THE DETERMINATION OF ATOMIC RATIOS IN THE URANIUM-OXYGEN SYSTEM BY A THERMOGRAVIMETRIC METHOD'

A description is given of a thermogravinletric osidation-reduction method for determining U-0 ratios, especially suited to oxygen-sensitive materials of undetermined purity, and equally applicable to pure stoichiometric and non-stoichiometric osides and their mixtures with each other and with uranium metal. Provided that no uncombined base metals other than uranium, and no oxidizable or reducible matter other than its oxides, are present, it is not essential to laow the detailed composition of the sample, since the assay is based on interconversion of the stoichiometric oxides U02 and U308 within that sample. Incidental advantages of this method are: the avoidance of manipulation of the sample during the determination, precise reaction control, and immediate recording of thecompletion or otherwise of the required reaction. One of the chief characteristics of the uranium-oxygen system is the extent to which excess oxygen can be absorbed into the crystal lattice to form non-stoichiometric oxides. By far the most important of the oxides which has this ability is the dioxide, UO2, often termed brown oxide. In view of its function as an intermediate in the production of the metal, and being itself a potentially valuable reactor fuel, a knowledge of its exact U-0 atomic ratio is often necessary. Current research into new methods of production has extended this need to materials, such as freshly precipitated oxide, which are extremely sensitive to oxidation by exposure to air and may contain inert impurities besides a large percentage of water. It appeared likely that thermogravimetry offered a solution to the problems connected with this type of product, and would also provide a coilvenient general method for U-0 ratios. A controlled-atmosphere apparatus (I) incorporating a Stanton milligram recording thermobalance was used in the investigation: the advantages of exact control over reaction conditions and a visible continuous record of the weight changes associated with a reaction make it possible to carry out a complete analytical operation within the assembly, thereby avoiding excessive manipulation of sensitive samples. The standard chemical methods for determining atomic ratios, namely oxidation to UBOs (2) or reduction to UOs (3), have been evaluated and refined by Bright et al. (4): these latter investigators point out the necessity of preparation and weighing iri an oxygen-free atmosphere and recommend the use of an argon-filled dry box for these operations. Assumptions are usually made that the material under investigation contains only uranium oxide after a preliminary heat treatment to drive off water, etc. Though justifiable with familiar samples, such assumptions could not be made in the case of unusual products, and preliminary assays of material subject to aging should be avoided. The method to be described postulates only that oxidizable and reducible matter other than uranium and its oxides shall be absent, and uses interconversion of stoichiometric UO2 and U3OS as an internal standard on which to base the assay.