Abstract
Solid/liquid equilibria in the system UO2ZrO2 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)O2+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 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 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 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.
Highlights
Measurements were performed on small amounts (5 mg) of powdered samples mixed with boron nitride (BN) and pressed into pellets
At the beginning of a laser pulse, the sample surface temperature recorded by the 655 nm pyrometer increases very rapidly due to the contribution of thermal radiation emitted by the hot surface
No inflection in the heating flank is visible since thermal equilibrium conditions were not achieved in this part of the measurement, where the laser delivers a large amount of heat in a short time interval on the sample surface [23]
Summary
Mastromarino et al / Journal of Nuclear Materials 494 (2017) 368e379 coolant accident (LOCA), the fuel is expected to be exposed to temperatures higher than 3000 K At this temperature the fuel will melt and react with the surrounding structural materials of the reactor such as the cladding, usually made of Zircaloy in water cooled thermal reactors, to form a reaction mixture commonly termed “corium” [1]. Samples of initially stoichiometric (U, Zr)O2.00 across the full U/Zr compositional range were prepared using sol gel methods employed at the European Commission's Joint Research Centre of Karlsruhe (Germany) Such preparation approach gives similar structures as those obtained during a melt reaction in a reactor core. Main goals of the present analysis are a deeper understanding of the phenomena leading to the formation of corium in a LOCA, and new assessment of corium properties at temperatures near the solid-liquid transition
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