The development of a new, neutron, time correlated, interrogation method for measurement of 235U content in LWR fuel assemblies

This paper presents the first application of a new technique for the measurement of the 235U content in fresh fuel assemblies. The technique, called time correlated induced fission (TCIF), uses a 252Cf neutron source to irradiate the fuel assembly, and the subsequent induced fission events in the fissile material are measured by multiplicity counting. The doubles and triples rates are enhanced by having the trigger events from both the 252Cf source and the induced fission neutrons in the same time gate in the coincidence analysis. The average neutrons per fission (ν) of the 252Cf source is 3.76 and the induced fission ν for 235U is 2.44, so the combined ν is ∼5.2 with one neutron removed by the fission reaction. This high effective ν significantly increases the multiplicity counting rates and reduces the statistical error. The background coincidence counts from the 252Cf have been minimized by neutron shielding between the source and the detector. This method of active neutron interrogation has been applied to the measurement of fresh pressurized water reactor (PWR) fuel assemblies. The neutron uranium collar (UNCL) that is routinely used for 235U verification in PWR reactor fuel assemblies is used to compare the TCIF method with the typically used AmLi neutron interrogation source. This paper presents both the experimental verification of the TCIF method for a PWR mockup assembly and the MCMPX simulations to optimize the detector geometry.