(214b) A Full-Scale Mixing and Transport Feasibility Study Using Non-Radioactive Solid Simulants in a Mock-up Electrolytic Dissolver to Enable the Processing of Spent Nuclear Fuels

As part of a directive to disposition all of the spent nuclear fuel (SNF) remaining in storage at the Savannah River Site (SRS). The H-Canyon processing facility at the SRS is being adapted to readily process legacy SNFs. H-Canyon will dissolve and neutralize the SNF within the facility and discard the acid-soluble portion of the SNFs to the Concentration, Storage and Transfer Facilities (CSTF), where the discards will be blended and mixed with other tank waste from across the SRS. The blended waste will form sludge batches to be processed in the Defense Waste Processing Facility (DWPF) and turned into a vitrified-glass waste-form.

A risk assessment was performed for the disposition and handling of the legacy SNFs, which includes a group of legacy non-aluminum SNFs (NASNF). Several risks associated with electrolytically dissolving zirconium (Zr) clad NASNF were identified. Of which, a significant risk for the buildup of solid residue in the electrolytic dissolver over time was emphasized, as ~85% of the Zr processed is likely to be converted into insoluble residue that is mainly composed of ZrO₂. The risk for this task is elevated further owing to the fact that the legacy electrolytic dissolver (last run in the 1980’s) was recently found to contain a significant amount of legacy insoluble solids residue residing in the bottom of the dissolver. This indicates that the legacy residue removal campaigns were not successful, and the residue was never removed from the dissolver bottom post-processing. Therefore, SRNL was tasked with developing a full-scale mock-up rig to evaluate the feasibility of the handling and removal of the dissolver residue by using non-radioactive simulants and to provide operational recommendation for the removal campaigns.

This work will touch on the development and use of non-radioactive simulants for the feasibility of performing insoluble solids removal campaigns post-dissolution in an electrolytic dissolver. Emphasis will be on the density and morphology dependence of the simulant, as well as mixing performance and removal efficiencies of the simulants at various solids loading. The evaluation suggests that the removal campaigns will result in a high likelihood of success in managing the buildup of solids within the dissolver. Some opportunities were identified to optimize solids transfer and water usage by configuring the operating parameters of the mock-up dissolver tank. The cycling of the spray ring (the method was used to mix the tank using water as the motive fluid) in the dissolver bottom, water level in the tank, and the outlet suction height were some of the key parameters identified to influence the mass transfer and mixing of the simulants in the tank.