Purifying a nitric acid solution containing U/Pu ions and contaminated by metal impurities by(a) oxidizing the U/Pu ions of the solution to the hexavalent form,(b) conducting the solution after oxidation through a cation exchanger column in which the impurities, especially americium, are retained,(c) flushing the cation exchanger column to remove impurities therefrom, which may be taken to waste processing or utilization plant.