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 (DWP F) was constructed to vitrify high-level radioactive liquid waste in b orosilicate glass for permanent storage. Formic acid, which serves as both an acid and a reducing agent, is used to treat the washed alkalin e sludge during melter feed preparation primarily to improve the proce ssability of the feed and to reduce mercury to its Zero state for stea m stripping. The high-level sludge is composed of many transition meta l hydroxides. Among them, there are small quantities of platinum group metals such as ruthenium, rhodium, and palladium that are fission pro ducts. During the treatment of simulated sludge with formic acid, sign ificant amounts of hydrogen were generated when the platinum group met als were included in the sludge. Apparently the noble metals in the sl udge were reduced to their zero states and caused formic acid to decom pose catalytically into hydrogen and carbon dioxide, usually with an i nduction period. The production of hydrogen gas presented the DWPF wit h a safety issue. Therefore, the objective of this research was to gai n a fundamental understanding of what controlled the hydrogen evolutio n 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 gener ation rate dramatically; once the catalysts were activated, the hydrog en generation rate decreased significantly with a lowering of the temp erature of the sludge; the relative catalytic activities of the noble metals in the sludge-decreased in the following order: rhodium > ruthe nium much greater than palladium; ammonium ions were generated catalyt ically from the reaction between formic acid and nitrate; and when pre sent, the noble metals caused higher upward drifts of the sludge pH. B ased on these bench-scale results, in conjunction with a pilot-scale s tudy, a forced air purge and hydrogen monitoring system, along with a temperature controlled safety shutdown algorithm, were developed.