In the event of a severe accident in a nuclear reactor, the loss of cooling leads to high-temperature heating of the UO2 fuel and surrounding materials. The fuel can then react with the zirconium alloy cladding and steel vessel to form a complex mixture of solid and liquid phases, known as in-vessel corium. In a second step of the accident, if the vessel is breached, this corium can flow out and react with concrete. The silicate (U,Zr)SiO4 known as chernobylite (present in the Chernobyl corium) can then form by reaction between the mixed oxide (U,Zr)O2 and silica SiO2.
To predict the heat exchanges as well as the phases formed in the corium, severe accident codes are coupled with thermodynamic calculation software using databases describing the thermodynamic properties of the corium. These calculations are performed using the CALPHAD method, by minimizing the free enthalpy of the system. In these databases, the UO2-ZrO2-SiO2 pseudo-ternary diagram is one of the important to be well described in order to predict the phases formed in the corium. However, the solubility limit of uranium in zircon (ZrSiO4) and the thermodynamic properties of chernobylite (Zr,U)SiO4 are poorly known. Moreover, the mixed oxide (U,Zr)O2 remains poorly characterized from a thermodynamic point of view. We therefore propose to study the stability of these phases to overcome the lack of data and to improve thermodynamic models of the UO2-SiO2-ZrO2 and U-Zr-O systems.
After optimizing the conditions for synthesis by wet chemistry route and purification of the two solid solutions, the samples will be characterized from a physico-chemical and thermodynamic point of view. All these experimental results will serve as input data for modeling the UO2-ZrO2-SiO2 and U-Zr-O systems.
The candidate will have a Master's degree or an engineering diploma in radiochemistry, separative chemistry or materials science. During his/her work, he/she will be required to master a wide range of techniques related to the solution chemistry particularly that of uranium and zirconium), the solid-state chemistry, the physical chemistry and the characterization of materials, including their thermodynamic properties. This will allow him/her to leverage his/her skills not only in the nuclear field, but also more broadly in that of materials elaboration and characterization.