Effects of Irradiation and Thermal Ageing on the Nanoscale Chemistry of Steel Welds

Abstract: Structural materials of nuclear power plants degrade during operation due to thermal ageing and irradiation from the reactor core. Effects on the materials are an increase in hardness and tensile strength, and a decrease in ductility and fracture toughness, i. e. embrittlement. The degradation of the mechanical properties stems from changes in the microstructure. In this thesis, the effects of thermal ageing and irradiation on the nanoscale chemistry has been studied using atom probe tomography (APT).   During irradiation, nanometre sized clusters are formed in the reactor pressure vessel (RPV) welds. As the RPV is a life-limiting part of a nuclear power plant, neutron irradiation with high flux is attractive for accelerated studies. Here, the effect of high flux is found to result in a higher number density of smaller NiMnSi-rich clusters for the high Ni and Mn - low Cu welds from Ringhals R4, resulting in similar hardening compared to surveillance material. It is also found that there are some stable matrix defects formed in the high flux material, contributing to the embrittlement. The cluster evolution showed no signs of late blooming phases (an accelerated degradation at high fluences). Furthermore, thermal ageing during operation for 28 years of a weld from the former Ringhals R4 pressurizer with similar composition is found to result in  clusters forming mainly on dislocations, hardening the weld metal.   In ferrite with higher Cr-content, such as the ferritic parts of the mainly austenitic welds from the core barrel of the decommissioned Spanish reactor José Cabrera, spinodal decomposition occurs as well as G-phase (Ni16Si7Mn6) precipitation. Weld metals irradiated up to 2 dpa are compared with thermally aged welds, confirming that the irradiation is considerably contributing to the changes in the microstructure. After 0.15 dpa, the spinodal decomposition was well developed, and the Cr concentration in the ferrite was found to influence the wavelength more than the difference in irradiation (0.15 to 2 dpa). The G-phase precipitates were more well-developed after 1 dpa neutron irradiation, but no difference could be distinguished between the material irradiated to 1 and 2 dpa.