Corrosion prevention and control at Sellafield nuclear fuel reprocessing plant

Abstract

After discharge from the reactor and transport to Sellafield, irradiated nuclear fuels are stored in water filled ponds before being reprocessed. BNFL has established fuel management strategies and a pond water chemistry which ensures minimal corrosion of stored fuels. BNFL's materials selection policy for its reprocessing plants has developed from over 35 years’ experience with Magnox reprocessing. An extensive R&D programme evaluated materials for the thermal oxide reprocessing plant (Thorp). The original stainless steels used in Magnox reprocessing plants, 18Cr–13Ni–Nb and 18Cr– 8Ni–Ti, have been observed to show deficiencies in operation such as end grain corrosion, knifeline attack at welds, susceptibility to corrosion in HNO₃ vapours, and poor resistance to iodine vapour corrosion. Titanium plant has also shown corrosion performance deficiencies. For Thorp, nitric acid grade 18–10L and 304L stainless steels have replaced 18–13–Nb and 18–8–Ti respectively, whereas 310L has been used to construct fuel dissolvers. In preference to titanium, Thorp makes extensive use of zirconium where minimal corrosion is required, e.g. heat transfer surfaces and demisting column packing. For reprocessing plants, BNFL's materials selection procedure begins with a corrosion audit, in which plant designs are submitted to a corrosion specialist who either selects the appropriate material or identifies where further R&D or design changes are required. To augment the materials selection procedures, BNFL has instituted a policy of remote inspection and on line corrosion monitoring for its reprocessing plants. Facilities for remote repair are also being developed. Decommissioning of plant may involve corroded and degraded structures. Methods have been developed for the non-destructive examination of concrete and rebars to aid decommissioning scheduling. High level and intermediate level radioactive waste, arising from the reprocessing operations, will be stored at Sellafield until afinal disposal policy is formulated. Corrosion resistant container materials have been selected for all major waste streams. A concurrent R&D programme is aimed at understanding the corrosion nzechanisms pertinent to reprocessing plants and seeking improved methods and materials.