The aim of this thesis is to develop sitinakite-type materials compatible with a continuous treatment process for strontium-contaminated effluents.
Sitinakite is a poorly crystalline silico-titanate phase with ion exchange properties. In particular, the sodium atoms present in the channels of this structure are mobile and can exchange selectively with strontium ions. This means that exchange with strontium will take priority even in the presence of other competing cations from the family of alkaline earth elements, such as calcium.
However, for sorption materials to be suitable for continuous effluent treatment involving a high flow rate through the filter element, they need to be shaped. In fact, fine powders are not suitable for such continuous processes because of the clogging phenomena of the filtering elements.
Consequently, the research teams proposing this thesis topic have developed a protocol for shaping millimetre-sized sitinakite granules. This involves converting millimetre-sized TiO2 granules into sitinakite via a hydrothermal pseudomorphic transformation reaction. However, if the phase conversion works, it leads to a loss of efficiency with a consequent slowdown in the rate of exchange between sodium and strontium compared with sitinakite in powder form.
This thesis therefore proposes to adapt the transformation protocol so that the sodium - strontium exchange rates are faster, equivalent to the powder system. This will involve treating sitinkaite granules or pre-treating precursor TiO2 granules prior to processing in order to increase the specific surface area of the final materials and thus improve the accessibility of the exchange sites.
The PhD student will also analyse the effects of irradiation on the sorption properties of materials induced by the presence of radioactive Sr in the materials. In particular, this will involve finding out whether irradiation can lead to the release of strontium, for example. To this end, material irradiation campaigns will be carried out on electronic irradiators (LSI, Polytechnique), the aim of which will be to simulate the presence of a beta emitting element such as 90Sr.
The applicant profile we're looking for is based on a Master 2 and/or school of engineering student with a specialization in solid-state chemistry, particularly materials. Ideally, the candidate will have notions of the physical chemistry of interfaces. The PhD student will benefit from the expertise of the two host laboratories in the field of porous materials and nuclear decontamination. These two aspects will help the candidate in his or her post-doctoral job search and enable him or her to apply for offers in the field of decontamination, whether nuclear or in other sectors (water treatment, soil decontamination, etc.).

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.