JPH0324499A - Removal of ruthenium in radioactive waste gas - Google Patents

Abstract

PURPOSE:To enable keeping the amount of silica gel which turns into a radioactive waste afterword at a rather low level by controlling the composition of nitrogen oxides in a generating waste gas so that a nitrogen dioxide may be a dominant component. CONSTITUTION:A spent fuel is treated mainly by Purex process, and a uranium and a plutonium are recollected and concurrently fission products are separated and solidified, and then the former two are stored. After controlling the composition of nitrogen oxides in a waste gas which is generated during the process, so that a nitrogen dioxide may be a dominant component, this waste gas is made to be absorbed by a silica gel. A treatment process mainly consisting of Purex process, is a process, for instance, that is constituted by crushing and sieving process of the spent fuel, a dissolution process by a conc nitric acid, a solvent extraction process of a tributyl phosphate by an n-dodecan solution, a reverse extraction process by a thin-nitric acid, a concrete soldification and a vitrification process of radioactive wastes, and the like. A method to control the composition of nitrogen oxides so that the nitrogen dioxide may be a dominant component, could be an ordinary method.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for removing ruthenium contained in waste gas generated from a spent nuclear fuel reprocessing plant.

[Conventional technology]

At a spent nuclear fuel reprocessing plant, spent nuclear fuel containing uranium (U), plutonium (Pu), and various fission products is dissolved in concentrated nitric acid, and then reprocessed using a n-dodecane solution of tributyl phosphate (TBP). The treatment is carried out by the so-called Purex method, in which after solvent extraction of and Pu is carried out, back extraction is performed again with dilute nitric acid.

In the above process, NO2. A gas containing radioactive ruthenium (Ru), which is an element that is easily volatile among nuclear fission products, is generated along with nitrogen oxides (NOx) such as NO, moisture, and dust. These waste gases are subjected to dry and wet treatment using cyclones and scrubbers, and then passed through an adsorption tower filled with an adsorbent such as silica gel. Although most of the above-mentioned N O x and dust are removed by dry and wet processing, waste gas containing fission products is only removed by an adsorbent such as silica gel.

[Problem to be solved by the invention]- However, since the adsorption amount of radioactive Ru to silica gel is extremely low (for example, 4 to 1 g of Ru is adsorbed to silica gel).
5mg/g), and must be constantly replaced with new silica gel, which generates a large amount of radioactive waste.
Processing and storage were extremely difficult.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, an object of the present invention is to provide a method for removing ruthenium from radioactive waste gas, which can reduce the amount of silica gel generated as radioactive waste.

[Means to solve the problem]

The method of removing ruthenium from radioactive waste gas according to the present invention involves processing spent nuclear fuel using a method mainly based on the Purex method, recovering uranium and plutonium, and at the same time separating and solidifying fission products, which are then stored. The process is characterized in that the composition of nitrogen oxides in the waste gas generated in the process is controlled so that nitrogen dioxide is the main component, and then the waste gas is adsorbed onto silica gel.

[Effect]

What is the process of processing using the Purex method as the main method?
For example, the process consists of pulverization of spent nuclear fuel, sieving, dissolution with concentrated nitric acid, solvent extraction with n-dodecane solution of tributyl phosphate, back extraction with dilute nitric acid, solidification of radioactive waste into concrete and vitrification, etc. In addition to the equipment used in the above process, it naturally includes auxiliary equipment such as waste gas treatment equipment and waste liquid treatment equipment. The waste gas generated in the above process refers to nitrogen oxides (NOx
). It consists of dust and gas containing moisture, fission products, and air, which is the main component. In addition, according to a survey of waste gas composition at a normal spent nuclear fuel reprocessing plant, the ratio of nitrogen dioxide concentration (NO2) to nitrogen monoxide concentration [NO] [
:NO. )/[NO] is approximately 1/2, and nitrogen dioxide (
NO2) is low.

Therefore, the present inventors investigated the effects of [NO] in nitrogen oxides in waste gas.
It was discovered that when the proportion of z) was increased, the amount of Ru adsorbed onto the silica gel increased significantly. This is thought to be because Ru forms a complex compound with NOx and is adsorbed on the silica gel, and the more NOx there is, the greater the amount of the complex compound. In practice, in carrying out the method of the present invention, it is necessary to make the ratio [NO2]/[NO] more than 1/2, and more preferably 1 or more. The nitrogen oxide composition can be controlled so that NO2 is the main component, such as by blowing NO2 into the waste gas delivery system or
Examples include a method of oxidizing o with a catalyst or the like to convert it into N O 2 . Waste gas consisting mainly of NO2 is treated with general waste gas processing equipment, such as cyclones, scrubbers, and electrostatic precipitators, to remove NOx to a level of 100 ppm or more as NOx.
After adjusting the concentration to 50,000 ppm or less, Ru is adsorbed and removed by venting into an adsorption tower filled with silica gel. A silica gel adsorption tower for water drying may be used.

〔Example〕

An adsorption tower with an inner diameter of 25 mm and a height of 780 mm was filled with silica gel, and waste containing Ru in the form of R u O 4 was introduced into the adsorption tower at a temperature of 70°C and a linear velocity of 0.3 m/'sec. Gas was supplied.

The composition of the waste gas is N2 75 to 80 v o 1. %
+0215~20vol. %, moisture 4vol. %,NO
x2 0 0 0 0ppm ((NO2
] / (No)=1/2), Rub. The amount of Ru adsorbed on silica gel was about 4 to 5 μ/g when the concentration was 3 μ/Nl, but the amount of Ru adsorbed on the silica gel was about 4 to 5 μ/g.
1) 1) m ((NO2) / (No) = 1/0)
The amount of Ru adsorbed on the silica gel is approximately 17 to 1
It was 8■/g.

Therefore, according to the method of the present invention, the amount of silica gel generated as radioactive waste containing Ru can be significantly reduced.

[Effects of the Invention] As mentioned above, according to the method for removing ruthenium from radioactive waste gas according to the present invention, the amount of silica gel that becomes radioactive waste can be suppressed to a low level, and therefore its processing and storage become easier. It has extremely important practical advantages.

Claims (1)

[Claims] In the process of processing spent nuclear fuel using a method mainly based on the Purex method, recovering uranium and plutonium, and at the same time separating and solidifying fission products and storing them, nitrogen oxidation occurs in the waste gas generated in the process. A method for removing ruthenium from radioactive waste gas, which comprises controlling the composition of the waste gas so that it is mainly composed of nitrogen dioxide, and then adsorbing the waste gas onto silica gel.