Abstract: For the operation of material-test-reactors, recently there have been considered fuels having lower degrees of enrichment in U-235. For this purpose, there is needed U.sub.3 O.sub.8 powder having a crystal size of 40 to 90 .mu.m, which is producible economically and in whose production there is developed as little secondary waste as possible and a pore-free product. For this purpose, uranium compounds of the stage of oxidation +4 to +6 are calcined in an oxygen containing atmosphere whereby the calcining is carried out above 1250.degree. C. in an atmosphere containing more than 30 volume % oxygen.