Publications

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This progress report describes work performed during FY21 at Sandia National Laboratories (SNL) to assess the localized corrosion performance of canister materials used in the interim storage of spent nuclear fuel (SNF). Of particular concern is stress corrosion cracking (SCC), by which a through-wall crack could potentially form in a canister outer wall over time intervals that are shorter than possible dry storage times. In FY21, modeling and experimental work was performed that further defined our understanding of the potential chemical and physical environment present on canister surfaces at both marine and inland sites. Research also evaluated the relationship between the environment and the rate, extent, and morphology of corrosion, as well as the corrosion processes that occur. Finally, crack growth rate testing under relevant environmental conditions was initiated.

This report summarizes the current actives in FY21 related to the effort by Sandia National Laboratories to identify and test coating materials for the prevention, mitigation, and repair of spent nuclear fuel dry storage canisters against potential chloride-induced stress corrosion cracking. This work follows up on the details provided in Sandia National Laboratories FY20 report on the same topic, which provided a detailed description of the specific coating properties desired for application and implementation on spent nuclear fuel canisters, as well as provided detail into several different coatings and their applicability to coat spent nuclear fuel canisters. In FY21, Sandia National Laboratories has engaged with private industry to create a Memorandum of Understanding and established a collaborative R&D program building off the analytical and laboratory capabilities at Sandia National Laboratories and the material design and synthesis capabilities of private industry. The resulting Memorandum of Understanding included four companies to date (Oxford Performance Materials, White Horse R&D, Luna Innovations, and Flora Coating) proposing six different coating technologies (polyetherketoneketone, modified polyimide/polyurea, modified phenolic resin, silane-based polyurethane hybrid with and without a Znrich primer, and a quasi-ceramic sol-gel polyurethane hybrid) to be tested, evaluated, and optimized for their potential use for this application. This report provides a detailed description of each of the coating systems proposed by the participating industry partners. It also provides a description of the planned experimental actives to be performed by Sandia National Laboratories including physical tests, electrochemical tests, and characterization methods. These analyses will be used to identify specific ways to further improve coating technologies toward their application and implementation on spent nuclear fuel canisters. In FY21, Sandia National Laboratories began baseline testing of the base metal material in according with activities of the Memorandum of Understanding. In FY22, Sandia National Laboratories will receive coated coupons from each of the participating industry partners and begin characterization, physical, and electrochemical testing following the test plan described herein.

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Additively manufactured (AM) stainless steels (SSs) exhibit numerous microstructural differences compared to their wrought counterparts, such as Cr-enriched dislocation cell structures. The influence these unique features have on a SSs corrosion resistance are still under investigation with most current works limited to laboratory experiments. The work herein shows the first documented study of AM 304L and 316L exposed to a severe marine environment on the eastern coast of Florida with comparisons made to wrought counterparts. Coupons were exposed for 21 months and resulted in significant pitting corrosion to initiate after 1 month of exposure for all conditions. At all times, the AM coupons exhibited lower average and maximum pit depths than their wrought counterparts. After 21 months, pits on average were 4 μm deep for AM 316L specimen and 8 μm deep for wrought specimen. Pits on the wrought samples tended to be nearly hemispherical and polished with some pits showing crystallographic attack while pits on AM coupons exhibited preferential attack at melt pool boundaries and the cellular microstructure.

Stress corrosion cracking (SCC) is an important failure degradation mechanism for storage of spent nuclear fuel. Since 2014, Sandia National Laboratories has been developing a probabilistic methodology for predicting SCC. The model is intended to provide qualitative assessment of data needs, model sensitivities, and future model development. In fiscal year 2021, improvement of the SCC model focused on the salt deposition, maximum pit size, and crack growth rate models.

During typical atmospheric conditions, cathodic reduction reactions produce hydroxyl ions increasing the pH in the cathodic region. Therefore, cathodic reduction reactions are investigated on platinum and stainless steel 304 L (SS304L) in NaOH solutions ranging in pH from 13.6 to 16.5. It was found that in solution pHs less than 16.5 the cathodic reduction reaction on Pt and SS304L was ORR with an electron transfer number less than two due to superoxide formation as an intermediate. Increasing pH decreased the number of electrons transferred. At a pH of 16.5, the cathodic reduction reaction on SS304L is no longer ORR and the cathodic current on the surface of the alloy is due to oxide reduction occurring on the surface as indicated by the creation of multi-component Pourbaix diagrams. The results of this study have important implications for predicting corrosion in atmospheric environments.

The critical pitting temperature (CPT) of selective laser melted (SLM) 316 L stainless steel in 1.0 M NaCl was measured and compared with a commercial wrought alloy. Potentiostatic measurements determined a mean CPT value of 16 ± 0.7 °C, 27.5 ± 0.8 °C and 31 ± 1 °C for the wrought alloy, the SLM alloy normal to the build direction and parallel to the build direction, respectively. The lead-in pencil electrode technique was used to study the pit chemistry of the two alloys and to explain the higher CPT values observed for the SLM alloy. A lower critical current density required for passivation in a simulated pit solution was measured for the SLM alloy. Moreover, the ratio of the critical concentration to saturated concentration of dissolving metal cations was found to be higher for the SLM alloy, which was related to its different salt film properties, possibly as a result of the SLM's distinct microstructure.

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