JPH087280B2 - 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.

(Prior art) Concentrated effluents and sludges generated from nuclear facilities and nuclear fuel reprocessing facilities and their disposal standards have not yet been established. The intermediate storage method of storing is adopted. Conventionally, as a technique relating to such an intermediate storage method, a method of pelletizing radioactive waste with 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.

Therefore, the inventors of the present invention have conducted repeated studies to solve such problems, and as a result, in order to accelerate the curing of this binder with an inorganic binder made of silica sol, lithium silicate or a mixture thereof in dry powder of radioactive waste. It was found that pellets having a high mechanical strength and a small amount of dust can be produced by mixing a curing accelerator of (1) and a water resistance improving agent for improving the water resistance of the product and press molding.

By the way, when making pellets with such a composition,
It was found that the properties of pellets depended largely on the homogeneity of the mixture. Binders such as the above-mentioned inorganic binder, curing accelerator, water resistance improver, etc. are extremely small relative to the amount of waste due to volume reduction of the waste, so that it is further necessary to disperse them uniformly. However, since silica sol and lithium silicate are liquid and the curing reaction is dehydration condensation, when the binders are mixed in the powder at the same time, the curing reaction between the binders in the powder starts first, so that a uniform dispersion occurs. Becomes difficult and good pellets cannot be obtained. This tendency is particularly strong when the curing accelerator and the water resistance improver are less than the binder.

[Structure of the Invention] (Means for Solving the Problems) As a result of repeated studies to solve the above problems, the present inventors first found that a curing accelerator and a water resistance improver were added to dry powder of radioactive waste. The present invention has been completed, and it was found that a homogeneous mixture can be obtained by adding and mixing the above with an inorganic binder and then achieving the present invention.

That is, the present invention, after adding the curing accelerator and the water resistance improver to the dry powder of radioactive waste and mixing them uniformly,
The present invention relates to a method for solidifying a radioactive waste, which comprises mixing an inorganic binder made of silica sol, lithium silicate or a mixture thereof, and press-molding the obtained mixture into a pellet to cure the mixture.

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. When it is less than this range, the bond hardening is low, while when it is more than this range, the volume reduction property becomes 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 these, 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. The addition amount of these is preferably 0.5 to 15% by weight.

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) Since both the curing accelerator and the water resistance improver used in the present invention are powders, they can be easily and uniformly mixed with the radioactive waste powder. If silica sol or lithium silicate, which is a water-dispersible inorganic binder, is added after forming such a uniform powder mixture, a mixture in which the binder is uniformly dispersed is obtained, and the curing failure of the binder does not occur. Good pellets are made by pressure molding such a homogeneous mixture. In order to mix the binder uniformly in the powder, it is necessary to use a mixer having a strong stirring power to some extent.

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 curing. 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.

Further, the curing accelerator acts on the silanol group 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.

Magnesium silicate used as a water resistance improver is commonly known as talc. It promotes thickening and gelation aggregation of silica sol and lithium silicate to further multimerize them.
It has the effect of strengthening the mesh structure. In addition, magnesium aluminometasilicate and ultrafine silica powder are porous substances having a very large surface area and have a very high adsorption capacity, so that the elution of the produced pellets is suppressed and the deliquescent is also effective.

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.

A liquid containing sodium nitrate as a main component simulating radioactive waste liquid generated from a nuclear fuel reprocessing plant was made into powder by a dryer. As shown in Table 1, silica sol or lithium silicate as a binder, aluminum dihydrogen tripolyphosphate as a curing accelerator, Portland cement, and magnesium metasilicate as a water resistance improver were mixed in a weight ratio of 300 kgf / cm as shown in Table 1. ^It was molded into pellets at a pressure of ² .

At this time, the binder is mixed with the waste powder (A) simultaneously with a binder, a curing accelerator and a water resistance improver. (B) A method in which a curing accelerator and a water resistance improver are mixed with the waste powder at the same time and uniformly dispersed, and then a binder is mixed. It was done in two ways.

The drop damage rate was examined for each of the produced pellets.
As for the drop damage rate, the pellets were naturally dropped onto the iron plate from a height of about 5 m, and the degree of damage was compared by the weight reduction rate. The results are shown in Table 2. In the table, ○ indicates that the weight loss due to breakage was small, and × indicates that the weight loss was large. Further, in the table, upper-1, upper-2, middle-1, middle-2, lower-1, lower-2 represent the position of the sample in the mixer when the mixture was sampled from the mixer. .

As is clear from Table 2, the pellets made by the method A have weak drop strength when sampled from the lower part of the mixer, but the pellets made by the method B are good pellets regardless of which part of the mixer they are sampled. It was found that

[Effects of the Invention] As described above, according to the present invention, a homogeneous molding mixture can be obtained, and by pressure molding the mixture, radioactive waste that is chemically and mechanically stable for a long period of time is obtained. Pellets 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.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuaki Matsuo 1 Minamito, Ogakie-cho, Kariya city, Aichi Toshiba Ceramics Co., Ltd., Kariya Plant (72) Inventor, Takanao Sugino 1 Minamifuji, Ogakie-cho, Kariya city, Aichi prefecture Address inside Toshiba Ceramics Co., Ltd. Kariya Plant (56) Reference JP-A-63-187196 (JP, A)

Claims (6)

[Claims] 1. A dry powder of radioactive waste, to which a curing accelerator and a water resistance improver are added and uniformly mixed, and then an inorganic binder composed of silica sol, lithium silicate or a mixture thereof is mixed to obtain a product. A method for solidifying radioactive waste, which comprises press-molding the mixture into pellets and curing the mixture. 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 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.