Phase Transformations in the Cs2U2O7 Cesium Diuranate at Temperatures up to 1000°C.

Abstract—One of the methods for high-temperature treatment (HTT) of nitride spent nuclear fuel (SNF) of fast neutron reactors is its direct oxidation. At this stage of processing, it is desirable to remove radioactive cesium, which is in the form of uranates in an oxidizing atmosphere, from SNF. The behavior of the Cs₂U₂O₇ cesium diuranate upon heating to 1000°C is investigated by of synchronous thermal analysis, isothermal holding, and vacuum sublimation. Samples were analyzed by X-ray diffraction, microstructural, and chemical analyses. The data of thermal analysis and isothermal holding at 150, 400, 700, and 900°C indicate that the decomposition of cesium diuranate to form monouranate and volatile products begins at temperatures of 150–200°C, the complete conversion of diuranate into monouranate ends by 700°C, and complex uranium oxides form. The sample mass loss upon holding at 700°C is ~8%. At 900°C, monouranate partially decomposes into diuranates, complex polymer uranates, and volatile products. The total mass loss is ~14%. During high-temperature vacuum sublimation at 950°C in a vacuum of 2 × 10^–2 mmHg, cesium diuranate decomposes into monouranate, uranium(IV) oxide, polyuranates, and volatile products; the mass loss does not exceed 20%; and the main sublimate component is metallic cesium (>99%). Thus, cesium is incompletely removed during direct oxidation of nitride SNF, and the resulting cesium uranates would significantly complicate HTT. [ABSTRACT FROM AUTHOR]