This progress report describes work done in FY19 at Sandia National Laboratories (SNL) to assess the localized corrosion performance of container/cask 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. Work in FY19 refined our understanding of the chemical and physical environment on canister surfaces and evaluated the relationship between chemical and physical environment and the form and extent of corrosion that occurs.
This report discusses several possible sources of water that could persist in SNF dry storage canisters through the drying cycle. In some cases, the water is trapped in occluded geometries in the cask such as dashpots or damaged fuel. Persistence of water or ice in such locations seems unlikely, given the high heat load of the canistered fuel; this is especially true in the case of vacuum drying, where a strong driver exists to remove water vapor from the headspace of such occluded geometries. Water retention in Boral® core material is a known problem, that has in the past resulted in the need for much extended drying times. Since the shift to slightly higher porosity "blister resistant" Boral®, water drainage appears to be less of a problem. However, high surface areas for the Boral® core material will provide a trap for significant amounts of adsorbed water, at least some of which is certain to survive the drying process. Moreover, if corrosion within the cores produces hydrous aluminum corrosion products, these may also survive.
The phonon, infrared, and Raman spectroscopic properties of zirconium tungsten phosphate, Zr2(WO4)(PO4)2 (space group Pbcn, IT No. 60; Z = 4), have been extensively investigated using density functional perturbation theory (DFPT) calculations with the Perdew, Burke, and Ernzerhof exchange-correlation functional revised for solids (PBEsol) and validated by experimental characterization of Zr2(WO4)(PO4)2 prepared by hydrothermal synthesis. Using DFPT-simulated infrared, Raman, and phonon density-of-state spectra combined with Fourier transform infrared and Raman measurements, new comprehensive and extensive assignments have been made for the spectra of Zr2(WO4)(PO4)2, resulting in the characterization of its 29 and 34 most intense IR- and Raman-active modes, respectively. DFPT results also reveal that ν1(PO4) symmetric stretching and ν3(PO4) antisymmetric stretching bands have been interchanged in previous Raman experimental assignments. Negative thermal expansion in Zr2(WO4)(PO4)2 appears to have very limited impact on the spectral properties of this compound. This work shows the high accuracy of the PBEsol exchange-correlation functional for studying the spectroscopic properties of crystalline materials using first-principles methods.
This study was initiated to quantify and characterize the uncertainty associated with the degradation mechanisms impacting normal dry storage operations for used nuclear fuel (UNF) and normal conditions of transport in support of the Spent Fuel and Waste Science & Technology Campaign (SFWST) and its effectiveness to rank the data needs and parameters of interest. This report describes the technical basis and guidance resulting from the development of software to perform uncertainty quantification (UQ) by developing and describing a holistic model that integrates the various processes controlling Atmospheric Stress Corrosion Cracking (ASCC) in the specific context of Interim Spent Fuel Storage Installations (ISFSIs). These processes include the daily and annual cycles of temperature and humidity associated with the environment, the deposition of chloride-containing aerosol particles, pit formation, pit-to-crack transition, and crack propagation.
The High Burn-Up Demonstration Project was recently initiated by the Department of Energy (DOE) to evaluate the effects of fuel drying and long term dry storage on high burn-up spent nuclear fuel. As part of the project, samples of the He backfill gas were collected 5 hours, 5 days, and 12 days after completion of drying. The samples provide information on the state of the fuel at closure, and on the environment within the cask. At Sandia National Laboratories, the samples were analyzed by gamma-ray spectroscopy to quantify fission product gases and by gas mass spectrometry to quantify bulk and trace gases; water content was measured via humidity probe. Gamma-ray spectroscopy results indicated no detectible 85Kr, indicating no failed fuel rods were present after drying. Mass spectrometry indicated build-up of CO2 to 930 ppmv over two weeks, attributed to oxidation of organic compounds (possibly vacuum grease or vacuum pump oil) within the cask. H2, generated by either radiolysis or metal corrosion, also increased up to —500 ppmv. Water contents in the cask were higher than anticipated, increasing to —17,400 ppmv ±10% after 12 days. Measuring water content proved challenging, and possible improvements to the method for future analyses are proposed.
The DOE and industry collaborators have initiated the high burn-up demonstration project to evaluate the effects of drying and long-term dry storage on high burn-up fuel. Fuel was transferred to a dry storage cask, which was then dried using standard industry vacuum-drying techniques and placed on a storage pad to be opened and the fuel examined in 10 years. Helium fill gas samples were collected 5 hours, 5 days, and 12 days after closure. The samples were analyzed for fission gases (85Kr) as an indicator of damaged or leaking rods, and then analyzed to determine water content and concentrations of other trace gases. Gamma-ray spectroscopy found no detectible 85Kr. Sample water contents proved difficult to measure, requiring heating to desorb water from the inner surface of the sampling bottles. Final results indicated that water in the cask gas phase built up over 12 days to 17,400 ppmv ±10%, equivalent to ∼100 ml of water within the cask gas phase. Trace gases were measured by direct gas mass spectrometry. Carbon dioxide built up over two weeks to 930 ppmv, likely due to breakdown of hydrocarbon contaminants (possibly vacuum pump oil) in the cask. Hydrogen built up to nearly 500 ppmv. and may be attributable to water radiolysis and/or to metal corrosion in the cask.
For long-term storage, spent nuclear fuel (SNF) is placed in dry storage systems, commonly consisting of welded stainless steel canisters enclosed in ventilated overpacks. Choride-induced stress corrosion cracking (CISCC) of these canisters may occur due to the deliquescence of sea-salt aerosols as the canisters cool. Current experimental and modeling efforts to evaluate canister CISCC assume that the deliquescent brines, once formed, persist on the metal surface, without changing chemical or physical properties. Here we present data that show that magnesium chloride rich-brines, which form first as the canisters cool and sea-salts deliquesce, are not stable at elevated temperatures, degassing HCl and converting to solid carbonates and hydroxychloride phases, thus limiting conditions for corrosion. Moreover, once pitting corrosion begins on the metal surface, oxygen reduction in the cathode region surrounding the pits produces hydroxide ions, increasing the pH under some experimental conditions, leads to precipitation of magnesium hydroxychloride hydrates. Because magnesium carbonates and hydroxychloride hydrates are less deliquescent than magnesium chloride, precipitation of these compounds causes a reduction in the brine volume on the metal surface, potentially limiting the extent of corrosion. If taken to completion, such reactions may lead to brine dry-out, and cessation of corrosion.
Appropriate waste-forms for radioactive materials must isolate the radionuclides from the environment for long time periods. To accomplish this typically requires low waste-form solubility, to minimize radionuclide release to the environment. However, radiation eventually damages most waste-forms, leading to expansion, crumbling, increased exposed surface area, and faster dissolution. We have evaluated the use of a novel class of materials-ZrW2O8, Zr2P2WO12 and related compounds-that contract upon amorphization. The proposed ceramic waste-forms would consist of zoned grains, or sintered ceramics with center-loaded radionuclides and barren shells. Radiation-induced amorphization would result in core shrinkage but would not fracture the shells or overgrowths, maintaining isolation of the radionuclide. We have synthesized these phases and have evaluated their leach rates. Tungsten forms stable aqueous species at neutral to basic conditions, making it a reliable indicator of phase dissolution. ZrW2O8 leaches rapidly, releasing tungstate while Zr is retained as a solid oxide or hydroxide. Tungsten release rates remain elevated over time and are highly sensitive to contact times, suggesting that this material will not be an effective waste-form. Conversely, tungsten release rates from Zr2P2WO12 rapidly drop and are tied to P release rates; we speculate that a low-solubility protective Zr-phosphate leach layer forms, slowing further dissolution.