Dilute chemical decontamination of PHWRS Indian experience

Pressurized Heavy Water Reactors (PHWRs) are the mainstay of Indian Nuclear Energy Programme. Already twelve such reactors are under operation/construction. In order to operate these reactors with maximum output with minimum exposure to radiation field for the operating personnel, a systematic effort was made to optimise a chemical decontamination process so that it can be applied to decontaminate the Primary Heat Transport (PHT) system periodically. Systematic studies relating to corrosion of PHT system structural materials, dissolution of corrosion product oxides and dominant activated and fission product contaminants in candidate decontamination formulations have led to the choice of an Ethylene Diamine Tetra Acetic Acid (EDTA) based chemical solvent for decontaminating the PHT systems of PHWRs. The formulation consists of EDTA, an organic acid, a reductant and a corrosion inhibitor. The results obtained in applying the regenerative dilute chemical decontamination process in Indian PHWRs are discussed in this paper. This process was applied twice in Madras Atomic Power Station, unit-1 (MAPS-1) and once in MAPS-2 with good radiation field reduction and MAN-REM savings. The DFs observed at different locations, duration of run, amount of activity and iron removed are given. Variations in EDTA concentration, pH, conductivity, DF, activity in solution and crud in the system water as a function of time are described briefly. Significant differences were observed in ion exchange and oxide dissolution behaviour in normal and heavy water medium and this paper discusses the salient observations. (J.P.N.)