The heat and off-gas generation behavior was experimentally examined during a safe chemical denitration, pre- and mild-denitration,
of simulated HLLW with a nitric acid concentration of 2 to 7.5 M. The maximum heat and off-gas generation were no more than
100 cal/s·1 and about 0.8 l/min, respectively. The solution temperature does not reach boiling temperature and no solution
was squirted out from the denitration vessel. The pre-and mild-denitration technique could be considered as one of safe methods
for removing nitric acid from the HLLW with various nitric acid concentrations. The pre- and mild-denitration also has an
advantage to improve the filtration characteristics of precipitates produced by the denitration of simulated HLLW. The denitration
of HLLW with 7.5M nitric acid concentration induced formation of “very easy-to-filter” solid. Moreover, a good filter cake
washing is possible.