Direct thermal denitration to prepare mixed oxides for nuclear fuel fabrication

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Abstract

Direct thermal denitration of nitrate solutions containing uranium, plutonium, and about 2 mol of ammonium nitrate per mol of heavy metals produced a mixed oxide powder which was well suited to the production of pellets. The nitric acid concentration of the solutions was maintained at greater than 1 M to prevent hydrolysis of the plutonium. The resulting UO{sub 3}-PuO{sub 2} powder had a bulk density of about 1.2 g/cm{sup 3} and required preslugging before pellet pressing. Pellets prepared at the Oak Ridge National Laboratory (ORNL) by pressing at 241 MPa (35,000 psi) or 310 MPa (45,000 psi) and sintering at 1450{sup 0}C for 4 h had densities of 95% of theoretical, good external appearances, and good microstructures. The pellets were completely soluble in refluxing 7 M HNO{sub 3}. Pellets were also pressed from this powder at Battelle Pacific Northwest Laboratory (PNL) with similar results. A sinterable PuO{sub 2} product was obtained in comparable denitration tests with plutonium solutions, although the powder had poor flowability and produced pellets with scattered, relatively large porosities. The results, however, encourage further development of better plutonia powder by this process.

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