Natural geochemical isolation of neutron-activated waste: Scenarios and equilibrium models

A coupled geochemical/geohydraulic model is used to discuss and interpret possible mechanisms for contaminant transport and accumulation in inorganic environments. The geochemical part of the code, the triple layer model for adsorption, allows one to estimate the variation of the contaminant distribution coefficient with solution composition. The hydraulic part of the model establishes a deterministic correlation of the spatial variation of the distribution coefficient. Scenarios are constructed incorporating computed system behavior. A comparison of potential contaminant concentrations with acceptable ones allows one to quantify the degree of geochemical isolation of the contaminant which a chosen environment provides. Long lived waste, activated in the thermal neutron flux of a light water reactor, is classified using the proposed methodology and a very conservative scenario: beryllium, lead, molybdenum, selenium, tin and zirconium activated in the bulk of the reactor decommissioning waste (the bioshield) might be sufficiently isolated by the chemistry in common soils. The concentration of nickel in oxidizing inorganic noncomplexing drinking water has an upper limit given by the precipitation of nickel minerals. Above pH = 7 is an effective geochemical barrier for nickel activated anywhere in the reactor, except the high neutron flux region.