Publications

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One of the primary concerns with the long-term performance of storage systems for spent nuclear fuel (SNF) is the potential for corrosion due to deliquescence of salts deposited as aerosols on the surface of the canister, which is typically made of austentic stainless steel. In regions of high residual weld stresses, this may lead to localized stress-corrosion cracking (SCC). The ability to detect and image SCC at an early stage (long before the cracks are susceptible to propagate through the thickness of the canister wall and leaks of radioactive material may occur) is essential to the performance evaluation and licensing process of the storage systems. In this paper, we explore a number of nondestructive testing techniques to detect and image SCC in austenitic stainless steel. Our attention is focused on a small rectangular sample of 1 × 2 in2 with two cracks of mm-scale sizes. The techniques explored in this paper include nonlinear resonant ultrasound spectroscopy (NRUS) for detection, Linear Elastodynamic Gradient Imaging Technique (LEGIT), ultrasonic C-scan, vibrothermography, and synchrotron X-ray diffraction for imaging. Results obtained from these techniques are compared. Cracks of mm-scale sizes can be detected and imaged with all the techniques explored in this study.

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This progress report describes work done in FY15 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. In order for SCC to occur, three criteria must be met. A corrosive environment must be present on the canister surface, the metal must susceptible to SCC, and sufficient tensile stress to support SCC must be present through the entire thickness of the canister wall. SNL is currently evaluating the potential for each of these criteria to be met.

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The objectives of this work are to (1) Develop a detailed understanding of the performance characteristics of current chromate conversion coatings utilized in the stockpile, (2) Develop a detailed understanding of the performance characteristics of non-chromate conversion coatings proposed to replace their chromate containing counterparts, (3) Provide an informed assessment of the aforementioned coatings and a recommended course of action for the replacement of chromate containing coatings, and (4) establish subject matter expertise in the area of replacement coatings to provide as a resource to the systems organization. These tasks will be accomplished by performing a battery of traditional corrosion performance tests (in accordance with the relevant MIL specifications governing chemical conversion coatings), combined with more detailed analytical evaluation of the materials. Coatings will be evaluated in the as-received and artificially aged condition. Four different coating systems were evaluated on four different aluminum alloys in FY16. The focus this year was to continue populating the test matrix initiated in previous years, evaluating key coatings in the as received and thermally stressed condition, as compared to the baseline chromate conversion coating (Bonderite (formerly Alodine) 1200S). The non-chromate conversion coatings included Bonderite T 5900, Chemeon TCP-HF, Luster-On Aluminescent, and MacDermid-Enthone Iridite NCP. A second effort that was completed this year was to address a bottleneck that existed for the electrical evaluation of the coatings, assessing their conformance with the MIL specification. An electromechanical test system was assembled and automated, eliminating the need to rely on an already heavily utilized mechanical test system and operator in Materials Mechanics and Tribology.

This report summarizes the outcome of an expert panel review of a key degradation phenomenon identified for atmospherically exposed austenitic stainless steel containers used for the interim dry storage of used nuclear fuel - specifically, chloride induced stress corrosion cracking due to the presence of atmospherically deposited salts. The expert panel consisted of Dr. Peter Andresen (GE Corp Research & Development), Dr. Robert G. Kelly (University of Virginia), Dr. John R. Scully (University of Virginia), and Dr. Alan Turnbull (National Physical Laboratory) and was moderated by Dr. David G. Enos (Sandia National Laboratories). In addition to the above subject matter experts, participants from Sandia National Laboratories, Savannah River National Laboratory, Pacific Northwest National Laboratory, and the Southwest Research Institute will be present. Input from the panel members for a series of preliminary questions dealing with the subject area, along with the meeting minutes, presentation materials, and final recommendations are included here.

While the low cost and strong safety record of lead-acid batteries make them an appealing option compared to lithium-ion technologies for stationary storage, they can be rapidly degraded by the extended periods of high rate, partial state-of-charge operation required in such applications. Degradation occurs primarily through a process called hard sulfation, where large PbSO4 crystals are formed on the negative battery plates, hindering charge acceptance and reducing battery capacity. Various researchers have found that the addition of some forms of excess carbon to the negative active mass in lead-acid batteries can mitigate hard sulfation, but the mechanism through which this is accomplished is unclear. In this work, the effect of carbon composition and morphology was explored by characterizing four discrete types of carbon additives, then evaluating their effect when added to the negative electrodes within a traditional valve-regulated lead-acid battery design. The cycle life for the carbon modified cells was significantly larger than an unmodified control, with cells containing a mixture of graphitic carbon and carbon black yielding the greatest improvement. The carbons also impacted other electrochemical aspects of the battery (e.g., float current, capacity, etc.) as well as physical characteristics of the negative active mass, such as the specific surface area.

Stress corrosion cracking (SCC) of interim storage containers has been indicated as a high priority data gap by the Department of Energy (DOE) (Hanson et al., 2012), the Electric Power Research Institute (EPRI, 2011), the Nuclear Waste Technical Review Board (NWTRB, 2010a), and the Nuclear Regulatory Commission (NRC, 2012a, 2012b). Uncertainties exist in terms of the environmental conditions that prevail on the surface of the storage containers, the stress state within the container walls associated both with weldments as well as within the base metal itself, and the electrochemical properties of the storage containers themselves. The goal of the work described in this document is to determine the stress states that exists at various locations within a typical storage canister by evaluating the properties of a full-diameter cylindrical mockup of an interim storage canister. This mockup has been produced using the same manufacturing procedures as the majority of the fielded spent nuclear fuel interim storage canisters. This document describes the design and procurement of the mockup and the characterization of the stress state associated with various portions of the container. It also describes the cutting of the mockup into sections for further analyses, and a discussion of the potential impact of the results from the stress characterization effort.

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In July, 2016, the Electric Power Research Institute and industry partners performed a field test at the Maine Yankee Nuclear Site, located near Wiscasset, Maine. The primary goal of the field test was to evaluate the use of robots in surveying the surface of an in-service interim storage canister within an overpack; however, as part of the demonstration, dust and soluble salt samples were collected from horizontal surfaces within the interim storage system. The storage system is a vertical system made by NAC International, consisting of a steel-lined concrete overpack containing a 304 stainless steel (SS) welded storage canister. The canister did not contain spent fuel but rather greater-than-class-C waste, which did not generate significant heat, limiting airflow through the storage system. The surfaces that were sampled for deposits included the top of the shield plug, the side of the canister, and a shelf at the bottom of the overpack, just below the level of the pillar on which the canister sits. The samples were sent to Sandia National Laboratories for analysis. This report summarizes the results of those analyses. Because the primary goal of the field test was to evaluate the use of robots in surveying the surface of the canister within the overpack, collection of dust samples was carried out in a qualitative fashion, using paper filters and sponges as the sampling media. The sampling focused mostly on determining the composition of soluble salts present in the dust. It was anticipated that a wet substrate would more effectively extract soluble salts from the surface that was sampled, so both the sponges and the filter paper were wetted prior to being applied to the surface of the metal. Sampling was accomplished by simply pressing the damp substrate against the metal surface for two minutes, and then removing it. It is unlikely that the sampling method quantitatively collected dust or salts from the metal surface; however, both substrates did extract a significant amount of material. The paper filters collected both particles, trapped within the cellulose fibers of the filter, and salts, while the sponges collected only the soluble salts, with very few particles. Upon delivery to Sandia, both collection media were analyzed using the same methods. The soluble salts were leached from the substrates and analyzed via ion chromatography, and insoluble minerals were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The insoluble minerals were found to consist largely of terrestrially-derived mineral fragments, dominantly quartz and biotite. Large (mm-sized) aggregates of calcium carbonate, calcium silicate, and calcium aluminum silicate were also present. The aggregates had one flat, smooth surface and one well crystallized surface, and were interpreted to be efflorescence on the inside of the overpack and in the vent, formed by seepage of cement pore fluids through joints in the steel liner of the overpack. Such efflorescence was commonly observed during the boroscope inspection of the storage system at the site. The material may have flaked off and fallen to the point where the dust was collected, or may have brushed off onto the sponges when the robot was retrieved through the inlet vent. Chemical analysis showed that the soluble salts on the shield plug were Ca- and Na-rich, with lesser K and minor Mg; the anionic component was dominated by SO 4 and Cl, with minor amounts of NO 3 . The cation composition of the soluble salts from the overpack shelf and the canister surface was similar to the filter samples, but the anions differed significantly, being dominantly NO 3 with lesser Cl and only trace SO 4 . The salts appear to represent a mixture of sea-salts (probably partially converted to nitrates and sulfates by particle-gas conversion reactions) and continental salt aerosols. Ammonium, a common component in continental aerosols, was not observed and may have been lost by degassing from the canister surface or after collection during sample storage and transportation. The demonstration at Maine Yankee has shown that the robot and sampling method used for the test can successfully be used to collect soluble salts from metal surfaces within an interim storage system overpack. The results were consistent from sample to sample, suggesting that a representative sample of the soluble salts was being collected. However, it is unlikely that the salt samples collected here represent quantitative sampling of the salts on the surfaces evaluated; for that reason, chloride densities per unit area are not presented here. It should also be noted that the relevance to storage systems at the site that contain SNF may be limited, because a heat- generating canister will result in greater airflow through the overpack, affecting dust deposition rates and possibly salt compositions.

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