Ricerc@Sapienza

Solid/liquid equilibria in the system UO2eZrO2 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 (U0.6Zr0.4)O2þy, 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 X-ray
diffraction. Investigations under 0.3 MPa air (0.063 MPa O2) revealed a significant decrease in the melting
points down to 2500 Ke2600 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 2493 K.
The current results are important for assessing the thermal stability of the system fuel e cladding in an
oxide based nuclear reactor, and for simulating the system behaviour during a hypothetical severe
accident.