[en] Nuclear fuels, such as uranium trioxide, are prepared by thermal decomposition of a nitrate in a fluidized bed reactor. The fluidized bed reactor for preparing uranium trioxide from uranyl nitrate by thermal decomposition has (a) a rectangularly shaped bed which satisfies the critical safety shape of ²³⁵U, (b) plural holes to supply an aqueous solution of uranyl nitrate into the reactor, (c) at least two reaction rooms divided by barriers, the bottoms of the barriers being capable of being lifted to control their distance from the floor and a mechanism by which uranium trioxide powder is taken out mainly by being overflowed from the top of the barriers through the reaction rooms, (d) heating means inside and outside of the fluidized-bed, and (e) a head structure which is upwardly V-shaped. Continuous operation can be attained with ease, and uranium trioxide can be made efficiently