Decommissioning of a nuclear power plant: Determination of site-specific sorption coefficients for Co-60 and Cs-137

Assessment of radiological risks in strategies for decommissioning of nucle ar installations have to consider not only technical concepts such as cutti ng and decontamination techniques but, even more important, requirements fo r input of reliable information on the hydrological situation and retardati on capabilities of relevant radionuclides specific to the respective decomm issioning operation. In this paper we describe appropriate methods for obtaining site-specific s orption data and present results achieved from a case study performed as a commercial contractual work preliminary to the planned decommissioning of a nuclear power plant. A detailed mineralogical study of the sediment used i n our sorption experiment highlights the necessity of a thorough sample hom ogenization and characterization. Batch experiments using radiotracer techn iques for the determination of site-specific sorption coefficients show sig nificant retardation for Co-60 and Cs-137 after only 2h of equilibration be tween the preconditioned groundwater and sediment. Sorption is more effecti ve in the groundwater of a deeper aquifer containing a higher amount of col loidal clay (illite) particles < 0.63 mu m. The Co-60 radiotracer is more c ompletely sorbed than the Cs-137 radiotracer. Equilibration of radionuclide distribution is slow, particularly for Co-60. Presence of EDTA reduces sor ption of Co-60 efficiently while Cs-137 sorption remains unaffected.