The Properties of Uranium-Zirconium Nuclear Fuels and Methods for Improving Burnup Capability

Abstract

Metallic U-Zr fuels have received renewed global interest due to their potential use in Generation-IV sodium-cooled fast reactor systems. Previous work conducted during the Experimental Breeder Reactor-II (EBR-II) programme have shown that metallic fuels are highly compatible with liquid sodium and have the potential to reach high burnups. They offer a means to close the nuclear fuel cycle by the incorporation of minor actinide elements that are otherwise discarded. This improves the fuel utilisation while simultaneously reducing the waste burden. Currently there are limitations preventing their chemical compatibility with cladding constituents due to the inter-diffusion of lanthanides with iron and nickel. A range of U-xZr (x = 5, 10, 15, 30, 50 wt%) alloys were manufactured by arc-melting and their microstructures were observed both in as-cast and annealed states. A coexistence of α-U and δ-UZr2 phases was identified in samples beyond 10 wt% but could not be accurately determined below 10 wt%. Thermophysical analysis was used to confirm the coexistence below this zirconium content. Phase transformations were determined, the results of which showed a disagreement with well-established phase diagrams. Steam oxidation tests were performed showing a correlation between oxidation resistance and zirconium content. Thermal conductivity measurements were obtained for the binary alloys and pure zirconium samples showing a reduction in conductivity according to an increased quantity of the δ-UZr2 phase. To minimise the effects of fuel-cladding chemical interaction (FCCI), aluminium and lanthanides (Ln = Nd, Ce, Pr, La) were incorporated into alloys of U-10Zr to study the microstructural features with regard to Al-Ln intermetallic formation. No such phases were identified in the system suggesting the ineffective behaviour of aluminium as a fuel dopant. The microstructure of the U-Zr-Al alloys were vastly different to the binary structure and compositional analysis identified the increased presence of higher symmetry phases which may improve certain properties of the fuel. Antimony was also studied as a fuel dopant for lanthanide immobilisation. In fresh fuel, the antimony bound with zirconium, forming SbZr2 intermetallics and depleting the microstructure of zirconium. In the presence of lanthanides, antimony formed intermetallics of a mixed composition of SbLn and Sb3Ln4. The stability of the Sb-Ln intermetallics was confirmed up to temperatures of 600°C as they were unaffected at elevated temperatures. It is suggested that in-pile examinations of antimony-doped fuels are conducted following the promising behaviour exhibited in this study.

Date of Award	31 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Preuss (Supervisor) & Timothy Abram (Supervisor)