In order to reduce heat and off-gas generation rates at early stage of the chemical denitration of high and medium level liquid
wastes from the reprocessing of nuclear fuel, a safety denitration method, Pre- and Mild-denitration technique, was originated.
In the Pre-denitration step, the formic acid of 0.06 to 0.3 times nitric acid concentration ([HCOOH]/[HNO3]=0.06 to 0.3) was poured into a nitric acid solution at 80°C and denitration was initiated (Pre-denitration). Then additional
formic acid was injected into the Pre-denitrated solution at a constant injection rate in 80°C and the solution mixture was
heated up to the boiling condition (Mild-denitration). In the Mild-denitration step, the denitration reaction was smoothly
initiated and a leak out of solution from a abrupt boiling has never occurred. The maximum heat and off-gas generation rates
were about 50 cal/s-l and about 1.01/min respectively even in the 10M nitric acid solution. These measured values were low
enough to ensure the safety operation of denitration.