JPH0772295A - Method for removing radioactive ruthenium - Google Patents

Abstract

PURPOSE:To enable the efficient removal of radioactive ruthenium to be effected by transforming volatile ruthenium into a water-soluble ruthenium compound through the action of the gas containing NO2 of 100 or more times of its quantity in molecular number and an overabundant quantity of water. CONSTITUTION:If ruthenium tetroxide contained in an exhaust gas comes in full contact with NO2 gas and water when they exist, a water-soluble nitrosyl ruthenium compound is generated. Then, ruthenium tetroxide easily shifts from a gas aspect side to a water aspect side and can be removed from the exhaust gas. For this purpose, an action is exerted by the gas containing NO2 of the 100 or more times of its quantity in molecular number and an overabundant quantity of water. Incidentally, it is expected that exhaust gas always contains either a sufficiently large quantity of steam or NO2 gas, compared with ruthenium. Moreover, if the exhaust gas should contain little water, a sufficient content of NO2 gas will avoid trouble in removing radioactive ruthenium, because it comes in contact with a large quantity of water in an ordinary water cleaning tower.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention mainly removes volatile radioactive ruthenium contained in waste gas emitted from nuclear facilities, especially spent nuclear fuel reprocessing facilities and uranium and plutonium conversion facilities. Regarding the method.

[0002]

2. Description of the Related Art In the reprocessing of spent nuclear fuel by the Purex method, a process of heating a nitric acid solution to a high temperature (dissolution process, product solution evaporation concentration process, acid recovery process, waste liquid evaporation concentration
There are many calcination / solidification processes, denitration conversion processes for uranium and plutonium, etc.). In these heating steps, a part of ruthenium contained in the solution is often oxidized to ruthenium tetroxide and volatilized, and there is a possibility that it will be transferred to the waste gas. Volatile ruthenium that has been transferred to such waste gas system is used for normal waste gas treatment equipment (condenser, water washing tower,
It is known that effective removal is difficult with a HEPA filter or the like.

As a well-known method of removing volatile ruthenium, there is a method of adsorbing ruthenium on silica gel, iron oxide or the like. This method has indeed been applied in some high level waste liquid treatment facilities. For example, in the waste liquid calcination process (WCF process) used in the Idaho Chemical Treatment Plant (ICPP) in the United States, a silica gel adsorption tower is installed at the end of the waste gas treatment system (immediately before the final filter). A ruthenium adsorption tower filled with iron oxide is also used in the Italian Fingal process.

As another method for treating volatile ruthenium, the waste gas containing volatile ruthenium is removed by passing it through a packed gas scrubber using a relatively high concentration alkali (sodium hydroxide solution) as a scrubbing liquid. There is a method to do so, and it is actually used in some plants. In this case, since ruthenium tetroxide in the waste gas is changed to sodium ruthenate and dissolved in the cleaning liquid, a considerably high decontamination factor (hereinafter referred to as DF) is obtained.

[0005]

In the conventional method for removing volatile ruthenium, in the case of the ruthenium removal treatment method by adsorption, in addition to the usual waste gas treatment equipment, an apparatus specifically intended to remove volatile ruthenium ( It is necessary to install an adsorption tower), the construction of the waste gas treatment equipment is large, and the treatment cost is high. In order to obtain a high DF for ruthenium in this adsorption tower, the operating temperature is 1
There are problems such as the need to raise the temperature to 00 ° C. or higher, and it cannot be said that this is an excellent treatment method.

On the other hand, in the case of the method for cleaning and removing ruthenium with an alkaline aqueous solution, there is a problem that the solution after cleaning becomes secondary waste containing a large amount of sodium salt, and this method is also an appropriate removal method. I can't say.

The present invention has been made in view of the above situation, and does not use volatile radioactive ruthenium contained in exhaust gas generated from a nuclear fuel reprocessing facility or the like, without using a special dedicated processing apparatus, and a sodium salt. An object of the present invention is to provide a method for removing volatile radioactive ruthenium that can be effectively removed without generating a secondary waste containing

[0008]

Means for Solving the Problems As a means for achieving the above-mentioned object, the present invention uses a large excess of a gas containing NO ₂ in a molar ratio of 100 times or more with respect to volatile radioactive ruthenium in waste gas. The volatile ruthenium is made into a water-soluble ruthenium compound by acting with water in an amount, and the water-soluble ruthenium compound is dissolved and removed in the acted water.

[0009]

In the method of removing volatile radioactive ruthenium according to the present invention, ruthenium tetroxide contained in the waste gas, when NO ₂ gas and water are present, is sufficiently contacted with nitrosyl, if NO ₂ gas and water are present. Generates a ruthenium compound. And the generated nitrosyl ruthenium compound is
Since it is water-soluble, it easily migrates from the gas phase side to the water phase side,
Removed from waste gas.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

In the present invention, as described above, the waste gas containing volatile ruthenium (ruthenium tetraoxide) is contained in NO _{2 in} an amount 100 times or more that of ruthenium contained in the waste gas. A volatile ruthenium compound is converted into a water-soluble ruthenium compound (nitrosyl ruthenium compound) by the action of a gas containing a gas and a large excess amount of water to remove the volatile ruthenium compound.

More specifically, in the process in which the migration of ruthenium into the waste gas as described above is likely to be a problem, the waste gas contains a sufficiently large amount of water vapor and NO in comparison with ruthenium. _At least one of the _two gases is expected to be included at all times.

Among the above-mentioned steps, particularly in the case of the step involving nitric acid reaction, the waste gas contains a very large amount of water vapor and NO ₂ gas. Therefore, in this case, a sufficiently high removal of ruthenium can be expected without newly adding water or NO ₂ to the waste gas.

Even in the case of a waste gas containing almost no water, if a sufficient amount of NO ₂ gas is contained, a large amount of water will be generated in the water washing tower that is always installed in the usual waste gas treatment system. Therefore, a high DF for ruthenium can be expected here as well.

Further, in the case of a waste gas such that the content of NO ₂ gas is not sufficient for ruthenium, when passing this gas through a condenser or a water washing tower,
If a small amount of NO ₂ gas is mixed, a sufficiently high DF can be secured. Since NO ₂ gas is used in large quantities as an oxidizer for plutonium in a solution in a reprocessing plant, no special new gas generating equipment is required if this is used.

Table 1 below shows the NO ₂ / Ru ratio dependence of DF in a volatile ruthenium scrubbing tower.

[0017]

[Table 1]

Experimental Example 1 The present inventors have found that ruthenium is about 12 mg (about 0.12).
Ruthenium tetroxide gas, about 0.32 Nl
About 400 N of air containing (about 14 mmol) NO ₂ gas
1 was used as a simulated waste gas, and this was passed through a water scrubber (irrigation filling water washing tower) filled with Raschig rings made of porcelain. Before and after the scrubber, part of the simulated waste gas was collected and collected through a gas collection bottle filled with an absorption liquid of ruthenium tetroxide, and the dissolved ruthenium in each liquid was quantified to be removed by the scrubber. An experiment was conducted to check the amount of ruthenium that was removed.

As a result, the amount of ruthenium in the simulated waste gas after passing through the scrubber was reduced to about 1/680 of the amount of ruthenium in the gas before passing through the scrubber, and the amount of ruthenium in the scrubber was volatile. 6 for ruthenium
It was confirmed that a DF of 80 or more was obtained.

After the completion of the test, all the droplets adhering to the wall surface of the device and the Raschig ring were washed off, and the amount of ruthenium dissolved in the liquid was analyzed together with the liquid accumulated in the scrubber liquid receiver.

As a result, in the solution, ruthenium was dissolved in an amount corresponding to about 60% of the removed amount calculated based on the ruthenium tetroxide content (analytical value) in the waste gas,
It was confirmed that most of the ruthenium tetroxide removed from the gas was converted to a water-soluble ruthenium salt and dissolved in the scrubber liquid.

Comparative Example 1 A simulated waste containing almost the same amount of ruthenium tetroxide as in Experimental Example 1 but containing only about 0.03 Nl of the NO ₂ gas content, which is 1/10 of that in Experimental Example 1. The gas was passed through the scrubber under exactly the same conditions as in Experimental Example 1.

However, in this case, the DF for ruthenium tetroxide in the scrubber dropped to 30.

FIG. 1 shows that in a similar scrubber test,
The DF value of the obtained ruthenium is shown with respect to the molar ratio of NO ₂ and ruthenium in the simulated waste gas.

As is apparent from FIG. 1, NO ₂ / Ru
The DF increases with an increase in the ratio, and a high DF of about several hundreds is obtained because the molar ratio of NO ₂ / Ru is 100.
It can be seen that this is the case.

Experimental Example 2 The inventors of the present invention also used about 28 simulated exhaust gases having the same composition as in Experimental Example 1 (containing the same amounts of ruthenium tetraoxide and NO ₂ gas).
About 120 Nl of saturated steam was mixed with 0 Nl and passed through a condenser. Then, in the same manner as in Experimental Example 1, a part of the gas was separated before and after the condenser, and the amount of ruthenium contained was analyzed.

As a result of three tests, a value of 400 to 700 was obtained as the DF for ruthenium tetroxide in the condenser. Moreover, as in the case of Experimental Example 1, most of the ruthenium removed from the gas was dissolved and present in the condensate.

Comparative Example 2 A simulated waste gas having the same composition as in Experimental Examples 1 and 2 was passed through a condenser as it was without mixing with saturated steam, but ruthenium tetroxide in the waste gas was not removed at all, and DF was produced. It was 1. From this, it was found that both a sufficient amount of NO ₂ and water are indispensable for removing volatile ruthenium.

[0029]

As described above, according to the present invention,
Since ruthenium can be efficiently removed from the waste gas containing volatile ruthenium, the amount of radioactive ruthenium released into the environment can be greatly reduced. In addition, since no special new treatment equipment is required for this removal, the ruthenium removal treatment equipment does not become complicated and the treatment cost does not increase, and there is no generation of secondary waste containing sodium salts. .

[Brief description of drawings]

FIG. 1 is a graph showing the DF value of ruthenium obtained with respect to the molar ratio of NO ₂ and ruthenium in waste gas.

Claims (1)

[Claims] 1. A gas containing NO ₂ in an amount 100 times or more the number of moles with respect to the volatile radioactive ruthenium in the waste gas,
A method for removing radioactive ruthenium, which comprises reacting volatile ruthenium with a large excess amount of water to form a water-soluble ruthenium compound, and dissolving the water-soluble ruthenium compound in the reacted water to remove it.