Tri-n-butyl phosphate (TBP) and nitric acid, which is commonly used in the PUREX method for reprocessing spent - nuclear fuel, is known to be detonable under extreme condition when fuming nitric acid (FNA) is employed and ignited by an explosive. The chemical properties of the TBP / FNA mixtures along with the elapsed time were evaluated by FT - Raman spectroscopy. Raman spectrum of the TBP / FNA mixtures after the first 2 hours showed no significant chemical change. The critical thickness of detonation propagation in the TBP / FNA mixtures with in the first two hours was evaluated by the wedge method. Nitromethane (NM) was used as a comparison. The critical thickness of the TBP / FNA mixtures were very small, determined as 0.5 - 0.6 mm, and independent of the mixture ratio. Dark waves, a common phenomenon in liquid explosive, were observed from the trace on the recording plate just short of the failure point. The detonation propagation in the wedge charge was confirmed by framing camera photography. The average detonation velocity was 6.24 ± 0.24 km/s.