Microgamma Scan System for analyzing radial isotopic profiles of irradiated transmutation fuels

The U. S. Global Nuclear Energy Partnership/Advanced Fuel Cycle Initiative (GNEP/AFCI) is developing metallic transmutation alloys as a fuel form to transmute the long-lived transuranic actinide isotopes contained in spent nuclear fuel into shorter-lived fission products. The AFCI program has irradiated and examined eleven metallic alloy transmutation fuel specimens to evaluate the feasibility of actinide transmutation in advanced sodium-cooled fast reactors and thermal reactor implementation. Initial results of postirradiation examinations indicated the irradiation performance of the actinide-bearing compositions is similar to uranium-plutonium-zirconium ternary metallic alloy fuels (U-xPu-10Zr). Further studies to characterize radial burnup profile, constituent migration, and fuel cladding chemical interaction (FCCI) are in progress. A microgamma scan system is being developed to analyze the radial distribution of fission products, such as Cs-137, Cs-134, Ru-106, and Zr-95, in irradiated fuel cross-sections. The microgamma scan system consists of a set of indexed sample collimator blocks and a sample holder, which interfaces with the INL Analytical Laboratory Hot Cell (ALHC) Gamma Scan System high purity germanium detector, multichannel analyzer, and removable collimators. The microgamma scan results will be used to evaluate radial burnup profile, cesium migration to the sodium bond and constituent migration within the fuel. These data will further clarify the comparative irradiation performance of actinide-bearing metallic transmutation fuel forms and uranium-plutonium-zirconium alloys. Preliminary measurements of the microgamma scan system will be discussed. A simplified model of the microgamma scan system was developed in MCNP and used to investigate the system performance and to interpret data from the scoping studies. Recommendations for improving the MCGS analyses are discussed.