Systematic Chemical Decontamination Using IF₇ Gas

Since 1979, Uranium enrichment technology has been researched through the gas centrifuge method, at Ningyo-toge Environmental Engineering Center of Japan Atomic Energy Agency (JAEA). In addition, the Demonstration Plant, that is final stage test facilities, was operating continuously from 1988 to 2001. As a result, a lot of residues accumulated in the plant. Most of this accumulation was found be uranium intermediate fluoride. The basic decommission policy of JAEA is that equipments of gas centrifuge will be decontaminated by sulfuric acid immersion method for clearance and reuse. In our plan, approximately 90% of metals will be cleared and reused, and then the remaining 10% will be disposed of radioactive waste. We propose a combination of sulfuric acid immersion method and the systematic chemical decontamination as an efficient method for decontamination of uranium enrichment facilities. This paper focuses on the method and performance of systematic chemical decontamination using IF₇ gas. The following (Figure 1) shows our decommission policy and position of systematic chemical decontamination by IF₇ gas for uranium enrichment plant. The IF₇ treatment technique belongs to the systematic decontamination technology. It has the high performance decontamination technique for the plant that accumulates the uranium intermediate fluoride, such as UF₄, UF₅, U₂F₉, and U₄F₁₇, which exist in the uranium enrichment plant through the Gas Centrifuge, called GCF. The one of characteristics of the IF₇ treatment, the secondary waste is just an IF₅ and little residues. In addition, this IF₅ can be reused as materials for making new IF₇ gas. The IF₇ treatment can also be performed in the room temperature and very low pressure like a 10–45hPa. Furthermore, the IF₇ treatment is a simple method using chemical reaction. For this reason, we hardly need to care about secondary reaction with the exception of the reaction with IF₇ gas and the uranium intermediate fluoride. This is a very important feature when applying to a large-scale plant. In order to carry out the IF₇ treatment, we only set up a few equipments in GCF uranium enrichment plant, which were IF₇ feeding equipment and two circulating pumps. IF₇ gas cylinders are seated in IF₇ feeding equipment. This is the only equipment. Figure 2 shows the IF₇ treatment system. We carried out the IF₇ treatment for the four cascades in the uranium enrichment Demonstration Plant. The weights of uranium residue in the cascades were approximately 700kgU per cascade prior to the IF₇ treatment. In the IF₇ treatment, we were able to find the near-optimal processing condition. As a result, we could confirm the IF₇ treatment period for one cascade which was 60 days. The main factor to determine the IF₇ treatment period is the pressure and the flow rate of reaction product gas (UF₆ and IF₅ mixture gas) exhausted from the cascade. Although we carried out the IF₇ treatment with the maximum value of the flow rate, which our facility has, it is possible to further shorten the IF₇ treatment period by setting a higher gas flow rate. Moreover, after the IF₇ treatment, we evaluated the uranium recovery rate for cascades and the residues’ uranium weight in the main equipment of GCF. In addition, in the evaluation of the uranium recovery rate, we enable to confirm the uranium recovery rate of all cascades achieved more tan 98%. Furthermore, the average of uranium recovery rate more than 99% in the cascade that has been processed at the end. As a result, radioactive concentration of uranium in the main parts of the GCF fell to 1.0B q/g and below.