IMPACT OF SB4 TANK 40 DECANT AND ARP/MCU ADDITIONS WITH/WITHOUT ADDED CAUSTIC ON DWPF CPC PERFORMANCE

The Savannah River National Laboratory (SRNL) was requested to investigate the impact of decanting supernate from the Sludge Batch four (SB4) feed in Tank 40. The specific questions concerned the potential impact on the stoichiometric acid window determined for SB4 with respect to overall hydrogen generation rates, nitrite destruction in the Sludge Receipt and Adjustment Tank (SRAT) and the rheology of the sludge, SRAT product, and Slurry Mix Evaporator (SME) product slurries. The scope included considering an addition of sodium hydroxide to Tank 40 to partially offset the sodium lost during decanting as well as considering the impact of bounding quantities of Actinide Removal Process (ARP) feed and Modular Caustic-Side Solvent Extraction Unit (MCU) feed on these same parameters. Simulated SB4 waste was first adjusted to match the dilution that has occurred in Tank 40 during the initial period of SB4 operations in the DWPF. The adjusted simulant was decanted an equivalent of 100,000 gallons relative to 413,740 gallons projected supernate volume. The decanted simulant was divided into two equal parts. One part received an addition of sodium hydroxide to increase the Na{sub 2}O content of the calcined sludge solids by about 3%. The baseline decanted simulant and caustic adjusted simulant were each tested in three pairs of DWPF process simulations of the SRAT and SME cycles. The simulations were at the nominal SB4 acid stoichiometry of 130% with and without bounding ARP/MCU additions and at 170% of acid without ARP/MCU. The 170% case without ARP/MCU was considered bounding relative to 170% with ARP/MCU based on calculated acid requirements. No significant negative impacts on the proposed acid operating window for the SRAT and SME cycles were noted in the simulations. Nitrite was successfully destroyed and mercury reduced in all six SRAT cycles. Hydrogen was produced in all six SRAT and SME cycles, but the levels were below the DWPF SRAT and SME cycle limits in all cases. Relatively insignificant rheological changes were noted other than the previously established thinning from higher acid additions. SRAT and SME processing at 130% acid stoichiometry with and without ARP/MCU additions were very similar. MCU nitric acid content, however, shifted the normal acid addition prior to boiling toward more formic acid and less nitric acid than without MCU. SRNL found no technical issues processing the proposed Tank 40 decant (with or without added NaOH) from the standpoint of the impact on the DWPF SRAT and SME cycles subject to the inherent uncertainties in modeling the ARP noble metal concentrations. The noble metals in the ARP were set to the same weight percents in the total solids that they were measured at in the SB4 waste, which was considered likely to be bounding on the actual noble metal content of the ARP transfer to DWPF. Assumptions for formate loss and nitrite to nitrate conversion can be left at the current levels