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.
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.
The negative thermal expansion (NTE) material Zr2(WO4)(PO4)2 has been investigated for the first time within the framework of the density functional perturbation theory (DFPT). The structural, mechanical, and thermodynamic properties of this material have been predicted using the Perdew, Burke and Ernzerhof for solid (PBEsol) exchange-correlation functional, which showed superior accuracy over standard functionals in previous computational studies of the NTE material α-ZrW2O8. The bulk modulus calculated for Zr2(WO4)(PO4)2 using the Vinet equation of state at room temperature is K0 = 63.6 GPa, which is in close agreement with the experimental estimate of 61.3(8) at T = 296 K. The computed mean linear coefficient of thermal expansion is -3.1 × 10-6 K-1 in the temperature range ∼0-70 K, in line with the X-ray diffraction measurements. The mean Grüneisen parameter controlling the thermal expansion of Zr2(WO4)(PO4)2 is negative below 205 K, with a minimum of -2.1 at 10 K. The calculated standard molar heat capacity and entropy are CP0 = 287.6 and S0 = 321.9 J·mol-1·K-1, respectively. The results reported in this study demonstrate the accuracy of DFPT/PBEsol for assessing or predicting the relationship between structural and thermomechanical properties of NTE materials.
This progress report describes work done in FY18 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). The work focuses on stress corrosion cracking (SCC), the only mechanism 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 FY18 continued several studies initiated in FY17 that are aimed at refining the understanding of the chemical and physical environment on canister surfaces, and evaluating the relationship between chemical and physical environment and the form and extent of corrosion that occurs. The SNL canister environment work focused on evaluating the stability of sea-salt deliquescent brines on the heated canister surface; an additional opportunity to analyze dusts sampled from an inservice spent nuclear fuel storage canister also arose. The SNL corrosion work focused predominantly on pitting corrosion, a necessary precursor for SCC, and process of pit-to-crack transition. SNL is collaborating with several university partners to investigate SCC crack growth experimentally, providing guidance for design and interpretation of experiments. The scope of these efforts targets near-marine Independent Spent Fuel Storage Installation environments which are generally considered to be most aggressive for pitting and SCC. Work to define the chemical and physical environment that could develop on storage canister surfaces in near-marine environments included experiments to evaluate the thermal stability of magnesium chloride brines, representative of the first brines to form when sea-salts deliquesce, with the specific goal of understanding and interpreting results of sea-salt and magnesium chloride corrosion experiments carried out under accelerated conditions. The experiments showed that magnesium chloride brines, and by extension, low RH sea-salt deliquescent brines, are not stable at elevated temperatures, losing chloride via degassing of HC1 and conversion to Mg-hydroxychlorides and carbonates. The experiments were carried out on an inert substrate to eliminate the effects of corrosion reactions, simulating brine stabilities in the absence of, or prior to, corrosion. Moreover, analysis of salts recovered from actively corroding metal samples shows that corrosion also supports or drives conversion of magnesium chloride or sea-salt brines to less deliquescent salts. This process has significant implications on corrosion, as the secondary phases are less deliquescent than magnesium chloride; the conversion reaction results in decreases in brine volume, and potentially results in brine dry-out. The deliquescence properties of these reaction products will be a topic of active research in FY19.
Stress corrosion cracks (SCC) represent a major concern for the structural integrity of engineered metal structures. In hazardous or restricted-access environments, remote detection of corrosion or SCC frequently relies on visual methods; however, with standard VT-1 visual inspection techniques, probabilities of SCC detection are low. Here, we develop and evaluate an improved optical sensor for SCC in restricted access-environments by combining a robotically controlled camera/fiber-optic based probe with software-based super-resolution imaging (SRI) techniques to increase image quality and detection of SCC. SRI techniques combine multiple images taken at different viewing angles, locations, or rotations, to produce a single higher- resolution composite image. We have created and tested an imaging system and algorithms for combining optimized, controlled camera movements and super- resolution imaging, improving SCC detection probabilities, and potentially revolutionizing techniques for remote visual inspections of any type.
Cubic zirconium tungstate (α-ZrW2O8), a well-known negative thermal expansion material, has been investigated within the framework of density functional perturbation theory (DFPT), combined with experimental characterization to assess and validate computational results. Using combined Fourier transform infrared measurements and DFPT calculations, new and extensive assignments were made for the far-infrared (<400 cm−1) spectrum of α-ZrW2O8. A systematic comparison of DFPT-simulated infrared, Raman, and phonon density-of-state spectra with Fourier transform far-/mid-infrared and Raman data collected in this study, as well as with available inelastic neutron scattering measurements, shows the superior accuracy of the PBEsol exchange-correlation functional over standard PBE calculations for studying the spectroscopic properties of this material.
Elastic and thermodynamic properties of negative thermal expansion (NTE) αα-ZrW2O8 have been calculated using PBEsol and PBE exchange-correlation functionals within the framework of density functional perturbation theory (DFPT). Measured elastic constants are reproduced within ~2% with PBEsol and 6% with PBE. The thermal evolution of the Grüneisen parameter computed within the quasi-harmonic approximation exhibits negative values below the Debye temperature, consistent with observation. The standard molar heat capacity is predicted to be C $O\atop{P}$=192.2 and 193.8 J mol-1K-1 with PBEsol and PBE, respectively. These results suggest superior accuracy of DFPT/PBEsol for studying the lattice dynamics, elasticity and thermodynamics of NTE materials.