The formation and the oxidation-reduction properties of mixed oxides in the Ce–U–O ternary system were examined by means of thermogravimetry and the X-ray diffraction technique. The mixed oxides were prepared by the reactions of CeO2 with U3O8 in air and in hydrogen. The composition of the products heated in air, followed by slow cooling to room temperature, was approximately expressed by this equation: yCeO2+(1–y)UO2.67. By X-ray diffraction, it was shown that the oxides formed at 1100 °C in air were mixtures of Ce0.6U0.4O2.3 and α-U3O8 in the y<0.6 range, while they were in single phases of solid solutions for y≥0.6. When the mixed oxides, MO2+X, where M indicates yCe+(1–y)U, were re-heated in air, weight loss due to oxygen liberation was observed above 500 °C, and it was enhanced with a rise in the temperature. The hydrogen reduction of the oxides proceeded in two steps; the first step was the reduction of MO2+X to MO2 below 600 °C, and the second one was the reduction of MO2 to MO2−X above 600 °C. The composition of the oxides obtained can be given by this equation: yCeO1.81+(1–y)UO2.00. Two separate linear relationships for the lattice parameter of the fluorite-type structure as a function of the cerium content were obtained; one is applicable to the solid solutions with y≤0.6, where the lattice parameter is nearly that of UO2, and the other to the solid solutions with y≥0.4. The latter obeys Vegard’s law between UO2 and CeO2. The oxides with y=0.4–0.6 were in the two-phase region of the solid solutions.
Criticality accident / Uranium solution / Fission / Neutron capture reaction / 238 U, 239 Np, 239 Pu / Ion exchange separation / α-ray spectrometry / γ -ray spectrometry Summary. Atom numbers of 239 Np and 239 Pu, which had been produced by the neutron capture reaction of 238 U in the uranium solution of the JCO criticality accident at Tokai-mura, Japan, were determined precisely by a radiochemical method. It was found that the measured atom ratio of 239 Np/ 239 Pu at termination of the criticality depends sensitively on the history of the neutron capture reaction during the accident (about 20-hours continuance). The history was evaluated by comparing the measured value and those calculated by a burnup calculation code. The percentage of the number of the neutron capture reaction at the initial stage of the criticality during 25 min against the total events was determined to be 24 ± 6%.
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