Sintering of CaF2 pellets as nuclear fuel analog for surface stability experiments

Highlights: ► We describe the microstructures of CaF2 that can be obtained by sintering. ► Microstructures that resemble nuclear waste pellets were obtained in the range 900 °C–1000 °C. ► We demonstrate the importance of surface orientation on dissolution behavior and surface properties of a mineral. ► New procedure for studying the role of crystallography and surface chemistry in dissolution using 3-D confocal profilometry. - Abstract: To enable a detailed study of the influence of microstructure and surface properties on the stability of spent nuclear fuel, it is necessary to produce analogs that closely resemble nuclear fuel in terms of crystallography and microstructure. One such analog can be obtained by sintering CaF2 powder. This paper reports the microstructures obtained after sintering CaF2 powders at temperatures up to 1240 °C. Pellets with microstructure, density and pore structure similar to that of UO2 spent nuclear fuel pellets were obtained in the temperature range between 900 °C and 1000 °C. When CaF2 was sintered above 1100 °C the formation of CaO at the grain boundaries caused the disintegration of the pellet due to hydration occurring after sintering. First results from a novel set-up of dissolution experiments show that changes in roughness, dissolution rate and etch pit shape of fluorite surfaces are strongly dependent on the crystallographic orientation of the expose surface. Consequently, the differences observed for each orientation will affect the overall dissolution rate and will lead to uncertainties in the estimation of dissolution rates of spent nuclear fuel.