The Effect of BWR Startup Environments on Crack Growth in Structural Alloys

During startup of a Boiling Water Reactor (BWR), the water chemistry and temperature are constantly changing. Special operational practices can be performed to control the dissolved oxygen level using, for example, vacuum deaeration. To assess the impact of startup practice on environmental cracking in the structural materials used in the BWR, a large program was performed to evaluate crack growth at representative environmental conditions for both conventional and vacuum deaeration startup practices. Five alloys were studied: Types 304 and 316 nuclear grade stainless steel, Inconel 600, carbon steel, and A508-2 low alloy steel. Tests were performed using fracture mechanics type specimens with constant load crack growth measured at appropriate stress intensity levels. The program intent was to compare the crack growth rates for the two practices. The results show that normal startup and startup deaeration environments had varied effects. Sensitized Type 304 stainless steel exhibited a decrease in crack growth rate and concomitant decrease in severity of intergranular fracture morphology at the low temperatures under deaeration. In contrast, the Type 316 nuclear grade displayed very little influence of startup deaeration due to its inherent resistance to stress corrosion cracking. Inconel 600 showed up to a factor of five reduction in crack growth rate in the deaeration environment. The response of the carbon steel to deaeration was mixed—limited benefit was seen at low temperature, none was seen at the higher temperature. Finally, the low alloy steel displayed some improvement in behavior at low temperature at the high stress intensity value investigated. In summary, the program showed that a modest benefit, in terms of stress corrosion cracking mitigation, could be attributed to deaeration during startup, particulary for Type 304 stainless steel and Inconel 600.