DOE OSTI2016
Dry cask storage and transport of used (spent) nuclear fuel rely upon the basket and shell of the cask to transfer the decay heat from the fuel assemblies. Cask licensees have considered the use of light metal (aluminum) oxidation to enhance the emissivity of aluminum alloy or aluminum based Metal Matrix Composites (MMC) used in the cask basket. However, traditional anodization techniques suffer from issues that make fabrication and regulatory acceptance problematic. The Micro Arc Oxidation (MAO) technology overcomes the problems associated with traditional anodization in the production of consistent, uniform and reliable high emissivity coatings of aluminum alloys. Samples of AA 2219 and an aluminum-based MMC were treated using a proprietary MAO process and subjected to a number of tests to confirm suitability of the coated material. The treated material was inspected using macro photography and Scanning Electron Microscopy (SEM). A consistent, uniform black, porous surface with a typical surface roughness of Ra = 7-8 μm was observed. Emissivity of treated surfaces with thicknesses ranging from 24 microns to 40 microns was measured using an integrating sphere. In all cases, the emissivity coefficient at room temperature in the ranges of 400 to 700 nanometers and 1000 to 1650 nanometers was above 0.8, reaching 0.94 depending on the thickness. The MAO process yield was tested by coating 400 spent fuel basket components to ensure a minimum specified thickness. 97% of the tested points met or exceeded the minimum thickness required. The defect density was very low and 100% of the components met the quality assurance criteria. In-service surface performance was tested by exposing the treated surface to a number of high temperature environments (air, vacuum, nitrogen) and thermal cycling in air. In all cases, the surface remained unchanged. Test samples were exposed to 90 Celsius degrees borated and demineralized water for 48 hours with no change to the sample treated surface. Finally, tests sample were exposed to 60 C. degrees borated water for 168 hours with no change to the sample treated surface. MAO technology appears to be a viable method to provide consistent, durable, and high emissivity coatings, that can be used in the fabrication and licensing of dry storage/transportation casks.