JP2525790B2 - Method for solidifying radioactive waste - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a method for solidifying waste generated from a radioactive material handling facility, and more specifically, solidification suitable for an intermediate storage system. Regarding the method.

(Conventional technology) Concentrated effluent and sludge generated from nuclear facilities and nuclear fuel reprocessing facilities have not been established for their disposal standards yet. The intermediate storage method of storing in is adopted. Conventionally, as a technique related to such an intermediate storage method, a method of pelletizing radioactive waste and an appropriate pelletizer has been developed. This is a method in which an extrusion molding granulator or a compression molding granulator is used to apply a pressure of several hundreds kg to several tons per unit area to the powder of waste to coagulate the powder.

The advantage of this method is that it is more volume-reducing than the conventional cement and asphalt solidification methods, and it is more corrosive than waste liquid storage and powder storage, which are considered as other intermediate storage methods. It can be mentioned that there is little concern about pollution due to splattering or scattering.

(Problems to be Solved by the Invention) The above-described method of pelletizing radioactive waste has an advantage that the waste can be pelletized by a simple process, but since the binder is not put in the waste, It was necessary to apply very high pressure in order to give certain physical properties. For example, the main component of the waste liquid generated from the nuclear fuel reprocessing facility is sodium nitrate, but a pressure of about 500 kgf / cm ² or more is required to form this into a certain pellet, and silica etc. in the waste is required. If it contains impurities that deteriorate the moldability, it is necessary to have about 1 ton / cm ² .
Therefore, there is a problem that not only the apparatus becomes very large, but also it becomes difficult to take out pellets.

Further, in the above method, the mechanical strength of the pellet is largely influenced by the physical properties of the substance constituting the powder, and since the binder is not contained, the edge of the pellet is broken and dust or the like is generated.

[Structure of the Invention] (Means for Solving Problems) As a result of repeated research to solve the above problems, the present inventors have found that a dry powder of radioactive waste is cured with an inorganic binder and the binder. It is possible to form a pellet having a high mechanical strength and a small amount of dust by mixing with a curing accelerator for promoting the formation of a water resistance improver for improving the water resistance of the product and press-molding the mixture. However, the inventors have found that pellets having particularly high mechanical strength can be obtained by using silica sol, lithium silicate, or a mixture thereof as the inorganic binder, and have reached the present invention.

That is, the present invention provides a mixture obtained by adding to the radioactive waste an inorganic binder comprising silica sol, lithium silicate or a mixture thereof, and then mixing a curing accelerator and / or a water resistance improver for the inorganic binder. The present invention relates to a radioactive waste solidification treatment method, characterized in that the solidified product is cured by pressure molding into pellets.

As radioactive waste, dry powder of concentrated waste liquid containing sodium sulfate as a main component generated from a boiling water reactor power plant, and drying of waste liquid containing sodium borate as a main component generated from a pressurized reactor power plant The powder is, for example, a dry powder of concentrated waste liquid containing sodium nitrate as a main component generated from a nuclear fuel reprocessing plant.

The amount of the inorganic binder used is appropriately 2 to 25% by weight with respect to the radioactive waste. If it is less than this, the binding effect is low, while if it is more than this, the volume-reducing property is poor.

Examples of the curing accelerator include magnesium aluminometasilicate, aluminum dihydrogen tripolyphosphate, an inorganic phosphate compound, fused magnesia, sintered magnesia, talc, fly ash, alumina cement, asbestos, NH ₄ F, Al.
(OH) ₃ , MgO, Mg (OH) ₂ , CaO, Ca (OH) _{2 and the} like can be mentioned, and among them, aluminum phosphate, calcium aluminate, calcium, particularly represented by aluminum dihydrogen tripolyphosphate A hydraulic cement containing silicate as a main component, or a combination thereof is preferable. The addition amount of these is preferably 0.1 to 15% by weight.

Examples of the water resistance improver include magnesium silicate, magnesium aluminometasilicate, and ultrafine silica powder. Addition amount of these is 0.5-15
Weight percent is preferred.

In the present invention, a pressure of about 150 kgf / cm ² or more is required to produce pellets by pressure molding. Below this, the binder in the mixture does not adhere properly, which may cause defective molding. The upper limit of molding pressure mainly depends on the equipment, but pellet lamination (pellet cracking)
To 4 ton / cm ² or less is desirable.

The formed pellets have sufficient strength immediately after pressure molding, but the strength gradually increases with time after molding. However, if it is desired to obtain the final strength of the pellet immediately after molding, it is effective to carry out a drying treatment at a temperature of room temperature or higher. The heating and drying temperature is preferably room temperature or higher and 100 ° C. or lower.

(Operation) The silica sol or lithium silicate used in the present invention is mainly composed of colloidal silica having a silanol group, and has a property of condensing by dehydration reaction to form a polymer multimer and cure. . The cured product has an irregular network and is capable of encapsulating radioactive waste powder within the network. Further, these cured products are so-called gel-like substances and have a very large surface area, and they tend to adsorb waste powder, which further stabilizes the product.

In order to smoothly carry out the dehydration condensation reaction of the binder, it is effective to add a curing accelerator. The above-mentioned various curing accelerators are used as the curing accelerator, and these curing accelerators act on the silanol groups of the binder to accelerate the dehydration reaction and form an irregular network structure. When cement is used as the hardening accelerator, the water generated by the dehydration reaction is absorbed by the cement and becomes crystallization water and is fixed by the hydration reaction, which is more effective.

Further, when the water resistance improver is used, stabilization of physical properties of the produced pellets is promoted. Magnesium silicate, which is used as a water resistance improver, is commonly known as talc, and has the action of promoting the thickening and gelling aggregation of silica sol or lithium silicate to further multimerize and strengthen the network structure. In addition, magnesium aluminometasilicate and ultrafine silica powder are porous substances with a very large surface area.
Since it has a very high adsorption capacity, it suppresses the elution of the produced pellets and is also effective in preventing deliquescent.

In the present invention, since such a binder is added and pressure-molded, a pellet having a strong and long-term durability can be molded with a molding pressure lower than the conventional one.

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

Example 1 As a representative example of radioactive waste, a simulated reprocessed waste powder containing sodium nitrate as a main component was selected to carry out the solidification method of the present invention.

The waste powder, the binder, the curing accelerator, and the water resistance improver were mixed at the weight ratios shown in Table 1 and molded into pellets at a pressure of 300 kgf / cm ² . The appearance, compressive strength, drop damage rate and deliquescent of each of the produced pellets were examined. Of these, the drop damage rate is obtained by letting the pellets fall naturally on a steel plate from a height of about 2 m, and the damage rate is expressed by the magnitude of the weight loss rate.
The pellets were held in a constant temperature bath of No. 1 and the degree of deliquescent resistance was judged from the appearance and weight change.

As is clear from Table 1, the pellets solidified according to the present invention are much better than the pellets obtained by compression molding only waste powder (reference).

Example 2 A mixture having the composition shown in Examples 1-4, 1-6, and 1-9 was prepared.
The pressing pressure of the press was changed to pelletize. Table 2 shows the results. From this, the molding pressure of this pellet is 150 kgf / c.
It turns out that m ² or more is suitable. Also, it can be seen that lamination occurs at 4 ton / cm ² or more.

Example 3 As waste, a simulated waste powder containing sodium sulfate, sodium nitrite and sodium carbonate as main components was mixed with various binders shown in Table 3 to obtain 300 kgf / cm ^2.
And pressure-molded into pellets. The results are shown in Table 3. It can be seen that any waste is well pelletized.

Example 4 1-3 was selected as waste, and pellets were formed at a molding pressure of 300 kgf / cm ² . These pellets are stored at room temperature 50 ℃, 80 ℃, 100 ℃
It was dried at temperatures of ℃ and 140 ℃, and its strength change was investigated. As shown in the figure, it can be seen that there is no difference in drying at 100 ° C. or higher as compared with drying treatment at a temperature higher than 100 ° C. Further, it can be seen that drying at room temperature is more advantageous for curing in a shorter time than when left at room temperature.

[Effects of the Invention] As described above, according to the method for solidifying radioactive waste of the present invention, radioactive waste pellets that are chemically and mechanically stable over a long period of time can be easily formed.

Moreover, since the radioactive waste pellets produced by the present invention are also excellent in water resistance, even if they are fixed by a solidified package with a hydraulic inorganic filler as a final treated body, the water content in the inorganic filler is reduced. A chemically stable solidified package can be formed without interacting with the solidified package. Further, even when the radioactive waste pellets treated according to the present invention are stored / stored as they are, they are excellent in water resistance, so that the humidity control of the storage area can be eased, and the burden on the air conditioning equipment can be reduced, resulting in economic It can be stored and stored with excellent properties.

[Brief description of drawings]

The drawings are graphs showing the relationship between compressive strength and time at various drying temperatures of pellets molded and solidified according to the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masakazu Ota 1-1-7 Uchisaiwaicho, Chiyoda-ku, Tokyo Within Japan Atomic Energy Company (72) Inventor Kazuaki Matsuo 1 Nanto, Ogakie-cho, Kariya City Toshiba Ceramics Co., Ltd. Company Kariya Factory (72) Inventor Fukao Sugino 1 Minamito, Ogakie-cho, Kariya City Toshiba Ceramics Company Kariya Factory (56) Reference JP-A-63-187197 (JP, A) JP-A-61-250598 (JP, A) JP 63-115099 (JP, A)

Claims (6)

(57) [Claims] 1. A mixture obtained by adding an inorganic binder made of silica sol, lithium silicate or a mixture thereof to radioactive waste, and then mixing a curing accelerator and / or a water resistance improver for the inorganic mixture. A method for solidifying radioactive waste, characterized in that the material is cured by pressure molding into pellets. 2. The method for solidifying radioactive waste according to claim 1, wherein the curing is carried out at room temperature or by drying at room temperature or higher and 100 ° C. or lower. 3. A radioactive waste is a powder obtained by drying a waste liquid containing sodium sulfate as a main component generated from a nuclear power plant or a nitrate-containing waste liquid containing sodium nitrate as a main component generated from a nuclear fuel reprocessing facility. A method for solidifying radioactive waste according to claim 1. 4. The radioactive waste according to claim 1, wherein the hardening accelerator is a hydraulic cement containing aluminum phosphate, calcium silicate and calcium aluminate as a main component or a mixture thereof. Solidification treatment method. 5. The method for solidifying radioactive waste according to claim 4, wherein the aluminum phosphate is aluminum dihydrogen tripolyphosphate. 6. The method for solidifying a radioactive waste according to claim 1, wherein the water resistance improver comprises at least one selected from magnesium silicate, magnesium aluminometasilicate and ultrafine silica powder.