Radiolytic Oxidation of Ruthenium Oxide Deposits

In the case of a hypothetical severe accident in a nuclear pressurized water reactor, the formation of radiotoxic RuO₄(g) may occur in the reactor containment building, resulting from the interactions of ruthenium oxide deposits with the oxidizing medium induced by air radiolysis. Consequently, this gaseous ruthenium tetroxide may be dispersed into the environment; therefore, the determination of the ruthenium deposits behavior is of primary importance for nuclear safety studies. An experimental study, performed by the French Institut de Radioprotection et de Sûreté Nucléaire (IRSN), using a gamma irradiator cell (EPICUR facility at IRSN/Cadarache) has been carried out in order to obtain experimental data on these interactions. The results showed that radiolytic oxidation of ruthenium oxide deposits leads to the formation of gaseous ruthenium tetroxide to a significant extent. A comparison between the revolatilized Ru fractions obtained experimentally and those obtained by calculations based on the rate laws modeling ozone irradiation effect, established in previous studies, is presented. The disagreement observed is discussed. It appears that the oxidation resulting from air/steam radiolysis products is enhanced in comparison with pure ozone effect.