PNNL

Abstract

Fuel for the U.S. high-performance research reactor fleet is undergoing significant development as the United States moves away from using highly enriched uranium dispersion fuels. The proposed fuel is a high-density, low-enriched uranium (LEU, 19.75 wt% U 235) alloyed with ten weight percent molybdenum fuel (known as LEU U 10Mo). The LEU U 10Mo fuel samples studied in this report were made by down-blending highly enriched uranium with a master alloy, which is cast from molybdenum rods and depleted uranium, via vacuum induction melting and then casting. The final cast fuel should have 10 wt% molybdenum and a uranium enrichment of 19.75 wt% U 235. From casting to final fuel element formation, the LEU U 10Mo material undergoes several thermomechanical processing steps to attain the desired form and meet the specifications of the fuel plate. The present work focuses on methods to detect, identify, and quantify the factors behind varying levels of loose contamination on bare LEU U 10Mo after heat treatment (homogenization). Contamination level variation is suspected to originate from ingrowth of uranium decay products over time, together with diffusion and accumulation on the outer surface. To characterize the uranium decay products ingrown in the alloy without interference from uranium and molybdenum, a source-sample of LEU U 10Mo (two years after processing, analyzed as-is) was dissolved in concentrated acid. The uranium and molybdenum were separated out by anion exchange, leaving the decay chain products in solution. The solution was analyzed by gamma and alpha spectrometry. The gamma spectra shown a cluttered spectrum with minor components of the decay chains and high uncertainty on the nuclide identification. The alpha spectrometry results clearly showed ingrowth of Th 228, Ra-224, Rn 220, Po 216 (daughters of U 232/236), and Th 230 from U 234. The level of ingrown of Th 230 activity is approximately equal to the ingrowth expected from U 234 after two years’ decay. Presence of polonium 215 in the alpha spectrum proves that Ac-227 and likely its parent Pa 231 were also present. It was also hypothesized that thermal processing influenced diffusion of uranium decay products from the interior to the surface of the alloy. To test this hypothesis, a sample was thermally treated in a furnace at 900°C for 144 hours under vacuum, while another sample (from the same material) was kept as control. Next, the samples underwent controlled etching to remove thin layers approximately 5 microns thick. This step was repeated two more times to quantify the progeny levels at three different depths in the sample. The surface beta/gamma and alpha activities of the homogenized sample were higher than the activities measured for the same sample before heat treatment (homogenization). Of significance, the homogenized sample had 51% less mass; however, it exhibited more surface activity (alpha and beta/gamma). The alpha activity increased by 12% and the beta/gamma activity increased by a factor of 2.5. The beta-to-alpha activity ratio also increased by a factor of 1.50. The thermal processing experiment showed that the surface of the heat-treated sample had more progeny activity than the surface of the non-heat-treated sample per layer etched. This behavior was noticed for Th 234/Pa 234m, Th 230, Th 228, and Po-215. The activity was higher at the first surface etched from the heat-treated sample, while subsequently etched layers from the same sample seemed to be “depleted” in decay products relative to proportions that would be expected. The mechanism involved can potentially be attributed to vacancy movements among the atoms. In these vacancy movements, atoms exchange positions or move collectively via grain boundary diffusion, chemical diffusion, and intrinsic diffusion. Future research will investigate which mechanism is dominant.