This International Standard specifies an analytical method for determining the neptunium concentration by spectrophotometry, with a standard deviation 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. The method is applicable to aliquots containing a concentration of neptunium between 10 mg⋅l−1 and 200 mg⋅l−1.