This document specifies an analytical method for determining the neptunium concentration by spectrophotometry, with spectrophotometer implemented in hot cell or glove box allowing the analysis of high activity solutions, with a standard uncertainty, with coverage factor k = 1 of about 5 %, in nitric acid solutions after the dissolution of nuclear reactor irradiated fuels, at different steps of the process in a nuclear fuel reprocessing plant or in other nuclear facilities. The method is applicable to sample from the process containing a concentration of neptunium between 10 mg·l^-1 and 400 mg·l^-1 and uranium concentrations of up to 300 g·l^-1.