This International Standard specifies a method of determining the mass fraction of uranium and the oxygen-to-uranium atomic ratio in hyperstoichiometric uranium dioxide (UO2+X) powders and pellets. This International Standard is used for the determination of the U mass fraction and the O/U-ratio of nuclear grade uranium dioxide. The precision and accuracy stated in 8.2 are valid, if the total amount of impurities in the sample does not exceed the figures given in 8.2. An impurity correction in the U mass fraction and the O/U-ratio determination should be performed, if the amount of total impurities in oxide form exceeds 1 500 µg per gram of sample. Lower impurity levels influence the O/U-ratio by less than 0,000 5 and can be neglected. The non-volatile impurities shall be determined by an appropriate technique, and the correction applied. If the total content of the non-volatile impurities in oxide form is greater than 1 500 µg per gram of sample, the overall precision of the method depends on the accuracy of these impurity measurements. If no impurity correction is made and if the enrichment of the sample is not considered as described in 8.11, the precision and accuracy figures will be poorer than in 8.2.