Eu3+ Luminescence in High Charge Mica: An In Situ Probe for the Encapsulation of Radioactive Waste in Geological Repositories

Isolation of high-level radioactive waste (HLW) in deep geological repositories (DGR) through a multibarrier concept is the most accepted approach to ensure long-term safety. Clay minerals are one of the most promising materials to be used as engineered barriers. In particular, high charge micas, as components of the engineered barrier, show superselectivity for some radioactive isotopes and a large adsorption capacity, which is almost twice that of the other low charge aluminosilicates. In addition, high charge micas are optimum candidates for decontamination of nuclear waste through two different mechanisms; namely an ion exchange reaction and a nonreversible mechanism involving the formation of new stable crystalline phases under hydrothermal conditions. In this work, we report a new in situ optical sensor based on the incorporation of Eu3 in these high charge micas for tracking the long-term physical-chemical behavior of HLW contaminants in DRG under mild hydrothermal conditions. The incorporation of Eu3+ into the interlayer space of the mica originates a well resolved green and red luminescence, from both the D-5(1) and D-5(0) excited states, respectively. The formation of new crystalline phases under hydrothermal conditions involves important changes in the Eu3+ emission spectra and lifetime. The most interesting features of Eu3+ luminescence to be used as an optical sensor are (1) the presence or absence of the Eu3+ green emission from the D-5(1) excited state, (2) the energy shift of the D-5(0) -> F-7(0) transition, (3) the crystal-field splitting of the F-7(1) Eu3+ level, and (4) the observed luminescence lifetimes, which are directly related to the interaction mechanisms between the lanthanide ions and the silicate network.