Design of a neutron irradiation campaign for electron beam welded reactor pressure vessel steel

Abstract

The objective of this study is to document a neutron irradiation experiment campaign intended to enable evaluation of embrittlement in electron beam (EB) welded SA508, Grade 3 reactor pressure vessel (RPV) steel for small modular reactors. Arc welds in RPVs have historically experienced severe irradiation embrittlement, but advanced techniques such as EB welding, offer promise for minimizing embrittlement. Here, EB welds are made on a forged SA508 ingot and a SA508 compact manufactured by powder metallurgy with hot isostatic pressing (PM-HIP). Two different post-welding heat treatments are applied. Miniature compact tension (CT) specimens are machined from each weld region (weld centerline, heat affected zone, base metal) then fatigue pre-cracked. The average number of cycles to achieve the pre-crack length is negatively correlated with grain size, but positively correlated with hardness and ferrite phase fraction. These results imply that ductile ferrite phases absorb more energy than harder martensite and/or bainite phases. Specimens are then neutron irradiated in the Advanced Test Reactor to an average dose of 0.17 ± 0.02 displacements per atom at 322.4 ± 7.8 °C, representative of high-fluence end-of-life conditions. Specimens will undergo slow-strain-rate fracture testing over multiple temperatures to determine irradiation-induced ductile-to-brittle transition temperature shifts. Microstructural underpinnings of this irradiation embrittlement across different weld regions and materials, will also be ascertained. Specimens will be made available to researchers worldwide for future studies; this manuscript will serve as a permanent reference for material provenance.