Washout Behaviour of Chromia-doped UO2 and Gadolinia Fuels in LWR Environments

To enhance the efficiency of their plants, nuclear utilities have requirements for power uprate, or operating cycle extensions and increase of fuel average discharged burnups. This general trend is accompanied with uncompromising towards safety margins. In this context, the AREVA chromia-doped UO2 fuel proved to bring significant margins in mitigation of Pellet-Clad Interaction risk of clad failure as well as to the reduction of Fission Gas Release and induced rod internal pressure at the end of life. The optimized Cr2O3-doped fuel characteristics provide additional fuel robustness by a reduced pellet chipping susceptibility and reliability features. This latter concerns fuel washout behaviour in case of defective rods after the occurrence of primary defects occasionally observed under operation. This paper presents results of the testing program carried out to clarify the washout behaviour of both chromia-doped UO2 and chromia-doped gadolinia fuel pellets with comparison to non-doped standard fuels. The pure reaction with oxygen has been first investigated by thermogravimetry studies under dry conditions. For a better understanding, pellets with different chromia additions and grain sizes were tested. Distinctly, increasing the matrix grain size and additionally the fuel density improved the fuel oxidation resistance. For non-doped fuels, optical examinations revealed the phenomenon proceeded by surface oxidation quickly followed by intergranular cracking and spalling of oxidized grains. On the contrary, with the AREVA large grain chromia-doped fuels, the formation of an outside layer delays oxygen diffusion and decelerates the oxidation rate. In a second study, fuel corrosion behaviour was studied by means of autoclave leaching tests under representative LWR conditions. The washout rate of the Cr2O3-doped pellets has been shown to be reduced up to a factor of 5 in comparison to non-doped fuel types. The doped sample surface remains nearly intact following the formation of a compact surface layer. The entire testing program clearly establishes the improved resistance to oxidation and corrosion of the AREVA optimized chromia-doped UO2 and gadolinia fuels. Such an enhanced operational behaviour is desired to struggle against disintegration of the fuel in case of defective rods and combined consequences to LWR primary coolant contaminations. (author)