Measured and calculated fission-product poisoning in neutron-irradiated uranium-233

Samples of 233U and of natural thorium have been irradiated in high neutron-flux facilities, in both soft and hard neutron spectra, and for both short and long exposure times. Included are exposures resulting in depletions of more than 90 percent of the 233U in the fissile material and burnups of more than 30,000 MWd/MT in the fertile material. Fission-product poison cross sections in two energy groups (thermal and epithermal) exhibit differences between measurement and calculation that are believed to be attributable to a lack of adequate information on important fission products in the literature. Experimental results for transient absorbers in irradiated 233U give at least 20,000 b for the neutron absorption resonance integral of 149Pm. This is a factor of 15 higher than that obtained by a 1/v extrapolation of the thermal cross sections. For transient 135Xe, the measured absorption is 7.5 percent higher than that calculated using ENDF/B-IV data. Information is also provided concerning such matters as fission yields and neutron absorption of neodymium isotopes, the existence of significant transient fission-product poisons other than 135Xe and 149Sm, and the shielding of 233U by 232Th. Such shielding suggests the need for a change in the energy dependence of the 232Th thermal-neutron cross section