The solidification behaviour of the UO₂-ThO₂ system in a laser heating study

The high-temperature phase diagram of the UO₂-ThO₂ system has been experimentally revisited in the present study for the first time since 1970, using a laser heating approach combined with fast pyrometry in a thermal arrest method. The melting/solidification temperature, which is of fundamental information for a reactor design was studied here. It was found that low addition of ThO₂ to UO₂ would result in a slight decrease of the solidification temperature. A minimum was found at 3098 K around a composition of 5 mol% ThO₂. The solid/liquid transition temperature was then observed to increase again with increasing ThO₂ fraction. The literature value of pure ThO₂ (around 3630 K) was well reproduced here. Important experimental difficulties, stemming from the high temperatures reached during the measurements, as well as a complete investigation with electron microscopy, Raman spectroscopy and powder X-ray diffraction, are extensively discussed. These results show the importance of the high-temperature oxygen chemistry in this actinide oxide compound.