OXIDATION OF UO2 FUEL BY THE PRODUCTS OF GAMMA-RADIOLYSIS OF WATER

The kinetics of UO2 fuel oxidation by the products of gamma radiolyis of water were studied as a function of absorbed dose rate. The oxidation of UO2 during radiolysis was monitored by recording the corrosion potential of a UO2 electrode as a function of time. Changes in the surface of UO2 were determined using cathodic-stripping voltammetry (CSV) and X-ray photoelectron spectroscopy (XPS). The effects of specific radicals on UO2 oxidation were determined by adding suitable scavengers to the solution to maximize the yield of a particular radical. The radical species formed during radiolysis of water are much more effective in causing UO2 oxidation than the molecular oxidants formed during radiolysis of water or present in water from atmospheric sources. The oxidation Of UO2 in the presence of oxidants produced by the gamma radiolysis of water occurs in two distinct stages: (a) the formation of a thin layer of UO2+x with a stoichiometry close to UO2.33 and a thickness similar to that obtained (over longer exposure periods) in unirradiated oxygenated solutions; and (b) the subsequent oxidative dissolution of this surface layer to produce soluble U(VI) species and secondary phases, probably hydrated schoepite (UO3.xH2O), on the UO2 substrate. The first stage occurs in the potential range -500 mV < E(CORR) less than or similar to - 100 mV (versus SCE). The second stage starts around E(CORR) approximately -100 mV and eventually achieves steady state at a value of E(CORR) determined by the gamma dose.