Uranium-233 Signatures

Uranium-233 is a fissile, direct-use nuclear material that is subject to safeguards. The ability to accurately verify the presence and quantity of 233 U from its radiation signatures using nondestructive assay (NDA) methods is an integral component of a safeguards program, including such a program applied to the civilian thorium-uranium ( 232 Th/ 233 U) nuclear fuel cycle. However, NDA methods are hampered in part by the limited availability and poor qu ality of the existing 233 U basic nuclear data. For example, a literature review reveals that the absolute emission probabilities for a majority of the characteristic decay gamma-rays from 233 U have relative uncertainties of 10– 15% or greater. In the worst cases, the absolute emission probabilities have relative uncertainties exceeding 40%. Furthermore, there is limited da ta for the active neutron interrogation of 233 U using well-characterized reference materials. If uncertain ties in nuclear data are not reduced, they will continue to have a sign ificant adverse impact on 233 U assay results, total measurement uncertainty, and the ability to draw safeguards conclusions. The Next Generation Safeguards Initiative (NGSI) 233 U signatures project is researching the NDA of 233 U from a nuclear data perspective. M easurements of high-isotopic-purity 233 U items (>99.4% 233 U) have been performed at Oak Ridge Nationa l Laboratory to acquire and determine accurate radiation signatures (gamma-ray a nd neutron) for this fissile isotope. Measurements are being conducted in parallel with recovery a nd dispositioning activities for the NGSI 233 U preservation program. Measured data has been used to reevaluate 233 U gamma-ray absolute emission probability data, acquire neutron data, and investigate the NDA of this isotope when assayed in various physical forms. This paper reports on the measur ed gamma-ray and neutron data, resulting nuclear data findings, and observations re levant to the development of ND A methods and analysis tools for the implementation of 232 Th/ 233 U nuclear fuel cycle safeguards.