TANNIX loaded with uranium and nitric acid ions has accumulated as a secondary waste of radioactive water decontamination. Establishment of treatment procedure of the spent TANNIX is an important issue for facility management. We focused on subcritical water treatment due to its high decomposition performance of organic compounds in our previous study. This technology utilizes dissolution of organic compounds into subcritical water and chemical reactions with chemically activated species. After developing this technology, it is expected that organic compounds can be completely decomposed inside a closed reactor and uranium can be recovered as oxide precipitations after the operation. In this study, applicability of the subcritical water treatment for the spent TANNIX decomposition was experimentally examined. As the results, temperature conditions contributed significantly to the decomposition efficiency, and the solid–liquid ratio had a limited effect. Analysis of the solution showed water-soluble decomposition products. These products further decomposed into gaseous products.