Discrimination of233U and235U by Delayed Neutron Signatures for Nondestructive Assay

Many advanced nuclear reactor designs utilize a fuel cycle based on the conversion of
²³²
Th to
²³³
U, where the latter is used to drive the fission chain reaction. While this fuel cycle has many benefits over traditional fuel cycles, it presents a challenge for nuclear safeguards applied under the Non-Proliferation Treaty (NPT). In particular, there is a lack of mature techniques for the non-destructive assay (NDA) of
²³³
U which must be addressed before thorium fuel cycles come online in states subject to international safeguards. This is further complicated by the fact that in most thorium reactor designs,
²³³
U is collocated with
²³⁵
U, both of which must be individually accounted for. In this work, a technique is demonstrated to utilize the β-delayed fission neutron decay time profiles to quantify the ratio of
²³³
U to
²³⁵
U in mixed-isotope items. An experiment was performed to measure the delayed neutron time profiles of
²³³
U,
²³⁵
U, and mixtures thereof using a modified active well coincidence counter (AWCC), in which the AmLi neutron source was replaced with a deuterium-deuterium neutron generator. The shape of the resultant delayed neutron decay time profiles shows a clear dependence on the relative abundance of each isotope. The measured profile shapes are quantified using a double-integration technique and are consistent with values obtained from an analytical model using the tabular six-group delayed neutron yield parameters. This technique represents the first proposed neutron-based method to discriminate between
²³³
U and
²³⁵
U.