Beyond CANLUB-graphite: An Improved Alternative Coating Development

Abstract

This effort surrounds the identification and procurement of some potential candidates for use as Stress Corrosion Cracking (SCC)-mitigation coating in nuclear fuel sheathing materials in Canadian reactors. A model proposed by Lewis et al. elucidates iodine-induced stress corrosion cracking (I-SCC) phenomena and the role of impurities (Sodium; Na2O) in scavenging iodine species. It is desired to develop a strategy to realize the multitude of benefits, safety and economics, by moving beyond a single-mitigation coating, CANLUB, to an improved coating on the inner surface of the fuel sheathing. The thin layer of graphite presently applied to the inner surface of fuel sheathing is thought to form stable ZrxIyC-type compounds in reaction with ZrIx species [Chan et al.]. Vacuum grease, a type of siloxane coating, had demonstrated good performance against SCC during the early phase of coating development. The technology on siloxane coating has improved greatly during the last two decades. These have propelled polysiloxane coatings to prominence as an improved alternative coating to mitigate fuel-sheath SCC. Polysiloxanes are polymers with the (Si-O) building blocks. The resilience of polysiloxanes stems from the high strength of Si-O repeat unit (445 kJ mol-1), making it much stronger than typical C-C blocks (358 kJ/mol) in organic coatings. The silicone (Si) may be bonded to 2, 3 or 4 oxygen (O) atoms in the repeating backbone of the various polysiloxanes, rendering the 50-100&percnt; oxidized silicone immune to oxidative degradation, common among all organic coating substrates with carbon-only linkages. The highly polarized Si-O linkage further protects any organic substituent (i.e.; R = methyl, ethyl, phenyl) that is present in the backbone configuration as Si-R and/or Si-O-R [Finzel and Vincent]. Polysiloxanes high resistance to radiation, high temperatures and combustion, corrosive and oxidative environments is paralleled by their cost effectiveness, lower volatile organic compounds (VOC) content, and low temperature cure capability and improved methodology for the application. The fluid viscosity and Thermogravimetric Analysis (TGA) of three alternative coatings in addition to DAG154N (CANLUB) have been determined. Characterizations of zirconium alloy sheathing bearing all four coatings and preliminary I-SCC results will be presented. In addition, DAG154N doped with Na2O (<1&percnt; w) will also be characterized and examined for the potential added benefits of Na as an iodine scavenger and the remaining oxygen to repair cracks in the Zr-oxide.