Radiation Induced Precipitation in Reactor Pressure Vessel Steel Welds

Licentiatavhandling, 2016

Radiation induced clustering and precipitation affect the mechanical properties of the reactor pressure vessel (RPV) of a nuclear power plant, i.e. the material becomes embritteled. The combination of low Cu (0.04 at%) and high Ni (1.6 at%) used in Ringhals RPV welds is known to further enhance the embrittlement in terms of ductile-to-brittle transition temperature (DBTT) shift during long time ageing. Material irradiated in the Halden test reactor to levels corresponding to 20 and 60 years of reactor operation was evaluated. This research focuses on further understanding the clustering of Ni, Mn, Si and Cu in these low-Cu high-Ni RPV steels, and how it evolves with time. As the clusters are in the nanometre-range, atom probe tomography (APT) is a highly suitable technique for this type of study, giving both position and chemical identity of individual atoms with near-atomic resolution. The evaluation of the clusters using APT is intricate and requires some careful considerations. In this thesis, the quality of the data is discussed, as well as the cluster finding algorithm maximum separation method (MSM), and the choice of algorithm parameters. Furthermore, materials science aspects are brought up, discussing the implications of the cluster composition, number density, size distribution, flux effects, and the connection to the mechanical properties, i. e. the embrittlement. It is found that a high flux results in smaller clusters with a higher number density than what was found in a material irradiated to a similar fluence but with lower flux. The cluster compositions are similar with an exception for high flux induced clusters having a higher Cu content. The size distributions of the clusters are slightly bimodal.