ABSTRACT

For long-term dry storage, most spent nuclear fuel in the United States is placed in welded 304 SS or 316 SS canisters that are stored within passively ventilated overpacks. As the canisters cool, sea-salt aerosols deposited on the canister surfaces will deliquesce to form potentially corrosive brines. We have used thermodynamic modeling to predict the chemical composition of the brines that form by deliquescence of sea-salt aerosols, and to estimate brine volumes and salt/brine volume ratios as a function of temperature and atmospheric relative humidity. We have also mixed representative brines and measured the physical and chemical properties of those brines over a range of temperatures. These data provide a matrix that can be used to predict the evolution of deliquescent brine properties over time on storage canister surfaces, as the canisters cool and surface relative humidity increases. Brine volumes and properties affect corrosion kinetics and damage distributions on the metal surface, and may offer important constraints on the expected rate and extent of corrosion and the timing of SCC crack initiation. The predicted brines do not consider reactions with atmospheric gases that are known to affect sea-salt particle and deliquescent brine compositions under field conditions. The potential effects of such reactions are discussed, and preliminary modeling and experimental data are presented.

INTRODUCTION

In the United States, spent nuclear fuel (SNF) from nuclear power reactors is eventually moved from storage pools to dry storage at near-reactor Independent Spent Fuel Storage Installations (ISFSIs). Typically, the dry storage systems are welded stainless steel (304 SS or 316 SS) containers enclosed in concrete, or steel and concrete, overpacks1. For most dry storage systems, passive ventilation is used to cool the canisters within the overpacks, and large volumes of outside air are drawn through the system by natural convection. Inherent in this passive design is the potential for dust and aerosols within the air to be deposited on the steel canisters, and, as the casks cool over time, salts in the dust will deliquesce (absorb water) to form brine on the storage container surface. If the salts contain aggressive components such as chloride, localized attack can occur. Most modes of corrosion, including general corrosion, pitting, and crevice corrosion, cannot significantly damage the steel canisters within anticipated storage times. However, one corrosion mechanism, chloride-induced stress corrosion cracking (SCC), may propagate relatively rapidly under some conditions and could potentially result in canister penetration during long-term interim storage. Chloride-induced SCC is a well-documented mode of attack for stainless steels in marine environments2, and many ISFSIs are in coastal areas.

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