(136c) Mercury-Nitrite-Rhodium-Ruthenium Interactions in Noble Metal Catalyzed Hydrogen Generation From Formic Acid During Nuclear Waste Processing at the Savannah River Site

Legacy nuclear 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. Typically less than 10% of the solid waste mass involves compounds of radionuclides (iron and aluminum are two of the major components). 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.

DWPF brings the washed sludge slurry into the Sludge Receipt and Adjustment Tank (SRAT) to begin batch processing. Acids are added to adjust the rheology, dissolve some of the alkaline earth and transition metal compounds, convert HgO to elemental mercury for steam stripping, destroy some of the existing anions such as carbonate and nitrite, and chemically reduce a portion of the MnO₂ to Mn²⁺. (Thermal reduction of Hg and Mn in the DWPF melter leads to reduced melter throughput and can also lead to foam formation.) Optimal performance of the DWPF waste glass melter requires a balance of chemical oxidizers and reductants in the feed in order to avoid foaming and prevent precipitation of metals. The balance is achieved by dividing the SRAT acid demand between nitric acid (oxidizer) and formic acid (reductant).

The legacy waste contains small concentrations of noble metals formed from ²³⁵U fission product decay chains. In particular, the waste contains Ru, Rh, and Pd in the approximate relative proportions of 3.75 to 1 to 0.52. These species become catalytically active during formic acid addition at 93 °C. HgO also reduces to elemental Hg once formic acid becomes available under acidic conditions. The general existence of catalytic activity was noted during lab-scale nuclear waste processing over fifteen years ago, and the potential for catalytic hydrogen generation was studied just prior to completion of the DWPF design. The planned air purge system, used to keep the equipment under slight negative pressure, was redesigned to permit a large flow of air through the SRAT in order to dilute hydrogen below the lower flammable limit (LFL). Much subsequent experimental work was performed with the goal of ensuring that operating conditions would be selected that produced hydrogen concentrations below the LFL in the available air purge.

Changes in the preparation of the legacy waste sludges have been made to reduce the amount of decanted wash water. These have resulted in DWPF feeds that require proportionally more acid to process. This evolution in operating doctrine has constrained the quantity of acid that can be added relative to the minimum required to accomplish various processing controls. This in turn has brought a renewed interest in understanding the nature of catalytic hydrogen generation and in seeking means to better control and/or eliminate it. Consequently, several new rounds of experimental work were authorized to better understand the process chemistry.

The most active catalytic form of Rh appears to be a nitro-Rh complex during this period. Rh was found to be catalyzing the destruction of nitrite ion to nitrous oxide and carbon dioxide during formic acid addition. Gradual destruction of nitrite ion leads to eventual substitution of one or more of the nitrite ligands with other species. New data now show that it is during this period that Rh becomes an excellent catalyst for converting formic acid into hydrogen and carbon dioxide. Catalytic hydrogen generation can be three or more orders of magnitude greater than radiolytic hydrogen generation at this time. This creates the potential for the formation of flammable or explosive air-hydrogen mixtures in the SRAT air purged off-gas system. DWPF checks the hydrogen concentration once per minute using a pair of redundant gas chromatographs. The kinetics of nitrite destruction appears to be controlling both the activation and the deactivation of the nitro-Rh complex form that catalyzes hydrogen generation. The nitro-Rh complex is currently the catalyst of primary concern.

New data show that ruthenium does not become catalytically active for hydrogen generation in the presence of an excess of nitrite ion, but that once nitrite ion is brought to concentrations below that of the ruthenium that it does become catalytically active. This occurs after the most active form of the nitro-Rh complex has lost significant activity. Consequently, the two catalysts are not at their most active states simultaneously. The slower activation of Ru for catalytic hydrogen generation often correlates with lower hydrogen generation rates than occurred during the period of maximum Rh activity, but this is not always true. A primary deactivation mechanism for Ru catalysis has not been fully identified, but adsorption and/or precipitation of both soluble Rh and soluble Ru onto the insoluble solids correlates linearly with reductions in the catalytic hydrogen generation rate.

Palladium has been found to be relatively ineffective at converting formic acid into hydrogen in SRAT simulations, which, coupled with its lower concentrations, makes it a minor factor for hydrogen generation. Pd has been found, however, to be a fairly effective catalyst for formic acid conversion of nitrite ion to nitrous oxide, carbon dioxide, and water. Tests with and without mercury have shown that the elemental Hg, formed by reduction of HgO with formic acid, catalyzes the conversion of nitrite ion to NO, carbon dioxide, and water. Therefore, both Pd and Hg affect the activation and deactivation processes for the nitro-Rh complex through their impact on the kinetics of nitrite ion destruction. A large fraction of the elemental mercury is also being simultaneously steam stripped out of the system during catalytic hydrogen generation in order to prevent mercury from overwhelming the melter off-gas system quencher, scrubbers, and high efficiency mist eliminators.

The new data on the individual roles of Rh, Ru, Pd, and Hg presented in this talk represent a considerable advance in the understanding of the reaction sequences and the interplay of the significant species during processing of the legacy waste slurry upstream of the DWPF melter.