Microstructure, mechanical properties and IASCC susceptibility of stainless steel baffle bolts after 30 years of operation in a PWR

Abstract

Neutron irradiation in the core of a nuclear power plant can change the material properties of stainless steel significantly. In this paper cold worked AISI 316 baffle bolts retrieved from Doel 1 PWR were investigated with the aim to assess the structural degradation due to neutron irradiation by analyzing microstructure, mechanical behavior and stress corrosion susceptibility. The baffle bolts had received neutron doses ranging from 6 to 59 dpa covering a major part of a nuclear power plant's lifetime. First, microstructural investigations were carried out to reveal the defect and dislocation density and the amount of void swelling. Then mechanical properties were determined using tensile tests and fracture toughness tests. The irradiation assisted stress corrosion cracking (IASCC) susceptibility was investigated by slow strain rate testing and crack growth rate testing. Results were compared with literature values and found to be in good agreement. Main conclusions were that void swelling was small even for the highest dpa values and that the fracture toughness values saturated above 15 dpa. The material was susceptible to IASCC as shown by SSRT and crack growth rate measurements exhibiting intergranular fracture morphology.