Accelerated Burnup Irradiation of U-10Mo at Intermediate Temperatures (250-500C) in the High Flux Isotope Reactor

Uranium molybdenum alloys have long been used successfully as both dispersion and monolithic nuclear fuels in low temperature research reactors where service temperatures do not exceed 200°C. Molybdenum is alloyed with uranium to stabilize the cubic γ phase of uranium which exhibits controlled isotropic swelling under irradiation. However, they exhibit excessive swelling and fission gas release when irradiated at temperatures above 500°C as typically used in sodium fast reactor applications. Their favorable properties including high uranium density and high thermal conductivity have prompted suggestion of their potential use as light water reactor fuels, where service temperatures will fall between the two regimes and no irradiation data exists. The stability of γ phase is also unknown under these conditions.

Accelerated burnup irradiations of U-10Mo were performed using the MiniFuel irradiation platform in the High Flux Isotope Reactor at Oak Ridge National Laboratory to address this gap. Four irradiation temperatures (nominally 250, 350, 450 and 500 °C) were fielded to span the gap between research reactor and sodium fast reactor conditions. Three burnups (nominally .5, 1 and 2 fissions per initial metal atom, FIMA) were achieved to capture the regime where breakaway swelling is encountered above 250°C. The results of post irradiation examination data collected will be presented to outline the transition to breakaway swelling observed above 350°C. Experimental observations will also be compared to simulations of swelling and fission gas release performed using the BISON fuel performance code.

This irradiation and its outcomes will be placed in the context of how this accelerated burnup irradiation fits within an Accelerated Fuel Qualification approach to nuclear fuel development. Finally, the results of ongoing post irradiation microscopy at scanning and transmission electron microscope scales will be discussed within the context testing performed at the Advanced Test Reactor located at Idaho National Laboratory as well as Argonne Tandem Linear Accelerator System at Argonne National Laboratory. This round robin comparison is important to support the long term objective of better understanding how the underlying physical processes imparted by accelerated burnup irradiations may govern performance evolutions of importance to nuclear fuels.