Microstructure Response of WWER-440 Reactor Pressure Vessel Weld Material After Irradiation and Annealing Treatment

Abstract

The aim of this publication is to contribute to the recent research and development efforts on mitigation of the reactor pressure vessel (RPV) embrittlement for operational lifetime extension. This research was done in the framework of the PRIMAVERA international project and it was focused on the WWER-440 reactor pressure vessel materials, specifically weld metals. The investigation primarily addressed the issues related to the re-irradiation effects and annealing treatment of “Russian-type” weld metals. A series of experimental data for WWER-440 reactor pressure vessel welds are reported and discussed to analyse the effect of the recovery annealing procedure. The specific annealing process has been developed and practically applied for “Russian-type” reactor pressure vessels to recover the negative effect of radiation embrittlement. Besides mechanical properties, more detailed information regarding microstructure evolution is described because it brings additional information which can help to understand the embrittlement–annealing–re-embrittlement processes. This study contributes to the experimental investigation of three different welds with high levels of copper (0.16 wt. %) and variable content of phosphorus (0.027–0.038 wt. %) at four different stages (unirradiated, irradiated, annealed after irradiation, and re-irradiated after annealing). The specimens were irradiated in the nuclear power plant (NPP) Rovno-1 and Rovno-2 up to a fluence of ∼1.2 × 1023 n/m2 and approximately 1.2 × 1024 n/m2 and 5.9 × 1024 n/m2 (En > 0.5 MeV) for the re-irradiation stage. After the first stage of irradiation, the samples were annealed in accordance with standard regime for WWER-440 RPVs (475°C, 100 h). The flux was about 4 × 1015 n/m2s (Rovno-1) and 2 × 1016 n/m2s (Rovno-2), respectively. The analysis of microstructure was done by atom probe tomography (APT) to obtain information concerning precipitation kinetics of Cu and P segregation, as well as interaction with other alloying elements. Further study of point-type defects and their clustering was analysed by positron annihilation spectroscopy techniques. The paper aims to report on behaviour of investigated RPV weld materials which were irradiated and annealed to analyse the response of main mechanical properties and microstructural parameters. The authors also believe that this study will be of value to the specialists and experts who are involved in the assessment of “Western-type” reactor pressure vessels, which is carried out as a part of ongoing international activities related to the safe and long term operation of present NPP fleet.