active ingredient that promotes a reaction of forming water from hydrogen and oxygen is supported on a powdery stable compound. In the radioactive waste processing method of Claim 6,
The method for treating radioactive waste, wherein the powdery stable compound is a stable compound that absorbs or releases oxygen in accordance with an oxygen concentration in an atmosphere. In the radioactive waste processing method of Claim 7,
The method for treating radioactive waste, wherein the powdery stable compound is an oxide containing at least a rare earth element.

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