Treatment of simulated high-level radioactive waste with formic acid: Bench-scale study on hydrogen evolution

At the Savannah River Site, the Defense Waste Processing Facility (DWPF) was constructed to vitrify high-level radioactive liquid waste in borosilicate glass for permanent storage. Formic acid, which serves as both an acid and a reducing agent, is used to treat the washed alkaline sludge during melter feed preparation primarily to improve the processability of the feed and to reduce mercury to its Zero state for steam stripping. The high-level sludge is composed of many transition metal hydroxides. Among them, there are small quantities of platinum group metals such as ruthenium, rhodium, and palladium that are fission products. During the treatment of simulated sludge with formic acid, significant amounts of hydrogen were generated when the platinum group metals were included in the sludge. Apparently the noble metals in the sludge were reduced to their zero states and caused formic acid to decompose catalytically into hydrogen and carbon dioxide, usually with an induction period. The production of hydrogen gas presented the DWPF with a safety issue. Therefore, the objective of this research was to gain a fundamental understanding of what controlled the hydrogen evolution so that a practical solution to the safety issue could be obtained. A bench-scale parametric study revealed the following: increasing the amount of formic acid added to the sludge increased the hydrogen generation rate dramatically; once the catalysts were activated, the hydrogen generation rate decreased significantly with a lowering of the temperature of the sludge; the relative catalytic activities of the noble metals in the sludge-decreased in the following order: rhodium > ruthenium much greater than palladium; ammonium ions were generated catalytically from the reaction between formic acid and nitrate; and when present, the noble metals caused higher upward drifts of the sludge pH. Based on these bench-scale results, in conjunction with a pilot-scale study, a forced air purge and hydrogen monitoring system, along with a temperature controlled safety shutdown algorithm, were developed.