Spectrophotometric determination of nonstoichiometry in hyperstoichiometric uranium dioxide

Abstract

Uranlum dioxide is widely used as a nuclear fuel and usually exists as hyperstolchlometrlc UO,+,. I n the descrlbed method UO,+, I s dissolved in warm concentrated phosphork add saturated with sodium sulfate at room temperature to prevent the alr Oxidation of U(IV). The concentratlons of U(V1) and U( IV) are directly determined by means of the absorbance of the two species at two speciflc wavelengths 420 and 665 nm, respectively. The results obtalned are predse to f0.002 O/U units and have been compared with those obtalned from gravimetric tltritnetry employing potentiometric end point detectlon. The O/U ratio determined by the two procedures agrees within 0.004 O/U units. The method has been employed to determine O/U ratio In near stdchlometric U02 (2.039) as well as in U30s (2.670). The developed spectrophotometric method is simple and fast and does not require inert atmosphere.Uranium dioxide is well-known as a nuclear fuel; its composition is close to the stoichiometric ratio of oxygen-touranium of 2:1, but the exact stoichiometry is seldom attained. Generally uranium dioxide exists as hyperstoichiometric U02+z because of the presence of interstitial oxygen. Knowledge of oxygen to uranium ratio is essential because it affects certain physical and chemical properties of the fuel that are very important to fuel performance under reactor conditions (1). All wet chemical methods for the determination of O/U ratio are based on the determination of either U(1V) or U(V1) since hyperstoichiometry can be directly related to the concentration of U(V1). Thus the determination of the U(V1) serves as an exact measure of the concentration of oxygen above 2:l ratio and, coupled with total uranium determination, enables determination of the exact composition of the oxide. The choice of the most convenient analytical procedure for the determination of U(V1) is related to the degree of hyperstoichiometry of UO,,,. For the near-stoichiometric UOz pellets, where a minute quantity of U(V1) has to be determined in the presence of a large excess of U(IV), polarographic methods are preferred (2). The potentiometric titration of trace amounts of U(V1) in the H3P04 solution is also possible, but special methods of end point detection, e.g., bipotentiometry or biamperometry, are required (3). Gravimetric titrimetry coupled with potentiometric end point detection has been employed in the case of U308 where U(V1) is present in larger quantity. A weighed quantity of U308 powder is added to a standard solution of Ce(1V) in 2 M H2S04 and is dissolved by keeping the beaker in a thermostated water bath at 70 "C for 15-20 min. After complete dissolution the solution is cooled to room temperature and a known excess of standard Fe(I1) solution is added. The remaining Fe(I1) is back-titrated against standard potassium dichromate solution ( 4 ) . The recently published method of Khatoon and Rao (5), which involves the dissolution of U30s in an excess of Ce(1V) in 2 M perchloric acid at room temperature followed by potentiometric determination of t...

Preparation and Characterization of Uranium Oxides in Support of the K Basin Sludge Treatment Project

“…The spectrophotometric analyses were performed by dissolution of the uranium oxide slurries in 85 wt% (14.6 M) phosphoric acid (H 3 PO 4 ) solution that is near saturation in dissolved sodium sulfate (Na 2 SO 4 ) and measurement of the spectrophotometric absorbances in comparison with the spectra of dissolved uranium of known concentration and oxidation state (Ahmed and Sreenivasan 1986). The Na 2 SO 4 -H 3 PO 4 solution composition was selected to minimize, by complexation, oxidation of U(IV) by atmospheric oxygen and eliminate the need for an inert atmosphere glovebox as practiced in other spectrophotometric methods.…”

Thermogravimetric study of the oxidation behaviour of uranium dicarbide

“…The details of the method are described e!8~here [6]. The details of the method are described e!8~here [6].…”