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

Abstract Solid/liquid equilibria in the system UO 2 ZrO 2 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. In addition, a minimum melting point is identified here for compositions near (U 0.6 Z r0.4 )O 2+y , 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 UO 2 (3130 K) on one side and pure ZrO 2 (2970 K) on the other. Solid state phase transitions (cubic-tetragonal-monoclinic) have also been observed in the ZrO 2 -rich compositions X-ray diffraction. Investigations under 0.3 MPa air (0.063 MPa O 2 ) revealed a significant decrease in the melting points down to 2500 K–2600 K for increasing uranium content (x(UO 2 )> 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 U 0.6 Zr 0.4 O 2.13 was observed to solidify at a temperature as low as 2493 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.