Swelling analysis of highly rated MX-type LMFBR fuels. I

Four MX-type fuels (U,Pu)C, (U,Pu)Csub(0.5)Nsub(0.5), (U,Pu)Csub(0.2)Nsub(0.8), and (U,Pu)N have been irradiated in helium-bonded pins at heat ratings up to 1300 W/cm. The swelling behaviour has been analyzed on the basis of the complex restructuring occurring during the irradiation. The swelling mechanisms in the different restructured zones have been investigated with the aid of laboratory annealing experiments. Finally the properties of the different fuels have been discussed. Carbides provide acceptable swelling behaviour only under restricted temperature and temperature gradient conditions, whereas nitrides exhibit homogeneous performance in a wide temperature range. Carbonitrides with high nitrogen content provide an unfavourable in-pile restructuring, while carbonitrides with low nitrogen content display a carbide-type restructuring, but with more favourable features. (Auth.)