In-situ chemical decontamination of a PWR primary loop large components with the MEDOC process

Shutdown in 1987 after 25 years of operation, the BR3-PWR was selected in 1989 as one of the four pilot decommissioning projects by the European Commission, in the framework of its five-year plan of Research and Technological Development on decommissioning of nuclear installations. The dismantling of a PWR type reactor leads to the production of large masses of contaminated metallic pieces, including structural materials, primary piping, tanks and heat exchangers. One of our main objectives is to demonstrate that we can minimise the volume of radioactive waste in an economical way by privileging alternative material routes, such as the clearance of materials after thorough decontamination. Therefore the SCK-CEN has developed its own chemical decontamination process, so-called MEDOC (MEtal Decontamination by Oxidation with Cerium), based on the use of Ce(IV) as strong oxidising species in sulphuric acid, which is continuously regenerated by ozone injection at high temperature. The industrial installation, which was designed and constructed in close collaboration with Framatome-ANP (France), started operation in September 1999. Initially designed to decontaminate stainless steel pieces, the process has been easily upgraded to allow the treatment of carbon steel using simply H2SO4. Up to now, more than 30 tons of contaminated materials, including primary pipes and primary pumps housings, have been treated batch wise with success. 69% of material can be directly cleared after treatment (Activity lower than 0.1 Bq/g in 60Co) 27% will be free released after melting (activity lower than 1 Bq/g) and less than 4% have to undergo an additional physical decontamination step prior to clearance. However, the working of our process is not restricted to batch wise operations. Thanks to minor adaptations on the existing plant, the SCK-CEN has recently performed the closed loop decontamination of the large components of the primary loop, namely the steam generator and the pressurizer. The decontamination of the steam generator was achieved in April 2002 and the pressurizer one in December 2002. In both cases, the transferable contamination was totally removed and the contact dose rates were drastically reduced. This paper focuses on the technical as well as financial aspects of the closed loop decontaminations of large components and details the benefits of these operations in an ALARA framework and in terms of waste volume minimization. (authors)