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A number of cans of spent nuclear fuel from the UK's Windscale advanced gas-cooled reactor have been recovered from over four decades of pond storage to be repackaged. All the cans were found to be flooded and those with cut sections of pins contained significant quantities of sludge; cans containing only intact and/or punctured pins were free from sludge. These observations necessitated a study of the sludge to understand its composition. The results will help inform the likely long-term behaviour of irradiated UO 2 in contact with water during interim or extended storage, or disposal. Inductively coupled plasma – mass spectrometry found the sludge to be heterogeneous between cans but to be largely uranium based. Iron dominated the composition of one sample. Scanning electron microscopy – energy dispersive X-ray, power X-ray diffraction and thermogravimetric analysis indicated that the uranium phases were almost entirely crystalline UO 2. No other uranium phases were detected and the UO 2 was assigned as shards of spent fuel. In contrast, the iron phases consisted of a mixture of different oxides and oxy-hydroxides that appear to have precipitated from solution. Caesium was commonly associated with the iron phases. The sludge was associated with a substantial amount of moisture when investigated by TGA. [ABSTRACT FROM AUTHOR]

Microstructure is a controlling factor in the behavior of sintered materials. This work presents a quantitative phase field model of thermal sintering that predicts the evolution of the microstructure by capturing the sintering stress, GB/vacancy interactions, non-uniform diffusion, and grain coarsening without introducing a separate rigid body motion term. The model provides a mechanistic description of sintering using the grand potential phase field approach. Small test simulations are used to verify the new model against sintering theory, and they show that 3D simulations predict faster densification and coarsening than 2D simulations. 3D simulations are compared against experimental data available in the literature. The results of this comparison show that the model provides a reasonable estimate of the sintering behavior, though it overpredicts the sintering rate. This may be due to uncertainty in the material parameters and the relatively small scale of the simulation. [ABSTRACT FROM AUTHOR]