PURPOSE: Powders of UO2 and/or U3O8 are fed into the fluidized bed of UO3, uranium nitrate is introduced into the bed and the denitration is effected by heating with microwaves, thus conducting denitration without external heaters.

CONSTITUTION: UO3 powders, UO2 and/or U3O8 powders are introduced into the fluidized bed 2 and air is fed to make a fluidized state, then uranyl nitrate solution is sprayed from nozzle 4. Microwaves are applied and the powders of UO2 or U3O8 generate heat, because of their large dielectric loss, to gasify the water and nitrogen oxides in the solution and the gases are removed from the exhaust gas treater 5, after UO3, UO2 or U3O8 powders accompanying the gases are separated. In the meantime, the dehydrated and denitrated uranyl nitrate solution is converted into a UO3 powder and the powder is recovered into the recovering tank 9, as it is mixed with UO2 or U3O8 powders. Further, UO3 is converted into U3O8 and UO2 with hydrogen in reductive calciner and UO2 is collected in the vessel 12. A part of the product is contained in the powder feeder 14 for reuse.