Interactions of Backfill Materials with Cesium in a Bittern Brine Under Repository Conditions

The following potential backfill materials have been studied: phillipsite, clinoptilolite, mordenite, montmorillonites, vermiculites, chlorite, kaolinite, labradorite, and shales. Each of these was hydrothermally reacted with Cs₂MoO₄, a possible cesium phase in spent fuel elements, in the presence of a bittern brine at 200°C for two months under a confining pressure of 300 bars. Analyses of the product solutions indicated that montmorillonites, vermiculites, and zeolites fixed (as determined by resistance to K⁺ washing) the greatest fractions of the added cesium while other minerals, labradorite, and shales fixed only about 10% of the added cesium. For example, montmorillonite from Arizona and phillipsite from California fixed 47 and 50%, respectively, of the cesium added. X-ray diffraction analysis of the solid products revealed that cesium was fixed in the interlayers of montmorillonite as indicated by the collapse of the c-spacing from 15.5 to 12.1 A. Cesium interaction with clinoptilolite and mordenite zeolites did not result in their alteration or in any new cesium minerals as observed by x-ray diffraction. The cesium aluminosilicate mineral, pollucite, was detected only with phillipsite-cesium interactions in brine unlike in the hydrothermal interaction of these materials with Cs₂MoO₄ in deionized water where the presence of pollucite was found earlier to be pervasive. Powellite, CaMoO₄, was the only new phase found in all these interactions by x-ray diffraction which resulted from the combination of calcium from brine with molybdenum from Cs₂MoO₄. Montmorillonites among clay minerals and zeolites such as clinoptilolite and mordenite seem to be the best backfill materials in salt based on these studies and based on our earlier studies of mineral stability under repository conditions.