Abstract: A method and apparatus for reconditioning waste solutions of the nuclear ustry which contain ammonium nitrate. The waste solution is continuously sprayed from above into a 300.degree. to 600.degree. C. hot zone, from the lower end of which the decomposition products are withdrawn and are separated into condensate and exhaust gas. The NO content is reduced by adding oxygen, especially at the exit of the decomposition zone. By the presence of reducing agents, such as CO(NH.sub.2).sub.2 or preferably NH.sub.3, in the solution to be sprayed, the NO.sub.2 content of the decomposition products and the NH.sub.4 NO.sub.3 residue in the condensate can be practically eliminated and an additional precipitation in the condensate which can be filtered off can be obtained. Optimal contents are 0.25-0.4 Mol NH.sub.3 and 0.15-0.25 Mol CO(NH.sub.2).sub.2 per Mol NH.sub.4 NO.sub.3.