Study of diluting and absorber materials to control the reactivity during a postulated core meltdown accident in generation IV reactors

In order to limit the consequences of a hypothetical core meltdown accident in Generation IV Sodium Fast Reactors, absorber materials in or near the core, such as boron carbide B4C, and diluting materials in the core catcher will be used to prevent recriticality within the mixture of molten oxide fuel and molten structures called corium. The aim of the PhD thesis was to select materials of both types and to understand their behaviour during their interaction with corium, from chemical and thermodynamic points of view. Concerning B4C, thermodynamic calculations and experiments agree with the formation of two immiscible phases at high temperature in the B4C - UO2 system: one oxide and one boride. This separation of phases can reduce the efficiency of the neutrons absorption inside the molten fuel contained in the oxide phase. Moreover, volatilization of a part of the boron element can occur. According to these results, the necessary quantity of B4C to be introduced should be reconsidered for postulated severe accident sequence. Other solution could be the use of Eu2O3 or HfO2 as absorber material. These oxides form a solid solution with the oxide fuel. Concerning the diluting materials, mixed oxides Al2O3 - HfO2 and Al2O3 - Eu2O3 were preselected. These systems being completely unknown to date at high temperature in association with UO2, first points on the corresponding ternary phase diagrams were researched. Contrary to Al2O3 - Eu2O3 - UO2 system, the Al2O3 - HfO2 - UO2 mixture presents only one eutectic and thus only one solidification path which makes easier forecasting the behaviour of corium in the core catcher. (author)