Joydipta Banerjee

Densification behaviour of ThO2–PuO2 pellets with varying PuO2 content using dilatometry

Abstract The shrinkage behaviour of ThO 2 , ThO 2 –30%PuO 2 , ThO 2 –50%PuO 2 and ThO 2 –75%PuO 2 pellets has been studied using a dilatometer in inert (Ar) and reducing atmospheres (Ar–8%H 2 ). The effects of dopants of CaO and Nb 2 O 5 on shrinkage of the oxides of the above Pu/(Pu+Th) ratios were also studied. Out of the two dopants studied, CaO was found to give larger shrinkage for all the Pu/(Pu+Th) ratios covered in this study. It was also found that the shrinkage was marginally larger in Ar–8%H 2 than in Ar atmosphere. Addition of PuO 2 to ThO 2 enhanced sintering. This was found to be true for both the dopants. During the sintering of ThO 2 , a prominent peak was observed in the shrinkage curve at around 100–300 °C. This peak was attributed to the pressure increase of the trapped gases which subsequently release at high temperatures.

Thermophysical Properties of Thoria-based Fuels

The behavior of nuclear fuel during irradiation is largely dependent on its thermophysical properties and their change with temperature and burnup. Experimental data on out-of-pile properties such as melting point, density, thermal conductivity, and thermal expansion are required for fuel design, performance modeling, and safety analysis. The variables that influence the out-of-pile properties are fuel composition, temperature, porosity, microstructure, and burnup. Among the above-mentioned properties, thermal conductivity of nuclear fuel is the most important property which influences almost all the processes such as swelling, grain growth, and fission gas release, and limits the linear power. The changes in thermal conductivity occur during irradiation by the formation of fission gas bubbles, porosities, build-up of fission products, and by the change of fuel stoichiometry. Melting point plays a crucial role in determining the power to melt the fuel and decides the operating linear heat rating. The coefficient of thermal expansion (CTE) is needed to calculate stresses occurring in the fuel and cladding on change in temperature. In safety analysis, the values of thermal expansion data are required in determining the gap conductance and the stored energy.