JPH02162298A - Solidification of waste - Google Patents

Abstract

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

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for solidifying radioactive waste, and particularly to a solidifying method suitable for solidifying water-soluble inorganic solids.

[Conventional technology]

Aqueous solutions of water-soluble inorganic salts containing powdered nuclides (concentrated waste liquid) from nuclear fuel reprocessing facilities or nuclear power plants, etc.
is generated as waste. After the concentrated waste liquid is finally reduced in volume, it is solidified with an inorganic solidifying material such as cement.
Managed and disposed of at dedicated management facilities or disposal sites. The most efficient way to reduce the volume of concentrated waste liquid is to powder it using a centrifugal thin film dryer, etc., and then compression mold it into pellets.
The volume reduction ratio is also high. Compression-molded pellets are simply filled into a 200Q drum and solidified by injecting a solidifying material into the gaps between the pellets, and the pellet filling rate is high. However, when a water-soluble inorganic hardening material such as cement is used as a solidifying material, the pellets dissolve in the water contained in the solidifying material, which adversely affects the durability of the solidified material. In particular, sodium nitrate pellets generated from nuclear fuel reprocessing plants have a high solubility in water and a high dissolution rate, so the problem is noticeable.

Specifically, when sodium nitrate pellets are solidified with ordinary Portland cement, many fine cracks occur in the solidified material due to dissolution of the sodium nitrate pellets, and this also causes leaching of radionuclides contained in the solidified material. It was extremely large and posed a practical problem.

As a method to solve this kind of bad luck, Japanese Patent Application Laid-Open No. 62-2
Publication No. 67699 describes a method of solidifying by using fast-setting alumina cement as a solidifying agent and adding aggregate, fluidizing agent, and dispersant to reduce the amount of water in the solidifying agent as much as possible. Proposed. in this way,
Because it hardens quickly and has a low water content, it is possible to reduce the dissolution of water-soluble pellets. Therefore, there is a problem in that the solidifying material is poorly poured into the solidifying container filled with pellets. For example, if any trouble occurs during the injection process of the solidifying material, the solidifying material will begin to harden during the injection process, making it difficult to produce a healthy solidified body.

[Problem to be solved by the invention]

The purpose of the present invention is to solve the problems of the prior art, namely, that it is possible to prevent the dissolution of water-soluble pellets, but they harden quickly, and that there are problems in pourability due to the large amount of aggregate. .

[Means to solve the problem]

The above purpose is to use a solidifying material whose main component is sodium silicate,
This is achieved by solidifying with a solidifying material containing a cement-based water absorbing agent that has a high water absorption rate.

Conventional cementitious solidifying materials use calcium silicate or alumina cement, which is made by adding water to cement powder to make a paste, injecting it into the gaps between pellets, or mixing it with the pellets. It is poured into a drum and solidified. In a cement paste made by adding water to cement powder, some of the cement particles dissolve, but most of them are only suspended, and if water-soluble pellets are present, they are easily dissolved by water. Therefore, when using a cement-based solidifying material,
As shown in Japanese Patent No. 99, it is difficult to prevent the water-soluble pellets from dissolving unless the moisture content is reduced by adding a large amount of aggregate and the curing speed is increased. The present invention aims to solve the problems of cement-based solidifying materials by using a solidifying material whose main component is sodium silicate, which is different from these cement-based solidifying materials.

[Effect]

Cement solidifying materials, such as ordinary Portland cement, alumina cement, and silica cement, undergo hardening as their main components, calcium silicate and calcium aluminate, react with water and turn into hydrate crystals. do. On the other hand, the solidifying material containing sodium silicate as a main component of the present invention forms a solidified body mainly having a three-dimensional network structure of silicic acid through the reaction of sodium silicate and a phosphoric acid curing agent, as shown in the following formula.

Na 2o ・(Si02)II-mHzo+ (Hard JD 4 (solidified) + mHzO Sodium silicate is commonly called water glass with strong water coordination, has low viscosity, and is a water-soluble inorganic solid. In other words, it does not dissolve sodium nitrate pellets, etc., and is an excellent solidifying material. Therefore, even if this solidifying material and sodium nitrate pellets come into contact or are mixed, the pellets do not dissolve, making it an excellent solidifying material. There is no need to accelerate the curing rate as in the case of cement-based solidifying materials, and there is no need to consider the problems described above with cement-based solidifying materials.

However, as can be seen from the curing reaction formula of sodium silicate, the water coordinated to sodium silicate is released at the same time as the solidification is formed. This water usually reacts with cement added as a water-absorbing agent and is fixed as water of crystallization, making it possible to produce a strong solidified body. However, when the waste to be solidified is a substance that is extremely soluble in water, such as sodium nitrate pellets, there is a race between water absorption by cement and dissolution of the pellets, and in some cases, dissolution of the pellets becomes the mainstream. This makes it difficult to produce a healthy solidified body. The object of the present invention is to provide a method for solidifying water-soluble inorganic substances, such as sodium nitrate pellets, under conditions in which water absorption by cement is the main stream, while preventing the dissolution of such pellets. We will discuss the experimental facts and ideas that led to the discovery of the conditions, as well as the solidification conditions.

The rate of dissolution of water-soluble inorganic substances such as sodium nitrate is highly dependent on temperature, and generally the higher the temperature, the greater the rate of dissolution. In the case of pellet-like substances, evaluation is difficult because the dissolution rate changes greatly depending on the surface condition of pellets 1 - density 1, but qualitatively it can be evaluated in the same way as powder. Figure 2 qualitatively shows the temperature dependence of the dissolution rate of sodium nitrate. This temperature dependence reduces the activation energy of dissolution to -15 kcaQ/■ol
This is an estimate of the degree. According to this, for example, when the temperature is lowered to 20°C compared to 40°C, the dissolution rate decreases to about 1/4. On the other hand, the water absorption rate of cement also depends on temperature, but this dependence is smaller than the dissolution rate.
This is shown qualitatively by the broken line in FIG. This curve was obtained assuming that the hardening speed of cement is equivalent to the water absorption speed of cement.

In addition, the water absorption rate of cement is determined by the specific surface area of cement powder (
S), as an example in Figure 2, S=3 + OO
Ocxr ” / g, 12 + OOOam ” /
g (7) The case was shown. S=3,000c++2/
g is that of ordinary Portland cement. Second
Comparing the dissolution rate of sodium nitrate shown in the figure with the water absorption rate of cement, it can be seen qualitatively that the higher the curing temperature, the higher the dissolution rate, and the lower the temperature, the higher the water absorption rate. Figure 3 shows sodium nitrate pellets, sodium silicate solidifying material, phosphoric acid curing agent and specific surface area 3.0OOaa.
The compressive strength of solidified pellets solidified with /g of cement at various curing temperatures is shown.

The strength of the solidified product reaches its maximum when the curing temperature is around 20°C, and the strength decreases whether the temperature is higher or lower than that. When the temperature is high, as shown in FIG. 2, the dissolution of sodium nitrate is faster than the water absorption rate of cement, which indicates that the sodium nitrate dissolves and the strength decreases. On the other hand, if the temperature is low, the curing speed of the solidifying material will also decrease, so
In practical use, the strength is low because the curing has not progressed sufficiently.
It is desirable that sufficient strength be achieved in about 30 days, and in this case it is desirable to cure at 15 to 30°C. Since the solidification reaction using the sodium silicate solidifying material and the phosphoric acid curing agent is an exothermic reaction, the temperature of the solidified product is expected to rise to around 40°C when working at normal room temperature, and cooling equipment is required as a solution. . Cooling equipment causes an increase in equipment costs, and further increases operating costs. It is clear from FIG. 2 that in order to avoid this, a cement with a higher water absorption rate, that is, a cement with a larger specific surface area, should be used. Based on a curing temperature of 20°C and a cement specific surface area of 3 + OOOcs''/g, when the curing temperature rises to 40°C, the dissolution rate of sodium nitrate quadruples, and the corresponding rate of water absorption quadruples. It is sufficient to use cement with a surface area of 12,000 m"/g, and under these conditions, a healthy solidified pellet with a compressive strength of 170 to 190 kg/(1 m") can be obtained. Also, it does not matter how fast the water absorption rate is. Therefore, the specific surface area 12.OOOa
There is no problem even if it is more than m"/g. This is because it absorbs water which is a by-product during curing and does not interact with the water coordinated with sodium silicate before curing.

Looking at the effects of the curing temperature on the dissolution rate of sodium nitrate, the rate of cement water absorption, and the strength of solidified pellets, we can see that if the cement specific surface area S and the curing temperature T are as shown in the following formula, the sodium nitrate A high-strength and healthy solidified body that prevents dissolution can be produced.

For example, when the curing temperature T is 30° C., it is sufficient to add a cement having a specific surface area S of 6,0 OOan2/g or more. The amount added must be at least enough to stoichiometrically absorb all the water produced in reaction formula (1). In addition, to measure the specific surface area of cement,
A technique called the BET method is used, but if measurement errors are taken into consideration, it is practical to assume that there is a variation of ±50%.

Furthermore, considering the temperature distribution between the center and side portions of the solidified body, it is practical to assume that the curing temperature has a maximum and minimum value within a range of ±5°C.

Next, an example will be described in which water-soluble waste solids were solidified based on the above findings obtained in the present invention.

〔Example〕

An embodiment of the present invention will be explained with reference to FIG. In this embodiment, a concentrated waste liquid 1 in which a water-soluble inorganic solid is dissolved in water is solidified, and mainly a dry powder machine 2.degree. granulator 3. Pellet receptor 5. Pellet metering feeder6. Hydraulic solidifying material8. Inorganic water absorbing agent9. Water 10° kneading tank 15. And solidification container 7
, and a heat exchanger 12. Concentrated waste liquid 1
is supplied to a dry powder machine 2, and inorganic substances such as sodium nitrate are turned into dry powder, which is then pelletized in a granulator 3 in the next step.

After the pellets 4 are stored in the pellet receiver 5 for - time, they are fed in a fixed amount to the solidification container 7 by the pellet metering feeder 6. On the other hand, hydraulic solidifying material 8, inorganic water absorbing agent 9. water 1
0 respectively from tank 11 to valve 13. Then, a fixed amount is supplied to a kneader 15 via a metering feeder 14 and kneaded by a stirring blade 17 to create a solidifying paste 16.

The solidifying paste 16 is injected into the solidifying container 7 filled with pellets through the valve 13 to fill the gaps between the pellets. The solidified body 18 is created by leaving the pellets and the solidifying container filled with the solidifying paste to cure. When controlling the solidifying paste to a specified temperature,
Heat exchanger 12 installed in water 10 or kneading tank 15
drive.

In addition, during the injection process of the solidifying paste, 4 pellets should be prepared in advance.
The solidifying paste 16 may be kneaded and then poured into the solidifying container. Next, examples will be described in which various physical properties, formulations, and curing temperature conditions of the hydraulic solidifying material 8, the inorganic water absorbing agent 9, and the water 10 are changed.

[Example 1] Hydraulic solidifying agent when curing temperature is 2oC,
Table 1 shows the composition of the inorganic water absorbing agent and water.

Table 1 Using the present composition, 200 kg of sodium nitrate pellets were solidified in a 200Q solidification container using the solidification apparatus shown in FIG. When a paste was created by mixing the hydraulic solidifying agent, inorganic water absorbing agent, and water, and during the curing process, a temperature increase was observed due to the heat of reaction. Therefore, water 10 and kneading tank 15
The mixture was cooled by a heat exchanger 12 and adjusted to 10 to 15°C, and then poured into a solidification container. The temperature at the center of the solidification container is
The temperature gradually rose due to reaction heat due to hardening of the paste, but the maximum temperature was about 20°C. The solidified material produced in this example did not dissolve the sodium nitrate pellets and had a compressive strength of 1.
It was a healthy weight of 80 kg/am2. Portland cement with a specific surface area of approximately 3.0OOao"7g was used as the inorganic water absorbing agent, but a similar healthy solidified body was obtained using cement with a larger specific surface area.Also, the inorganic water absorbing agent was A suitable amount is 10 wt%; below that, the water absorption effect decreases, and above that, problems such as kneading properties occur.

The hydraulic solidifying material is composed of sodium silicate and a phosphoric acid curing agent, and the appropriate ratio of the two is a phosphoric acid curing agent/sodium silicate ratio of 0.2 to 0.3.

[Example 2] The same operation as in Example 1 was carried out at a curing temperature of 40°C. However, when using water and a hydraulic solidifying agent of 20°C, the temperature rise during kneading was only a few degrees, and the temperature rise due to reaction heat during curing was about 40°C at most, so heat exchange No equipment was required. In this case, the specific surface area of the inorganic water absorbing agent is 3. When Portland cement with OOOam2/g was used, the pellet surface was dissolved and the solidified strength was less than 100 kg/a++''.On the other hand, when Portland cement with an inorganic water absorbing agent having a specific surface area of about 12,000 aa2/g or more was used. A sound solidified body similar to that of Example 1 was obtained.

[Example 3] During the production of a solidified body in the same manner as in Example 2, an inorganic water absorbing agent was applied using gypsum plaster of JISA6904 standard. Gypsum also solidifies and hardens through a hydration reaction, and the rate is fast, so it has sufficient performance as an inorganic water absorbing agent.
A healthy solidified material similar to that obtained was obtained.

[Example 4] During the production of the solidified bodies of Examples 1, 2 and 3, the waste pellets were mixed with sodium sulfate, sodium chloride, sodium acetate,
Even when using soluble salts of sodium hydrogen phosphate, sodium dihydrogen phosphate, and calcium pyrophosphate, a solid solid with high strength and sound quality similar to that of sodium nitrate was obtained.

〔Effect of the invention〕

According to the present invention, it is possible to create a solidified body that is simple, low cost, and highly safe.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of an embodiment of the present invention, Fig. 2 is a diagram showing the dependence of sodium nitrate dissolution rate and cement water absorption rate on curing temperature, and Fig. 3 is a diagram showing the curing temperature dependence of pellet solidified compressive strength. It is a diagram showing dependence. 2... Dry powder machine, 3... Granulator, 5... Pellet receiver, 6... Pellet metering and feeding machine, 7... Container for solidification, 10... Water, 11...・Tank, 12...
Heat exchanger, 13...Valve, 14...Measuring feeder, 1
5... Kneading tank, 16... Solidifying paste, 17...
- Stirring blade, 18...solidified material. Qin l Early hardening curing 8 degrees (゛ri first hardening curing 1 degree (°C)

Claims (1)

[Claims] 1. A water-soluble inorganic substance is a hydraulic inorganic solidifying agent, an inorganic water absorbing agent,
A method for solidifying waste with a paste mixed with water, characterized in that an inorganic water-absorbing agent having a suction rate higher than a dissolution rate of the water-soluble inorganic substance in the paste is used. 2. Add water-soluble inorganic substances to hydraulic inorganic solidifying agents, inorganic water absorbing agents,
In the method of solidifying with a paste mixed with water, the specific surface area S (cm^2/g) and curing temperature T (℃) of the inorganic water absorbing agent are S≧2^(^T^-^2^0^)^ /^1^0×3,0
A method for solidifying waste that solidifies under 00 conditions. 3. Disposal of the water-soluble inorganic substance according to claim 1 or 2 in the form of any one or a mixture of sodium salts, calcium salts, and barium salts of sulfuric acid, nitric acid, acetic acid, phosphoric acid, and boric acid. How things solidify. 4. A method for solidifying waste, wherein the water-soluble inorganic substance according to claim 3 is a powder or a solid pellet. 5. A method for solidifying waste, wherein the hydraulic inorganic solidifying material according to claim 1 or 2 is a mixture of sodium silicate and a phosphoric acid curing agent. 6. A method for solidifying waste, wherein the inorganic water absorbing agent according to claim 1 or 2 is a mixture of cement and/or gypsum. 7. The water-soluble inorganic substance according to claim 1 or 2 is any one of sodium salt, calcium salt, and barium salt of sulfuric acid, nitric acid, acetic acid, phosphoric acid, and boric acid containing radionuclides, or , a method for solidifying waste that is a mixture of multiple species. 8. The solidification method according to claim 1 or 2, comprising: a kneading tank for kneading the hydraulic inorganic solidifying material, the inorganic water-absorbing agent, and water, and from the kneading tank to the solidifying container. A method for solidifying waste, comprising a paste transfer facility and a facility for curing and solidifying the solidification container by leaving the solidification container at a temperature of 30° C. or lower. 9. In the solidification method according to claim 1 or 2, the waste is solidified by mixing the water-soluble inorganic substance, hydraulic inorganic solidifying agent, and water, and then transferring the mixture to a solidifying container and solidifying the waste. Method.