Nature of brittle-to-ductile transition in UO 2-20 wt% PuO 2 nuclear fuel

Abstract

The four-point bend test was used to determine the brittle-to-ductile transition temperature T c in a mechanically blended UO 2-20 wt% PuO 2 fuel as a function of strain-rate ε (0.085–0.73 h −1), and volume-fraction porosity P (0.034–0.12). T c was located at about half the melting temperature on the absolute scale, and increased as the strain rate increased according to the relation T c(°C) = 5.5×10 4 28.6− ln ε ̇ −273 , and decreased with an increase in porosity, according to the relation T c(°C) = 1487−1588 P . Brittle fracture originated at existing surface flaws, and, therefore, crack propagation was the important process. A reduction in fracture toughness was achieved by increasing the strain rate. Pores can act as “crack blunting” agents, and the effect of blunting partly offsets the reduction in strength due to a decrease in load-bearing surface. High-temperature deformation was described by the conventional Arrhenius expression, and the implication was that grain-boundary sliding was responsible for the enhanced ductility of the mixed oxide about 100 °C above T c. The results are discussed relative to an in-reactor fuel-cracking model.