5-Year Accelerated Corrosion Testing of MAXUS® for Spent Fuel Pool and Dry Cask Performance

Many pool fuel storage racks and fuel storage/transportation packages incorporate neutron absorbers to improve the space efficiency of nuclear fuel held therein. The dominant performance concern regarding metal-based neutron absorbers is oxidation, whether from long-term residence in a spent fuel pool rack or short-term pool exposure experienced by spent fuel storage and transportation canisters/casks. A number of dry cask designers/fabricators have leveraged high-temperature, immersion-based corrosion tests to bound absorber performance of dry cask pool exposure and response to residual moisture. MAXUS®, an aluminum-boron carbide neutron absorber metal matrix composite material, is qualified for use in spent fuel pools and dry casks. The authors identified LWR fuel pool environmental factors that affect aluminum-based, boron-10 (10B) neutron absorber performance. These include water temperature, pH and dissimilar materials. Further identified are production methods that address these factors. To confirm adequate in-service performance, corrosion resistance testing was done in simulated LWR pool environments. The material testing involved up to 5-year exposure to simulated PWR and BWR pool accelerated corrosion environments. Test material configurations include flat and bent coupons. The flat coupons are used to determine overall material corrosion performance and allows for established means of 10B areal density measurement. Bent coupons confirm the performance of absorber configurations requiring mechanical work to achieve final shape of the absorber. These bent coupons simulate rack inserts that have been used extensively in US spent fuel pools and dry cask absorber panel configurations requested by system designers. Post-immersion tests included: visual examinations, dimension measurements, neutron attenuation measurements of 10B areal densities and evaluation of the effect of forming work on non-flat absorber configurations. Test coupons were periodically removed, measured and compared to pre-characterized values. The physical configuration and 10B areal density for all coupons were found to be essentially unchanged, with no blisters or delamination noted. This paper shows that the results of the completed 5-year test program demonstrate MAXUS® has life of plant efficacy as a spent fuel storage rack neutron absorber. These results also address dry cask operation absorber material exposure to spent fuel pool water during cask loading and residual moisture after closure.