Method for removing fluoride ions from UO.sub.2 powders

A method of purification of a heavy metal oxide selected from uranium and plutonium oxides for use as a nuclear fuel contaminated with fluoride and hydrogen to produce a purified heavy metal oxide containing not more than 25 parts per million fluoride which comprises passing steam in contact with said contaminated heavy metal oxide to effect removal of substantially all the hydrogen and thereafter subjecting said heavy metal oxide to pyrohydrolysis in the absence of hydrogen by passing steam at a temperature above 650.degree.C in contact with said heavy metal oxide from which hydrogen has been removed to effect reduction of the fluorine content in said contaminated heavy metal oxide to no more than 25 parts per million.