Assessment of solid/liquid equilibria in the (U, Zr)O2+x system

Solid/liquid equilibria in the system UO2 – ZrO2 are revisited in this work by laser heating coupled with fast optical thermometry. Phase transition points newly measured under inert gas are in fair agreement with the early measurements performed by Wisnyi et al. in 1957, the only study available in the literature on the whole pseudo-binary system. However, a minimum melting point is identified here for compositions near (U0.6Zr0.4)O2, around 2800 K. The solidus line is rather flat on a broad range of compositions around the minimum. It increases for compositions closer to the pure end members, up to the melting point of pure UO2 (3130 K) on one side and pure ZrO2 (2970 K) on the other. Solid state phase transitions (cubic-tetragonal-monoclinic) have also been observed in the ZrO2-rich compositions by Raman spectroscopy and x-ray diffraction. Investigations under compressed air revealed a significant decrease in the melting points down to 2500 K – 2600 K for increasing uranium content (x(UO2)> 0.2). This was found to be related to further oxidation of uranium dioxide, confirmed by x-ray absorption spectroscopy. For example, a typical oxidised corium composition U0.6Zr0.4O2.13 was observed to solidify at a temperature as low as 2523 K.
The current results are important for assessing the thermal stability of the system fuel – cladding in an oxide based nuclear reactor, and for simulating the system behaviour during a hypothetical severe accident.