Characterization of Physical and Chemical Conditions for Atmospheric Corrosion of Spent Nuclear Fuel Dry Storage Canisters
Authors: Bryan Charles , Knight Andrew , Katona Ryan , Schaller Rebecca Filardo 
Published: December 2020

For long-term interim storage, spent nuclear fuel (SNF) is currently stored in stainless steel canisters at reactor sites. At near-marine sites, deposition and deliquescence of chloride-rich sea-salt aerosols may lead to localized canister corrosion and potentially, to penetration via chloride-induced stress corrosion cracking (CISCC). To assess the potential for CISCC, and to aid in development of appropriate corrosion testing protocols, expected physical and chemical conditions have been characterized for canisters at four near-marine independent spent fuel storage installations (ISFSIs) (Diablo Canyon, Maine Yankee, Calvert Cliffs, and Hope Creek).

First, appropriate diurnal temperature and relative humidity (RH) cycles for corrosion testing were determined. Diurnal weather fluctuations result in variations in brine volume and solute concentration that may affect initiation, rate, or extent of canister surface corrosion and CISCC. To determine appropriate testing conditions, daily cycles in canister surface environment were estimated by taking local weather data and applying an equation of state for water [1] to calculate RH values at elevated canister surface temperatures. Results indicated that even slightly elevated temperatures (5-10°C) greatly reduced the diurnal range of RH experienced on the canister surface, and that previous experimental studies had used simulated diurnal RH ranges that were much too large.

Second, dust samples collected from in-service canister surfaces were used to evaluate the composition of soluble salts present. While sea-salt aerosols were dominant at one ISFSI, at other sites, the soluble salts contained elevated concentrations of nitrate, sulfate, and calcium relative to sea-salts. Despite storage at near-marine locations, measured nitrate:chloride ratios in the dusts were as high as 1:1. Deliquescent brine compositions, calculated using the geochemical solubility and speciation code EQ3/6 [2], differed greatly from those produced by sea-salt aerosols. The importance of other anions (i.e. NO3
- and SO4
2
-) as possible corrosion inhibitors will be evaluated experimentally, using brine formulas based on the measured salt compositions.

Third, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) analysis of the dust samples indicated that salt aerosols comprised only a small fraction of the dust, with the largest fraction consisting of detrital silicate mineral grains. Such grains would largely be inert but may affect corrosion processes by spreading deliquesced brine over the surface through capillary effects. Larger particles may also act a crevice-formers, providing occluded regions for corrosion initiation. Inert dust particle size distributions have been determined from the SEM images and are being used to develop experimental testing protocols to evaluate these effects. Median particle sizes were in the 4-7 µm range, with the largest particles generally less than 100 µm. For laboratory testing, silicate particles with diameters of 5 µm and 74 µm (200 mesh) will be used, as well as a mixture to assess combined effects.

Diurnal variations in temperature and RH, and the presence of other anionic species, may significantly affect the corrosiveness of near-marine locations by impacting brine corrosivity (composition and concentration) and rates of corrosion initiation. Inert dust particles may promote corrosion initiation by creating crevices on the metal surface and will certainly affect the brine distribution on the surface and the resulting cathode size and cathode kinetics. These data provide the basis for developing realistic test matrices to evaluate these effects.

Acknowledgements:

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. This document is SAND2020-5449 A.

References:

[1] Wagner W. and Pruß A. (2002). "The IAPWS formulation 1995 for the thermodynamic properties of ordinary water substance for general and scientific use." Journal of Physical and Chemical Reference Data 31(2): 387-535.

[2] Wolery T.J., EQ3/6: A Software Package for Geochemical Modeling, Lawrence Livermore National Lab, Livermore, CA (2010): https://www.osti.gov/servlets/purl/1231666.