Chloride Induced Stress Corrosion Cracking

Chloride induced stress corrosion cracking CISCC) of spent nuclear fuel storage canisters is a serious concern since it can potentially lead unexpected premature failure of the canisters. The weldments in these canisters involving segregated and sensitized zones along with residual stress) are exposed to coastal environments including salty air, humidity and variable temperature), which make the canisters susceptible to failure by CISCC. The chloride concentration in the solution leads to localized pitting or crevice corrosion nucleation. While intergranular stress cracking is commonly observed in sensitized parts of the stainless steel structure at ambient to moderate temperature, transgranular stress cracking is observed regardless of material condition at relatively high temperature. The ability to accurately predict the crack growth rate CGR) is critical for the nuclear industry to efficiently schedule canister inspections and replacement needed for reliable ageing management programs. The objective of this Phase 1 SBIR program is to develop physics-based mechanistic models that predict the damage evolution rates for stainless steel canisters in a marine environment. In collaboration with DNV GL, Prof. John Scully from University of Virginia, and Dr. Peter Andresen at GE Global Research, QuesTek intends to develop a multistage model that will predict the service life of stainless steel canisters by accounting for various stages of damage evolution such as the onset of localized under-deposit corrosion e.g. pit, crevice) and subsequent crack growth. The growth rate of defects in each of these stages will be determined by mechanistic models that take into account the effect of the environmental variables. The crack growth mechanism for each of these stages will be validated by experimental program and combined into a unified framework to bridge the effect of the environment variables to the ultimate goal of life prediction for stainless steel canisters. The scheme will be integrated into QuesTeks Integrated Computational Materials Engineering ICME) approach to yield a generic life prediction tool which can be readily integrated with existing component modeling practices. Ultimately, this approach is intended to provide designers quantitative estimates for ageing management and extract the greatest extent of service life. The model development is the most significant task of this phase I program. This task involves developing models to predict crack growth rate that account for different mechanisms, such as slip-induced passive film rupture mechanism and hydrogen-induced interface decohesion mechanism, as well as their interactions. Relevant mechanistic models for each mechanism will be modified from their original form to suit the case of CISCC in coastal environments, and finally combined into a unified framework to provide accurate quantitative prediction of crack growth rate. The end use of this study will be the determination of the integrity and safe handling of the canisters after prolonged storage periods in a marine-like environment.