Fracture-Surface Energy And Fracture-Toughness Of (U,Pu)C And (U,Pu) (C,O)

The fracture surface energy, γf, and the fracture toughness, K1c, were measured for the advanced fast-breeder reactor fuels (U0.80Pu0.20)C and (U0.82Pu0.18) C0.9O0.1, using the method of Hertzian indentation with steel indentors. Elastic behavior was observed with transgranular cracks yielding K1c, values of ∼ 1.5 MN·m−3/2. The surface energy of the oxicarbide was smaller than that of the carbide. Annealing of the (U,Pu) (C,O) led to a completely different behavior. Permanent “quasi-plastic” deformation was observed, in addition to Hertzian ring cracks, due to pronounced grain-boundary weakening and grain separation. The plausible cause, i. e. precipitation of He formed by decaying Pu into grain-boundary bubbles, indicates that the in-pile behavior of a nuclear fuel after some burn-up does not necessarily correspond to the behavior observed in laboratory experiments, even in the absence of restructuring due to thermal gradients that exist during operation.