For safety research in spent nuclear fuel reprocessing, the study of the potential explosion hazards of TBP (tri-n-butyl phosphate)/nitric acid mixture was conducted. A worst condition of the mixture was assumed with the using of fuming nitric acid (FNA) and a shock ignition stimulus. The shock initiation sensitivity of the TBP/FNA mixture was investigated by the card gap test, which is based on the Japan Explosive Society standard. Separately, photographs using a streak camera and framing camera measured the shock passing time through a PMMA gap, and then, a pressure calibration curve was determined. From the card gap test, it was found that the shock initiation sensitivity of TBP/FNA depends on the mixture ratio. By using the pressure calibration curve, a critical shock pressure was estimated. The shock initiation sensitivity of TBP/FNA at stoichiometry was estimated three or four times higher than that of nitromethane.