Characterisation of corrosion products from degraded uranium dioxide fuel

Abstract 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 UO2 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 UO2. No other uranium phases were detected and the UO2 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.