The Temperature Dependence of Epithermal Neutron Capture and Fission in Uranium-235 Dioxide

The temperature dependence of epithermal neutron capture and fission in enriched uranium-dioxide rods has been measured in a 1/E epithermal spectrum. Experiments performed to 1525°C in the reflector of the Thermal Test Reactor (TTR) have studied the Doppler effect on ²³⁵UO₂ fission, and experiments to 1278°C in the core of the Union Carbide Research Reactor have investigated the Doppler effect on ²³⁵UO₂ capture and fission. The samples were cylindrical pellets, 0.341 in. in diameter, mounted between similar pellets to simulate a rod. This UO₂ rod was irradiated at various temperatures in a cadmium-covered furnace which was water-cooled so that the temperature of the furnace exterior was that of its environment. Three dilute gold-in-aluminum alloy wires were placed symmetrically about the furnace exterior for neutron flux normalization. The relative numbers of neutron-induced captures and fissions in the ²³⁵UO₂ samples were determined separately using mass spectrometry and gamma-ray spectrometric techniques. The number of neutron-capture events was determined by mass-spectrometrically measuring the ratio of ²³⁶U to ²³⁵U in irradiated and unirradiated samples. The number of neutron-induced fission events was determined by the gamma-ray counting of an appropriate fission product. The measurement of the Doppler effect on fission in ²³⁵UO₂ over four temperature intervals to 1525°C in two irradiation facilities determined that the Doppler effect increased the effective fission integral of this rod by 2.8 ± 1.0% over the temperature interval 24 to 1525°C. A Doppler fission effect of 4% over this temperature was calculated. The Doppler effect on ²³⁵UO₂ capture has been measured to be 15.5 ± 1.8% over the temperature range 241 to 1278°C, which is ∼2.7 times greater than the calculated effect. These results are consistent with previous ²³⁵U Doppler reactivity experiments in which the calculated Doppler reactivity effect was two to three times less negative than the measured value. The effective fission and capture integrals for the enriched UO₂ samples have been measured to be 151 ± 18 and 67 ± 8 b, respectively, at 24°C, and are in agreement with the calculated values of 144 and 60 b. Included in this paper is a review of previous measurements of ²³⁵U Doppler effects, reported in a consistent set of units. This summary is important in understanding the relative contribution of capture and fission on the ²³⁵U Doppler effect and the effect of the neutron energy spectrum on these contributions.