In a review of the safety basis for solvent extraction processes at the U.S, Department of Energy's Savannah River Site, a question was raised concerning the safety margin associated with a postulated accident involving a runaway tri-n-butyl phosphate (TBP)/nitric acid reaction due to the inadvertent heating of a tank. The safety margin was based on studies that showed that the maximum temperature would not exceed 128degreesC, as compared to 130degreesC, the minimum initiation temperature for runaway reaction established in the 1950s following damaging incidents at the Savannah River and Hanford Sites. The reviewers were concerned that the minimum initiation temperature was not conservative since data for solutions containing 20 wt% dissolved solids showed initiation temperatures at or below 130degreesC, and process solutions normally contain some dissolved solids.To address the safety concern, the initiation temperature for runaway reaction was measured as functions of the nitric acid and dissolved solids concentrations using a Reactive System Screening Tool(TM). The thermal stability studies showed a gradual decrease in the initiation temperature with increasing nitric acid concentration due to the increase in the available oxidant. Temperatures measured using solutions containing dissolved solids showed a small dependence on the salt concentration due to a salting-out effect and decreased linearly with the amount of nitric acid extracted by the TBP. The data illustrated that initiation temperatures measured in the 1950s using TBP contacted with nitric acid solutions containing 20 wt% dissolved solids were 10 to 15degreesC below predicted values; thus, the margin of safety is much larger than indicated by the previous work.