Phase chemistry of tank sludge residual components. 1998 annual progress report

'The proposed research will provide a scientific basis for predicting the long-term fate of radionuclides remaining with the sludge in decommissioned waste tanks. Nuclear activities in the United States and elsewhere produce substantial volumes of highly radioactive semi-liquid slurries that traditionally are stored in large underground tanks while final waste disposal strategies are established. Although most of this waste will eventually be reprocessed a contaminated structure will remain which must either be removed or decommissioned in place. To accrue the substantial savings associated with in-place disposal will require a performance assessment which, in turn, means predicting the leach behavior of the radionuclides associated with the residual sludges. The phase chemistry of these materials is poorly known so a credible source term cannot presently be formulated. Further, handling of actual radioactive sludges is exceedingly cumbersome and expensive. This proposal is directed at: (1) developing synthetic nonradioactive sludges that match wastes produced by the various fuel processing steps, (2) monitoring the changes in phase chemistry of these sludges as they age, and (3) relating the mobility of trace amounts of radionuclides (or surrogates) in the sludge to the phase changes in the aging wastes. This report summarizes work carried out during the first year of a three year project. A prerequisite to performing a meaningful study was to learn in considerable detail about the chemistry of waste streams produced by fuel reprocessing. At Hanford this is not a simple task since over the last five decades four different reprocessing schemes were used: the early BiPO{sup 4} separation for just Pu, the U recovery activity to further treat wastes left by the BiPO{sup 4} activities, the REDOX process and most recently, the PUREX processes. Savannah River fuel reprocessing started later and only PUREX wastes were generated. It is the working premise of this proposal that most of the phase chemistry in the wastes was defined when the acidic process fluids were first neutralized prior to storage. The only notable exception being that some silicates obviously formed later under highly caustic conditions. Waste stream chemistries for each process have been established and surrogate sludges prepared. Aging of these different recipes have begun at 25, 60, and 90 C and the phase chemistry of the different mixes is being monitored.