Denise Adorno Lopes

Oxidation of accident tolerant fuel candidates

ABSTRACT In this study, the oxidation of various accident tolerant fuel candidates produced under different conditions have been evaluated and compared relative to the reference standard – UO2. The candidates considered in this study were UN, U3Si2, U3Si5, and a composite material composed of UN–U3Si2. With the spark plasma sintering (SPS) method, it was possible to fabricate samples of UN with varying porosity, as well as a high-density composite of UN–U3Si2 (10%). Using thermogravimetry in air, the oxidation behaviors of each material and the various microstructures of UN were assessed. These results reveal that it is possible to fabricate UN to very high densities using the SPS method, such that its resistance to oxidation can be improved compared to U3Si5 and UO2, and compete favorably with the principal ATF candidates, U3Si2, which shows a particularly violent reaction under the conditions of this study, and the UN–U3Si2 (10%) composite.

Degradation of UN and UN–U3Si2 pellets in steam environment

ABSTRACT In this work, a systematic study of the degradation of UN pellets (density range 96%–99.9% and grain size of 6–24 µm) and UN-10%U3Si2 (wt%) composite in a steam environment is presented. Static steam autoclave tests were performed at 300 °C and 9 MPa for period of 0.5–1.5 hours. Microstructural analyses of UN pellets show that, in a high-pressure atmosphere, the fuel collapses principally by intergranular cracking generated by the precipitation of an oxide phase in the grain boundaries. This mechanism leads to a premature mechanical collapse of the fuel pellet, exposing fresh surfaces to steam, and ultimately accelerating the oxidation process. Increasing density (specifically eliminating open porosity) was found to delay the oxidation process, while increasing grain size was found to accelerate the degradation process due to a greater susceptibility to mechanical fracture by way of intergranular oxidation. The performance of the UN-10%U3Si2 composite proved to be better when compared to UN. The U3Si2 phase served to stabilize the UN grain boundary interface and reacted preferentially with the steam, thereby altering the failure mechanism. In this composite material, the cracking was predominantly intra-granular and the exposure of fresh surfaces was limited, resulting in a slower degradation process.

Competitive Precipitation and Recrystallization in U-7.5Nb-2.5Zr Alloy

Abstract. Metallic nuclear fuel plates are nowadays an alternative to the ceramic ones in the sense that the uranium density can be increased at lower enrichment. Higher thermal conductivity is also a key factor favouring such fuels for power reactors. Uranium reacts promptly with oxygen and nitrogen at high temperatures to catastrophic corrosion due to non-protective oxide layers, which imparts hot forming processes. The gamma phase body centred cubic structure can be retained at room temperature by annealing the U-7.5Nb-2.5Zr (wt.%) alloy followed by quenching, where the deformation can be extensive. The resulted highly deformed gamma supersaturated structure is subjected further to competitive recovery/recrystallization and phase precipitation phenomena whose are studied in the work. The U-7.5Nb-2.5Zr alloy was melted into plasma and induction furnaces and afterwards annealed to gamma phase. The normalized alloy was cold rolled and underwent isochronal and isothermal treatments. The microstructure evolution was monitored by optical microscopy, X-ray diffraction analysis and hardness measurements. The results show the precipitation events of α” and α+γ3 phases are dominant over recovery in the range 200oC < T < 500oC. Above 500oC the recrystallization is the main process leading to softening and initial Vickers hardness recovery. One refined gamma phase grain structure was obtained (~8.0 μm) after annealing at 700oC for 2.5 hours.