ISO 15651:2015 describes a procedure for measuring the total hydrogen content of UO₂ and PuO₂ powders (up to 2 000 µg/g oxide) and of U0₂ and (U,Gd)O₂ and (U,Pu)O₂ pellets (up to 10 µg/g oxide). The total hydrogen content results from adsorbed water, water of crystallization, hydro-carbon, and other hydrogenated compounds which can exist as impurities in the fuel.