PNNL

Abstract

Solid secondary waste materials expected to be generated during Hanford Waste Treatment and Immobilization Plant (WTP) operations were characterized (using representative materials) with respect to their ability to retain technetium-99 (Tc), iodine-129 (I), and mercury (Hg) as input to future updates to the Integrated Disposal Facility (IDF) performance assessments (PAs). The waste materials included granular activated carbon (GAC), silver mordenite (AgM), and high-efficiency particulate air (HEPA) filter media; all are used for removal of contaminants and radionuclides from the gaseous effluent treatment systems within the WTP. It is expected that the GAC and AgM will be mixed into a cementitious waste form in which the GAC and AgM particles will be blended/stabilized into the cementitious material. It is also expected that the HEPA filter media will be compacted and placed in a container that is then backfilled with a grout to macroencapsulate the medium. Here we report Kds for all waste materials of interest, examine the physical properties of grout specimens containing the waste materials, and provide solid-state characterization information to further understand the interaction of the waste materials with the encapsulating grout. The Kds are used in PA calculations that estimate contaminant release from the disposal site. Monolithic test specimens were prepared with the GAC and AgM mixed in either a Cast Stone grout formulation (8 weight percent [wt%] ordinary Portland cement [OPC], 45 wt% fly ash [FA], 47 wt% blast furnace slag [BFS]) or a Hanford grout mix 5 (HGM-5) formulation (25 wt% OPC, 75 wt% FA). As the specimens were being prepared, the fluid grout mixes were characterized for processing properties including flowability, set time, and the presence of free liquids during curing. After curing, compressive strengths of the waste forms were measured; the compressive strengths were above 2000 psi, easily exceeding the 500 psi minimum requirement. The cured waste forms were also characterized for their density, porosity, hydraulic conductivity, and water characteristic curve—all properties needed in PA calculations of radionuclide release from the waste forms in the IDF. Finally, EPA Method 1315 leach tests were conducted on the GAC and AgM waste materials encapsulated in the Cast Stone and HGM-5 grouts to determine the rate of release of contaminants from the waste forms. Effective diffusivities are provided, which can be used in future PA analyses of the disposal of these solid secondary waste materials.