Decontamination of whole primary-moderator circuit in Atucha I NPP (Nuclear Power Plant)

As a consequence of high dose rates prevailing in Atucha I Nuclear Power Plant primary system after the fuel element damages in August 1988, some kind of complete circuit decontamination procedure is recommended for the future inspections and repair works. There is no previous experience and special considerations have to be made because of the unique characteristics of the Atucha I pressure vessel reactor: a) the 60Co inventory is high because of 'in situ' cobalt alloys activation mechanism. b) the oxidative step of decontamination procedure attacks preferently chromium in these hard alloys. c) there are steady zones, like moderator tank and fuel channels isolation, where mass transport is almost null. d) the presence of tritium constitutes an additional radiological problem. e) care has to be taken to avoid heavy water degradation. It is required, therefore, to use soft, low concentration chemicals. The procedure consists of four steps: oxidation with permanganic acid, reduction of the reactive with the stoichiometric amount of organic acid, dissolution and complexing the oxides and conditioning of the heavy water thought ion exchange beds. The principal question about the technique is material behaviour, particularly in 'incore' and steady zones. The qualification method is described to be applied to metal probes of the differential materials present in the circuit. The program and facilities are available in CNEA (Argentine Republic). (Author)