Radiation and isotope effects on the corrosion of Zircaloy-2 in heavy and light water mixtures

Specimens of Zircaloy-2 have been exposed in H2O/D2O mixtures of varying composition (11-98 wt%D2O) in ampoules in a loop in the Swedish R2 reactor, both in-pile and out-of-pile. The specimens were exposed for 232 days at about 3200C. The total absorption and isotopic composition of the hydrogen in the metal was studied. Out-of-pile the total absorption is about the same in liquid and in vapour, but the absorption of D2 in Zircaloy immersed in 98 wt%D2O is about 25% lower than that of H2 in 89 Wt% H2O. In-pile the absorption is higher in liquid than in vapour. Large isotope effects α=(H2/D2)/(H2O/D2O) are observed both in-pile and out-of-pile (α=2-9). The observed in-pile acceleration of the corrosion (3-8 times) is highest at high D2O contents. The large isotope effect for the hydrogen absorption is explained by a radiation chemical mechanism with the participation of the H3O radical. A mechanism for the corrosion acceleration is proposed, based on the participation of the O-2 radical. (Auth.)