Thorium and cerium chemical behaviour in ion-irradiated alkali-borosilicate glasses

Abstract

Simple alkali-borosilicate glasses containing SiO2–B2O3–Li2O–Na2O and only one or two transition metal oxides (CeO2 and/or ThO2) have been synthesized by melting the stoichiometric powder mixture at 1100°C in a platinum crucible. Thorium and cerium were used as chemical analogs of minor actinides (Pu and Am). Th is a purely tetravalent element, although Ce can be tetravalent or trivalent. Glass samples were submitted to aqueous leaching tests at 90°C in deionised water for one week, with or without having previously been ion-irradiated. The irradiation experiments were conducted in the nuclear energy loss regime. Kr ions supplied by a 1MV electrostatic Van de Graaff accelerator, were used to produce displacement cascades in the first hundreds of nanometers beneath the sample’s surfaces. The leached samples were then characterized by scanning electron microscopy (SEM), electron microprobe analysis (EMA) and ion beam analytical (IBA) methods: Rutherford backscattering spectrometry and elastic recoil detection analysis (RBS and ERDA), proton-induced X-ray or gamma ray emission (PIXE and PIGE). Th and Ce are shown to be enriched in the near surface region of leached glasses due to the extremely low solubility of their hydroxides. The effect of surface damage on the chemical behaviour of Th and Ce is then detailed. The possibility for Ce(IV) to be reduced as Ce(III) during ion-irradiation just before leaching and its consequences on the relative solubility of corresponding chemical species is discussed in terms of hydroxide solubility thermodynamical equilibria.