Hydrothermal reactions of strontium and transuranic simulator elements with clay minerals, zeolites, and shales

Reactions of Sr, La, and Nd (the latter two elements simulating Am and Cm) with potential backfill minerals for radioactive waste storage and with repository wall rocks, such as shale, were investigated under simulated repository conditions of 200 degrees and 300 degrees C for 12 weeks under a confining pressure of 30 MPa. The solid and solution reaction products were characterized to determine the nature and extent of reaction. Chlorite, illite, kaolinite, montmorillonite, mordenite, and clinoptilolite and four shales removed as much as 61.2 and 98.5% of the added SrCl (sub 2) and Sr(OH) (sub 2) from solution, respectively, by ion exchange and/or by forming new strontium compounds such as SrAl (sub 2) Si (sub 2) O (sub 8) , Sr (sub 2) MgSi (sub 2) O (sub 7) , SrCO (sub 3) (strontianite), and SrAl (sub 2) Si (sub 4) O (sub 12) .2H (sub 2) O (Sr-wairakite). The formation of these sparingly soluble Sr phases by the reaction of the soluble Sr compounds with such backfill materials indicates that the backfill may serve as a barrier during the thermal period of the waste in the life of a repository. These same minerals and shales removed as much as 99.99% of the added La or Nd from solution at 300 degrees C by forming new phases such as LaOHCO (sub 3) , NdOHCO (sub 3) , and possibly La or Nd oxides and hydroxides. Zeolites reacted with La and Nd to form smectite. Thus, if La and Nd truly simulate the reactivity of Am and Cm, properly designed backfills can serve as a barrier to the migration of transuranic elements of nuclear wastes.