A Direct Comparison of Some Nuclear Properties of U-233 and U-235

Nuclear properties of U²³³ and U²³⁵ are compared using data obtained in a series of critical experiments. Aqueous solutions of uranyl oxyfluoride containing uranium enriched to about 90% in each of the two isotopes have been made critical in water-reflected spherical reactors having diameters of 26.4 and 32.0 cm. Assuming the reported nuclear constants for U²³⁵ are reliably known and assuming equality of the neutron leakage spectra of U²³³ and U²³⁵ for the same water-reflected critical sphere, the value of η(U²³³) at 0.026 ev was determined to be 2.31 ± 0.03. The critical masses for the two isotopes in these systems have been measured over the temperature range from 20°C to 100°C; corresponding values of the reactivity temperature coefficient are reported. Delayed neutron yields for the two isotopes were compared by noting the periods resulting from the withdrawal of a boron poison from the critical spheres. It is shown that the yield from U²³³ is about one-third that from U²³⁵, in agreement with other determinations.