ABSTRACT

Spent nuclear fuel (SNF) is currently stored in dry storage canisters (DSCs) at various reactor sites awaiting final disposal in a future repository site. Many DSCs are made from welded Type 304/304L stainless steel (UNS S30400/S30403) and then placed inside a concrete cask system with passive atmospheric air cooling. The most likely long-term degradation that may result in a breach of the DSC's containment system is chloride induced stress corrosion cracking (CISCC). A precursor to CISCC is localized corrosion in the form of pit initiation and propagation. This paper describes a model to quantify the time-based occurrence of CISCC on SNF DSCs that includes: weld residual stress, hourly time histories for site weather data, canister aerosol deposition density, SNF decay heat, canister surface temperature, deliquescence of mixed salts, droplet condensation, coalescence, and evaporation, dissolved species concentrations, pit initiation, growth, and repassivation, a criterion for pit-to-crack transition, and strain rate-based SCC crack growth. A supplemental and repeatable (for other designs and applications) experimental study to calibrate the model components is presented. The integrated model incorporates input parameter uncertainties to compute a probabilistic estimate of remaining life.

INTRODUCTION

Spent nuclear fuel (SNF) is currently stored in stainless steel dry storage canisters (DSCs) contained within concrete cask systems with passive ambient air cooling [1]. These systems are emplaced, either horizontally or vertically, at independent spent fuel storage installations (ISFSIs), located at utility reactor sites. The ambient air introduces moisture, aerosolized salt particles, and dust to the canister surfaces [2-4]. The composition of the aerosols depends on geographical factors, such as proximity to the ocean, industrial area, rural areas, and transportation corridors that use road salt for winterization. Condensation of aqueous solution (deliquescence) and drying out (efflorescence) occur from changes in ambient temperature and relative humidity (RH). Once a condensed environment is formed, if the RH continues to rise, greater wetting leads to a dilution of the solution [2,3]. Depending on the extent of wetting and the solute concentration in the condensed solution, localized corrosion can initiate [5-7]. The localized corrosion can transition to stress corrosion cracking (SCC) or can compete with SCC and prevail [7-12].

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