A 133Cs magic angle spinning nuclear magnetic resonance study of cesium environments in barium hollandites and Synroc

Abstract

Barium aluminum hollandite is a major phase in Synroc, a ceramic designed for the immobilization of high-level waste (HLW) from nuclear fuel reprocessing. Radioactive cesium substitutes into the channel sites, and such hollandites give 133Cs MAS nuclear magnetic resonance (NMR) spectra consisting of a single peak at 211 ppm in the absence of paramagnetic ions. However, the peak shifts to 640 ± 30 ppm and becomes extremely broad when Ti3+ replaces Al3+ in the channel walls of the hollandite structure, apparently because of Fermi contact interaction between the Cs nucleus and the unpaired electron of Ti3+. 133Cs MAS NMR of Synroc and hollandites is very sensitive to the presence of water-soluble CsAlTiO4 which would compromise the aqueous durability of Synroc. 133Cs MAS NMR spectra of Synroc-C, hot-pressed in metal bellows at temperatures as high as 1325 °C, do not indicate significant formation of CsAlTiO4. Synroc samples loaded with Cs and Sr only were shown by MAS NMR as well as electron microscopic techniques to be capable of incorporating nearly 10 wt.% Cs before CsAlTiO4 is formed.