In a review of the safety base for solvent extraction processes at the Savannah River Department of Energy site, a safety margin issue was raised associated with a postulated accident involving a tri-n- butyl (TBP) / nitric acid reaction due to involuntary heating of a tank. The safety margin was based on studies demonstrating that the maximum temperature would not exceed 128 ° C in comparison with 130 ° C, the minimum initiation temperature for the decomposition reaction established in the 1950s after the harmful incidents in the sites Savannah River and Hanford. The reviewers were concerned that the minimum temperature was not conservative since the data for solutions containing 20% by weight of dissolved solids showed initiation temperatures of or below 130 ° C and the process solutions usually contained some dissolved solids.
To address the safety problem, the initiation temperature for the quench reaction as functions of nitric acid and dissolved solids concentrations was measured using a Reagent System Review Tool. Thermal stability studies showed a gradual decrease in the initiation temperature with increasing nitric acid concentration due to the increase in available oxidant. Temperatures measured using solutions containing dissolved solids showed a small dependence on salt concentration due to a salinization effect and decreased linearly with the amount of nitric acid extracted by the TBP. The data illustrates that initiation temperatures measured in the 1950s using TBP in contact with nitric acid solutions containing 20% by weight dissolved solids were 10-15 ° C below the predicted values; therefore, the margin of safety is much higher than indicated by previous work.