Irradiation-induced clustering in a high-Ni, high-Mn, low-Cu boiling water reactor pressure vessel weld after 28 full power years

High-Ni, high-Mn welds are the life-time determining components in reactor pressure vessels (RPVs) of Nordic reactors at desired operating times of pressurized water reactors (PWR) of 60 or even 80 years due to embrittlement that is caused by pronounced clustering of Ni, Mn and Si. To understand early stages of clustering we performed atom probe tomography (APT) measurements on an axial weld of the boiling water RPV from Barsebäck Unit 2 decommissioned after 28 years of operation. Contrary to our previous work on the same weldment, here we report observation of clustering. The cluster number densities vary significantly between individual APT measurements, which we attribute to variations in local Ni and Mn concentrations, a trend even seen within single grains. Based on comparison with high fluence samples containing more and larger clusters we propose that NiMnSi cluster formation and growth is an irradiation-induced continuous process without a relevant threshold dose.