PRODUCTION OF NxOy-O2-CO2-H2O (steam) FOR THERMOCHEMICAL TREATMENT OF IRRADIATED UO2 FUEL

Abstract

The relevance of the research is based on the rationale for creating suitable environment for fuel thermochemical treatment to ensure simultaneous ceramic fuel conversion into powder-like material, complete recladding of reaction product, and volatile fission products entrapment (tritium, iodine-129, carbon-14, radioactive noble gases) from fuel composition prior to hydrometallurgical processes. The main aim is to find main process parameters of NxOy-O2-CO2-H2O (steam) oxidizing mixture which can be used for thermochemical treatment of cladded fuel containing ceramic uranium dioxide. Subjects: solution of nitric acid and oxalic acid composition, NxOy-O2-CO2-H2O (steam) oxidizing mixture, sample of new fuel element containing uranium dioxide. Methods: investigational studies, conductometric and potentiometric titration, gravimetric and and X-ray phase analysis, morphological studies, gas chromatography. The results. The main physical and chemical rules of fabricating NxOy-O2-CO2-H2O (steam) oxidizing mixture were studied for thermochemical treatment of fuel containing uranium dioxide. It was found that generation of gas bearing nitrogen dioxide (26 vol. %), nitrogen monoxide (20 vol. %), carbon dioxide (44 vol. %) and water vapor (10 vol. %) was the result of passing acids mixture (nitric acid – 380 g/l, oxalic acid – 80 g/l) at the rate of 5 column volumes per hour through Pt/Cr2O3/ZrO2 catalyst layer at 368–373 K. It is preferable to implement nitrogen monoxide full oxidation and gas catalytic activation by means of Pt/Cr2O3/ZrO2 packing at 413 K and height of packed bed to column diameter ratio as 5:1. Upon doing that, oxidizing mixture has the following composition: nitrogen dioxide (41 vol. %), nitrogen monoxide (6 vol. %), carbon dioxide (42 vol. %), water vapor (6 vol. %) and oxygen (5 vol. %). Ability in principle to convert ceramic fuel containing non-irradiated uranium dioxide in zirconium cladding into powder-like triuranium octoxide was proven by using NxOy-O2-CO2-H2O (steam) mixture as an oxidizing agent following catalytic activation. We made powder-like material separated from zirconium cladding in full which corresponds to molecular formula U3O8 from X-ray phase and gravimetric analyses, O/U=2,67.

Keywords

• mixture of nitric and oxalic acids
• catalytic decomposition
• oxidizing mixture
• catalytically-active material packing
• ceramic fuel
• powder-like material