Material characterisation and selection for the international research project “PRIMAVERA”

The international research project “PRIMAVERA” aims to develop a physically validated prediction model for the transition temperature shift dependence (ΔT_K) of VVER-400 weld metals versus neutron fluence after reactor pressure vessel annealing. The reactor pressure vessel of VVER type has several ring welds between the forgings, characterised by large overall dimensions, structural heterogeneity and mechanical properties variations. The weld metal is a mild low-alloy steel with rather complex structure depending on chemical composition, manufacturing technology and heat treatment and the selection of the weld sample is crucial for the success of the project. A weld with high phosphorus content manufactured following standard VVER-440/230 weld technology, has been identified and selected for the project. The detailed characterisation of the selected weld in the un-irradiated condition is the first phase of the research programme and is presented in this paper. Significant phosphorus content variations through the weld could be identified; from 0.022 to 0.044 wt%. Only small variations in mechanical properties were identified; a maximum of 13 °C difference in ductile to brittle transition temperature, mainly related to the distance to the fusion boundary and the base metal. The study also showed that testing of 14 Charpy specimens is sufficient for a correct evaluation of transition temperature. A larger number of tests are required for base metal compared with the weld metal.