Research Activities, Investigations and Monitoring Related to Wet Storage: United Kingdom

The storage regime for stainless-steel clad gas cooled reactor fuel from AGRs was developed in the 1980s when the typical fuel stringer burnup was in the region of 18–21 GWd/tU. Currently, stringer mean irradiation is substantially higher, with 33–35 GWd/tU being the typical range and a peak element burnup around 43 GWd/tU. With the transition of AGR fuel management from reprocessing to long term storage in 2018, a series of examinations have been undertaken to understand whether there have been any significant changes in cladding microstructure as a result of the higher burnup and associated longer in-reactor dwell time (presently, typically 6–7 years), that could affect fuel performance during storage. AGR fuel differs from LWR assemblies, in that each fuel channel in the reactor is fuelled with a stringer consisting of eight, axially stacked, fuel elements containing 36 fuel pins of 1 metre length. Thus, the irradiation conditions experienced by the individual fuel pins varies considerably, as shown. In subsequent paragraphs reference to an element number (e.g. element 4) refers to the position in the stringer, with position 1 being the lowest located at the coolant inlet to the core.