A failure modes and effects analysis (FMEA) for stainless steel canisters used in many dry cask storage systems (DCSSs) was recently performed to identify credible degradation mechanisms that may be active over the lifespan of the canisters (EPRI report 3002000815). The scope of the literature review described here was limited to corrosion mechanisms that may affect stainless steels exposed to ambient atmospheric conditions. Emphasis was on stainless steel alloys currently licensed for use in the confinement boundary of DCSS canisters: Types 304, 304L, 304LN, 316, 316L, and 316LN austenitic stainless steels. Chloride-induced stress corrosion cracking (CISCC) was the most credible corrosion mechanism identified in the FMEA for environments relevant to the performance of welded canisters in DCSSs at independent spent fuel storage installations (ISFSIs). This literature review provides supporting information for that determination.
Due to the delayed opening of a final geological repository for used fuel generated by U.S. nuclear power plants, the majority of plants have constructed an ISFSI to relieve crowding in the spent fuel pool using DCSSs. Plants are concerned that corrosion of the DCSS inner stainless steel canister may occur at some sites over an extended life of 120 years or longer. EPRI is developing an Aging Management Plan with Susceptibility Assessment Criteria to address the concern that extended storage in given conditions may lead to loss of confinement function of stored DCSSs.
To provide an updated summary of literature relevant to the potential for atmospheric corrosion of stainless steel canisters, particularly CISCC.
This literature review comprises six sections. The first section introduces the report. The second section is a review of information relevant to determining the environments and conditions at the canister surface, with emphasis on the behavior of chloride-containing aerosols. The third section discusses literature and operating experience related to CISCC, while the fourth section covers other potential corrosion mechanisms including pitting and crevice/under deposit corrosion in both low-chloride and chloride-laden environments. The last two sections provide a summary of the review's key findings and the cited references.
This review considers literature to better define conditions at the canister surface as well as literature regarding susceptibility and timescales for atmospheric corrosion. The review characterized conditions that may lead to CISCC initiation and typical rates of propagation for atmospheric CISCC, pitting, and other mechanisms. Included are evaluations of chloride concentration models, databases, and deposit data.
The literature review showed that although there are exceptions, pitting typically initiates under less aggressive conditions than CISCC but propagates at a significantly lower rate. Topics considered in this literature review include
Industry experience with stainless steels at ambient and near-ambient temperatures (25°C–80°C) in regard to the possibility of SCC affecting the canisters
Corrosion modes and scenarios that need to be considered when evaluating the possibility of corrosion-caused penetration of the stainless steel canister confinement boundary
Variation in the rates of initiation and progression of corrosion as a function of material composition and thermomechanical condition, temperature, stress, availability of water, and concentration of chlorides in the water
Characteristics of the deposited salts and other materials on the canister surfaces as a function of time, temperature, ISFSI geographical location, and canister local geometry as well as the effect of these characteristics on SCC initiation and growth
Applications, Value, and Use
The information in this literature review informs the FMEA for stainless steel canisters used in DCSSs and acts as input to the forthcoming canister flaw growth and flaw tolerance assessment. Subsequently, an industry Susceptibility Assessment Criteria report is being developed to address the major degradation concerns identified and prioritized by this FMEA. This series of EPRI documents will culminate in an Aging Management Plan guidance report to support long-term management of this issue.