Preliminary studies on volatile fission gas distribution in minor actinide-bearing metal fuel

The METAPHIX programme is a collaboration between the Central Research Institute of Electric Power Industry (CRIEPI, Japan) and the Institute for Transuranium Elements (ITU, part of the Joint Research Centre of the European Commission) with the support of the Commissariat à l´Energie Atomique et aux Energies Alternatives (CEA, France) devoted to study the safety and effectiveness of a closed nuclear fuel cycle in which Minor Actinides (MA) are incorporated in UPuZr fuel and irradiated in fast reactor. Four different metallic alloys were prepared at ITU. Rare Earths (RE) were included in some of the as-cast fuel to reproduce the output of a pyrometallurgical reprocessing facility. The Na-bonded fuel pins were irradiated in the Phénix reactor up to 2.5 at.%, 7 at.% and 10 at% burn-up. Scanning electron microscopy revealed the presence of some large precipitates rich in MA, RE and noble metals throughout the fuel radius in the low burn-up fuel initially containing MA and RE. The three species are not simultaneously present in all precipitate formations. Precipitates rich in RE and with similar compositions were identified also by Electron Probe Micro-Analysis (EPMA) investigation performed on a medium burn-up basic alloy sample. Comparing low- and medium burn-up fuel to unirradiated archive samples allowed observing that many precipitates experienced some degree of growth during irradiation. EPMA provided additional information about the elemental and phase distribution in the different regions of the fuel. Complementary spectroscopy investigations allowed addressing specific features identified by EPMA and, in particular, the behaviour of fission gases Xe and Kr retained in the matrix. Full understanding of behaviour and (re)distribution during irradiation of the main fuel phases and precipitates is necessary to avoid the occurrence of melting phases in the fuel and to limit fuel-cladding chemical interaction. These, in turn, are important components of the fuel safety analysisJRC.E.2-Safety of Irradiated Nuclear Material