(632f) Catalyzed Ammonium Ion Formation From the Reaction of Formic Acid with Nitrate Ion During Radioactive Waste Processing at the Savannah River Site

Legacy radioactive waste generated at the Savannah River Site (SRS) during production of enriched uranium and plutonium during the Cold War is currently being processed in the Defense Waste Processing Facility (DWPF) into a stable borosilicate glass waste form for long term storage. The majority of the legacy waste is stored as a mixture of hydroxide and hydrous oxide insoluble solids in large cylindrical storage tanks at SRS. These 3.5-4.5 thousand cubic meter (900,000-1,200,000 gallon) carbon steel storage tanks also contain 5-7M sodium solutions rich in hydroxide, nitrate, and nitrite anions. Most SRS tank farm processes involve the transport of two phase slurries between tanks and the decanting of excess aqueous phases for processing in the tank farm evaporators. Batch feed preparation for DWPF involves washing the aqueous phase of slurries obtained from one or two waste tanks to about a 1M sodium concentration.

The preparation of the sixth DWPF sludge batch (SB5) was the first to use large tank aluminum dissolution to reduce the aluminum content of the insoluble solids. This change coincided with the transition from the high iron Purex wastes of the first five sludge batches (SB1A, 1B, 2, 3, and 4) to high aluminum HM wastes. Removal of aluminum led to significant enrichment of other species in the remaining insoluble solids. The major impact was the increased concentrations of Hg, Rh, and Ru. These species are catalytically active during DWPF processing using formic acid. Unusually high concentrations of ammonium ion were found in the recycle condensate stream leaving DWPF shortly after the start of SB5 processing.

A lab-scale DWPF simulation program commenced in late 2009 to facilitate preparations for sludge batch 6 (SB6). The main waste component of SB6 is HM waste that has undergone aluminum dissolution like SB5. The lab-scale equipment was modified to include an off-gas ammonia scrubber. Increased sampling of the Sludge Receipt and Adjustment Tank (SRAT) vessel was included in the test plan to identify the timing of ammonium ion formation and to quantify the moles of ammonium ion in the vessel. Additional sampling also targeted the various SRAT cycle condensates and Slurry Mix Evaporator (SME) cycle condensates for ammonium ion. SRAT and SME cycle condensates are eventually sent to the DWPF recycle condensate stream that returns an aqueous solution to the SRS tank farm for processing through the tank farm evaporators.

The results of the test program show that ammonium ion is formed during the SRAT cycle. Formation starts later than catalytic hydrogen generation (presented at the 2009 AIChE Annual Meeting as the last major reaction in the SRAT chemistry timeline). The amount of ammonium ion produced was found to depend on the amount of excess formic acid added during SRAT processing, as well as on the initial mercury concentration of the radioactive waste feed. Most end of SRAT cycle ammonium ion was either in the SRAT slurry or in the circulating acid reservoir for the ammonia scrubber. Significant SRAT slurry ammonium was driven out of the slurry during SME cycle concentration periods and was absorbed into off-gas condensates before the off-gas reached the ammonia scrubber. Active ammonium ion formation continued during the SME cycle in parallel with the stripping of ammonia vapor into the off-gas system.

The proposed overall reaction for ammonium ion formation in DWPF is given by:

5HCO2H + NO3- --> NH4OH + HCO2- + 4CO2 + 2H2O

Analytical data from the process simulations support the loss of additional formic acid, the loss of nitrate ion, and the formation of additional CO2 gas. A proposed series of reactions that add to the overall reaction above includes formation of some nitrite ion as an intermediate. Off-gas species associated with nitrite destruction were seen during periods of ammonium ion formation. A shiny gray clay-like deposit was found on the agitator impeller in one of the higher acid runs. The deposit was analyzed and found to most likely be a mercury amalgam with low concentrations of copper, rhodium, palladium, ruthenium, and a trace of calcium. The deposit contained a quarter of the mercury in the initial SRAT feed slurry. The amalgam is a likely candidate to be the catalyst for ammonium ion formation.