JPS63115099A - Method of processing radioactive waste - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Field of Industrial Application) This invention relates to a method for disposing of radioactive waste generated in facilities handling radioactive materials such as nuclear fuel reprocessing facilities. The present invention relates to a method for manufacturing a pellet-shaped intermediate storage body for radioactive waste that has excellent stability, durability, and fire resistance over a wide range of conditions, and for processing medium- to low-level radioactive waste.

(Prior art) Most of the radionuclides contained in radioactive waste such as concentrated liquid waste and sludge generated from spent nuclear fuel reprocessing facilities are fission products, and some of them have long half-lives. For example, the half-life of C8 is 30 years, so
300 years to reduce radioactivity to 1/1000, 1/
It would take 600 years to reach 1,000,000.

Currently, medium- and low-level radioactive waste generated from reprocessing facilities is treated using the asphalt solidification method. However, even if a solidified body of radioactive waste is obtained through solidification treatment in this manner, a final disposal method has not yet been established for radioactive waste containing long half-life nuclides such as those mentioned above. is the current situation.

Therefore, until standards for the treatment of these radioactive wastes are established, a method is being considered in which the radioactive wastes are processed into pellets and temporarily stored in the form of stable intermediate storage bodies. As a pelletizing technology for radioactive waste in which radionuclides have a relatively short lifespan, a method has already been proposed in which wastes such as concentrated waste liquid and sludge generated from BWR nuclear power plants are temporarily stored as intermediate storage. According to this method, compressive force is applied to the powdered radioactive waste after drying to pelletize it, resulting in a high volume reduction rate.

In addition, in the case of radioactive waste from the BWR nuclear power plant, the radionuclides contained in it have a relatively short lifespan, so the radioactivity in the pellets decays during temporary storage, and it also decreases again in the processing of the pellets after storage. It has an excellent feature that it can be easily processed.

For radioactive waste with a long half-life generated from reprocessing facilities, after pelletizing, the radioactive waste pellets are fixed integrally with a filler made of an inorganic material, for example, as a solid package in a 200 g drum. Methods to make this possible are being considered. This is because radioactive waste generated from reprocessing facilities has a long half-life, so it is impossible to attenuate the radioactivity through temporary storage, and treatments such as re-reducing the volume of radioactive waste pellets after storage are impossible. This is difficult because solidified radioactive waste is required to be chemically and mechanically stable for hundreds of years or more.

(Problems to be Solved by the Invention) In the conventional method of forming intermediate storage pellets to treat radioactive waste, the dry powder of radioactive waste liquid is molded into pellets only by compression. There is no strong chemical bonding force, and the pellet itself has no strength. Therefore, there is a problem in that the pellet breaks during storage or transportation, and the radioactive powder is scattered. Furthermore, in order to use the pellet as a final disposal material, it is necessary to immobilize it as an assimilate package using an inorganic filler, etc., and with pellets made only by compression molding, the moisture contained in the inorganic filler in the assimilate is absorbed by the pellet. This causes the pellet to swell and dissolve, resulting in adverse effects such as deterioration of the solidified material and generation of cracks.

This invention was made based on the above background,
The objective is to provide a method for disposing of radioactive waste by forming intermediate storage pellets with excellent long-term durability.

(Means for Solving the Problem) As a result of various studies on methods for processing radioactive waste, the inventors of the present invention have found that a binder is used when pelletizing radioactive waste, and in particular, long-term durability is achieved. The present inventors have found that it is effective to achieve the object of the present invention by using a binder made of an inorganic substance with excellent properties and treating it under predetermined conditions, leading to the completion of this invention.

That is, the radioactive waste treatment method of the present invention involves mixing radioactive waste with an inorganic binder and, if necessary, further adding a curing accelerator and/or a water resistance improver to the inorganic binder. The clay-like composition thus obtained is molded into pellets, the pellet-shaped molded product is left to stand at room temperature to harden, and the resulting molded product is heated at a heating temperature of 80° C. or higher.

In a preferred embodiment of this invention, the radioactive waste is powder,
It consists of at least one type of concentrated waste liquid and sludge.

In a preferred embodiment of the present invention, the inorganic binder may be made of at least one of hydraulic cement containing calcium silicate and calcium aluminate as main components, alkali silicate, aluminum phosphate, and phosphate glass. .

In a preferred embodiment of the present invention, the curing accelerator is magnesium aluminate metasilicate, dihydric aluminum tripolyphosphate, an inorganic phosphoric acid compound, fused magnesia,
Sintered magnesia, talc, fly ash, alumina cement, asbestos, NH4F.

Al (OH)3, M g OSM g (OH) 2
, Cab, and Ca (OH) 2 .

In a preferred embodiment of the present invention, the water resistance improver may contain at least one of talc and ultrafine silica powder with a particle size of 5 μm or less.

Hereinafter, the method for disposing of radioactive waste according to the present invention will be explained in more detail.

In this method, an inorganic binder and, if necessary, a curing accelerator and/or a water resistance improver for the inorganic binder are mixed with radioactive waste.

The inorganic binder used in this invention may be hydraulic cement containing calcium silicate and calcium aluminate as main components, alkali silicate, aluminum phosphate, phosphate glass, or a mixture thereof. be.

Among these inorganic binders, hydraulic cements containing calcium silicate and calcium aluminate as main components include Portland cement, alumina cement, and blast furnace cement. There are various alkali silicates such as sodium silicate and potassium silicate. Regarding the form, for example, sodium silicate includes seed plates such as powdered sodium silicate and liquid sodium silicate, which is also called water glass. Similarly, aluminum phosphate has various compounds,
There are forms such as powder and liquid. Phosphate glasses are currently available commercially, usually in powder form. When using a powdered inorganic binder, it is preferred to add an appropriate amount of water to obtain the desired clay-like composition.

When mixing the radioactive waste and the inorganic binder, a curing accelerator for the inorganic binder or a water resistance improver, or both thereof may be added.

Examples of the hardening accelerator that can be used in the present invention include magnesium aluminate metasilicate, aluminum dihydrogen tripolyphosphate, inorganic phosphate compound, fused magnesia, sintered magnesia, talc, fly ash, alumina cement, asbestos, NH4F, Al (
OI() 3, M g O1Mg (OH) Ca
There is one consisting of at least one type of Ca(OH)2. Further, the water resistance improver that improves the water resistance after curing includes at least one of talc and ultrafine silica powder with a particle size of 5 μm or less. Selection of these curing accelerators and clay-like compositions is arbitrary, but it is desirable to select them appropriately depending on the other components.

For example, in a hydraulic cement whose inorganic binder is mainly composed of calcium silicate and calcium aluminate, the hardening reaction is carried out by a hydration reaction with water, and there is no need to add a hardening accelerator.

Furthermore, ultrafine silica powder may be added to improve water resistance after curing. When an alkali silicate is used as an inorganic binder, the alkali silicate can be cured at room temperature by using magnesium aluminate metasilicate, aluminum dihydrogen tripolyphosphate, an inorganic phosphoric acid compound, or a mixture thereof. can. At this time, talc may be added as a water resistance improver. When using aluminum phosphate as an inorganic binder, fused magnesia, sintered magnesia, talc, fly ash, alumina cement, asbestos, NH4F.

AI (OH) 3, MgO5Mg (OH) 2,
Ca 2 O, Ca (OH) 2, etc. can be used as a curing accelerator. When using phosphate glass as an inorganic binder, no curing accelerator or water resistance improver is required, and after pellet formation, strength is developed by drying and heating, and this heating causes the glassy coating of phosphate glass to develop. A pellet with excellent water resistance can be obtained.

The clay-like composition obtained after mixing is then shaped into a pellet. Examples of this molding method include extrusion molding and compression molding.

The pellet-shaped molded product is left at room temperature for a certain period of time, during which time the inorganic binder in the pellet progresses a hardening reaction and gradually becomes stronger from clay-like. This standing time is preferably changed as appropriate depending on the curing speed of the inorganic binder.

After curing, the pellet is heated to a temperature of 80° C. or higher and maintained for a predetermined period of time. This is because water in the inorganic binder cannot be evaporated at temperatures below 80°C. It is desirable to change the temperature range of this temperature as appropriate depending on the object to be processed. For example, if the radioactive waste contains sodium nitrate, it is preferable to heat it at a temperature of 300° C. or lower because sodium nitrate melts at 308° C. and decomposes above that point, releasing oxygen and forming sodium nitrite.

(Function) In this invention configured in this way, an inorganic binder,
Since a curing accelerator is added as necessary, it is possible to chemically bond the dry powder of radioactive waste, thereby imparting mechanical strength to the obtained pellet molded body. The water resistance of the inorganic binder itself is improved after curing by using a water resistance improver as required, and long-term durability is imparted to the pellet without being affected by the water content of the filler in the solidified body. Furthermore, 8
Heating is performed at a temperature of 0° C. or higher to evaporate the water in the inorganic binder hardened in the pellet to cause dehydration, thereby further strengthening the hardened bonding properties of the inorganic binder. Furthermore, free water remaining in the pellet is also evaporated by heating.

(Example) Hereinafter, the present invention will be specifically explained with reference to Examples.

Examples 1 to 4 Dry powder of sodium nitrate, which simulates dry powder of concentrated waste liquid of radioactive waste generated from reprocessing facilities, was treated with an inorganic binder, a hardening accelerator, and a water resistant powder with the composition shown in Table 1. A clay-like composition was obtained by adding water as necessary and uniformly mixing and kneading the properties improver.

This was compression molded in a formwork to a diameter of 15 mm and a height of 15 m.
A cylindrical pellet of m was obtained.

After this pellet was left in an atmosphere at 25° C. for 24 hours, it was heated under the heating conditions shown in Table 1, and subjected to dehydration and drying treatment to obtain a strong pellet.

96 at a relative humidity of 80% for the resulting pellet.
Water resistance was tested by exposing it to water for 15 hours. Deliquescent sodium nitrate absorbs moisture from the atmosphere under the above temperature and humidity conditions,
Although it ultimately becomes an aqueous solution, the pellets obtained in Examples showed sufficient water resistance without any deliquescent change. Furthermore, since the pellets of the examples are entirely made of inorganic components, they have excellent long-term durability.

Table 1 Composition Example 1 2 3 4 Simulated waste (NaNO, powder) 52 60 70
80 Alumina cement 520 Powdered sodium silicate 30 Monobasic aluminum phosphate 4030 Phosphate glass 20 Aluminum dihydrogen tripolyphosphate 7 Electrofused magnesia 1 Talc
6 Water 1015 Heating conditions 110℃200℃110℃110℃
10 hours 3 hours 10 hours 10 hours Examples 5 to 8 An inorganic binder having the composition shown in Table 2 was applied to a 50 wt% sodium nitrate aqueous solution simulating the concentrated waste liquid of radioactive waste generated from a reprocessing facility. A curing accelerator and a water resistance improver were uniformly mixed and mixed to obtain a viscous composition.

Berets were prepared in the same manner as in Examples 1 to 4 above and tested for water resistance. As a result, it was confirmed that it did not show deliquescent property and showed sufficient water resistance.

Table 2 Composition Example 5 6 7 8 Simulated waste C10wL%NaN0a liquid> 200 200 20
0 200 Alumina cement 83 47 7
5 55 Sodium silicate powder 1020 Monobasic aluminum phosphate 20 Aluminum dihydrogen tripolyphosphate 23 Talc 5 10 Ultrafine silica powder 2525 Heating conditions
110'C 110℃ 300℃ 110℃ IO time 10 hours 3 hours 10 hours [Effects of the invention] As explained above, in the radioactive waste treatment method of the present invention, using an inorganic binder, chemical and mechanical A physically stable radioactive waste pellet can be easily formed.

In addition, the radioactive waste pellets produced by the present invention have excellent water resistance, so even if they are fixed in a solidified package with a hydraulic inorganic filler as a final treatment, the water content in the inorganic filler A chemically stable solid package can be formed without any interaction with other substances.

In addition, even when radioactive waste pellets treated according to the present invention are stored as they are, they have excellent water resistance, making it possible to ease humidity control in the storage area and reduce the burden on air conditioning equipment, making it economical. It can be stored and preserved with excellent properties.

Claims (1)

[Claims] 1. A clay-like composition obtained by mixing radioactive waste with an inorganic binder and, if necessary, further adding a curing accelerator and/or a water resistance improver to the inorganic binder. 1. A method for disposing of radioactive waste, which comprises forming a material into a pellet, curing the pellet-shaped molded product by leaving it at room temperature, and heating the obtained molded product at a heating temperature of 80° C. or higher. 2. A first method characterized in that the radioactive waste is in the form of at least one of powder, concentrated waste liquid, and sludge.
Radioactive waste disposal method described in Section 1. 3. Item 1 or 2, wherein the inorganic binder is made of at least one of hydraulic cement containing calcium silicate and calcium aluminate as main components, alkali silicate, aluminum phosphate, and phosphate glass. Radioactive waste disposal method described in Section 1. 4. The curing accelerator is magnesium aluminate metasilicate, aluminum dihydrogen tripolyphosphate, inorganic phosphate compound, fused magnesia, sintered magnesia, talc, fly ash, alumina cement, asbestos, NH_4F, A
l(OH)_3, MgO, Mg(OH)_2, CaO,
The method for treating radioactive waste according to any one of items 1 to 3, characterized in that the method comprises at least one type of Ca(OH)_2. 5. The method for treating radioactive waste according to any one of items 1 to 4, wherein the water resistance improver contains at least one of talc and ultrafine silica powder with a particle size of 5 μm or less.