Behaviour of uranium-plutonium mixed nitride and carbide fuels irradiated in Phenix

Carried out in collaboration between the Institute for Transuranium Elements (ITU) and the Commissariat a l'Energie Atomique (CEA), the Nimphe 1 and 2 irradiation experiments performed in the Phenix reactor around 1990 were only just subjected to destructive testing. Nimphe 1 was a rig composed of 11 helium-bounded mixed nitride fuel pins and 8 mixed oxide fuel pins that were designed according to Phenix pin geometry and irradiated at a linear power of 42 kW/m to a burn up of 6.9 at.%. Nimphe 2 was a rig composed of 5 mixed nitride fuel pins and 2 mixed carbide fuel pins, all helium-bounded, designed according to Super-Phenix pin geometry and irradiated at a linear power of 73 kW/m to a burn up of 5.8 at.%. The carbide and nitride fuel swelling observed during the experiments complies with swelling results specified in literature. However, the high fission gas releases in Nimphe 1 - which may be explained by the largely apparent porosity resulting from manufacturing - were less significant in Nimphe 2, which was nevertheless operated at a higher linear power. The effect of carbide and nitride fuel manufacturing conditions is also briefly discussed. (author)