Status of radionuclide sorption-desorption studies performed by the WRIT program

Abstract

The most credible means for radionuclides disposed as solid wastes in deep-geologic repositories to reach the biosphere is through dissolution of the solid waste and subsequent radionuclide transport by circulating ground water. Thus licensing activities must consider the physicochemical interactions between radionuclides present in ground water with package components, rocks and sediments since these processes can significantly delay or constrain the mass transport of radionuclides in comparison to ground-water movement. This paper focused on interactions between dissolved radionuclides in ground water and rocks and sediments away from the near-field repository. The primary mechanism discussed is adsorption-desorption, which has been studied using two approaches. Empirical studies of adsorption-desorption rely on distribution coefficient measurements while mechanism studies strive to identify, differentiate and quantify the processes that control nuclide retardation. The status of sorption mechanism studies are discussed with emphasis on delineating the usefuleness of ideal ion exchange, site-binding electrical double-layer and redox controlled sorption constructs. Studies to date on the applicability of ideal ion exchange and site-binding electrical double-layer models for describing nuclide adsorption onto geologic media show greater potential for the latter construct. Future efforts will concentrate on evaluation site-binding electrical double-layer models. Laboratory studies are discussed which corroborate the importancemore » of redox reactions in causing nuclide retardation for multivalenced elements such as Tc, Np, Pu and U. Results suggest both solution mediated reduction such as the Fe(II)-Fe(III) couple and solid/solution heterogeneous reduction reactions such as reduction of solution Pu(VI) at the mineral surface by structural Fe(II) occur.« less