WHY MONAZITE STRUCTURE APPEARS AS A PROMISING LONG-TERM RADWASTE MATRIX ? CONSEQUENCE OF STRUCTURAL FLEXIBILITY AND HIGH CHEMICAL DURABILITY

In the last decades, several phosphate-based ceramic materials have been envisaged as potential matrices for the long-term confinement of radioactive waste coming from the back-end of the nuclear fuel cycle. Among them, monazite (LnPO₄) appears probably as the most promising candidate owing to its physico-chemical properties associated to an unique structural flexibility allowing the incorporation of various elements. Indeed, the original nine-fold coordination of the cation in the monazite structure allows the formation of solid solutions through substitution mechanisms either on cationic or phosphate site. A large literature then related the formation of monazite phases incorporating various di-, tri or tetravalent elements as well as several polyoxoanions such as vanadates, silicates or even chromates. The simultaneous incorporation of tri- and tetravalent actinides could then be performed in cheralite-type phases with general formula An^III_1-xM^II_x-yM^IV_yPO₄.

Furthermore, the monazite phase was usually reported to present several properties of interest in the field of waste management. First, it was found to be very highly durable compared to several other materials (including britholites) studied with the same objective. As instance, the normalized leaching rate of Ca_0.5U_0.5PO₄ reached 4.10^-6 g.m^-2.day^-1 which appears several order of magnitude lower than borosilicate glasses. Moreover, the concentrations of the elements released in solution were usually controlled by the rapid precipitation of neoformed phases which led to the decrease of dissolution rates through the apparition of diffusion phenomena.

The study of geological samples of monazites, naturally containing large amounts of thorium and/or uranium also allowed to evidence the good resistance of such compounds to radiation damages. Indeed, the total amorphization of the monazite structure was only obtained for very high doses (7.10⁸ α.g^-1) and the crystalline state was easily recovered after annealing between 250 and 450°C.

On this basis, this talk aims to give an overview on the studies dedicated to monazite as a potential radwaste matrix, mainly concerning its essential qualities such as high loading capability in actinides associated to a good resistance to dissolution.