The microstructural stability of Nb-stabilised steels under neutron irradiation and proposed methods of improvement

FV548 has been considered as a suitable alternative to 316 for Fast Reactor fuel pin cladding. Prolonged irradiation, however, has on a number of occasions shown cold-worked FV548 to be susceptible to microstructural instability at temperatures above 5600C; this is seen in the form of recovery of the cold-worked structure and partial recrystallisation. These observations have raised doubts as to the suitability of 20% cold-worked FV548 for Fast Reactor cladding applications since both the void swelling behaviour and mechanical properties of the recovered and recrystallised material are inferior to those of the original cold-worked alloy. Optimisation studies have been carried out to improve the performance of the alloy at high temperatures. This has included an investigation into the effects of cold-work level on recovery and recrystallisation and a study on the effects on undissolved NbC particles on microstructural stability. These have led to proposals for modifications to the original 20% cold-worked FV548 clad specification and neutron irradiations of these optimised versions are currently under way before full scale pin trials begin. This paper reviews the neutron irradiation data available on CW FV548 with emphasis on its microstructural instability and the out-of-pile studies which have been used to develop optimised versions of the steel. (Author)