An Integrated Delayed-neutron Differential-Die-away Instrument to Quantify Plutonium in Spent Nuclear Fuel

This paper evaluates the use of a neutron generator source to interrogatespent nuclear fuel to measure the plutonium content.The differential die-away (DDA) technique preferentially measures239Pu relative to 235U while a delayed neutron (DN) measurementis complementary in that it provides a unique signaturemore sensitive to 235U. The first section of this paper discussesthe integration of a 14 MeV deuterium-tritium (DT) neutrongenerator with a system of neutron detectors to measure both delayedand prompt neutron (PN) signals in the same instrument.An evaluation of the design’s ability to measure the DN for awide range of virtual spent fuel assemblies (SFAs) is provided.Both DN and PN detections are active techniques that measurethe signal emitted most prominently from the fissile isotopes of235U, 239Pu and 241Pu. The application of both the DN and PNsignals will be evaluated for the potential discrimination of the235U and 239Pu components in the fuel assemblies. This paperbuilds on previous conceptual design studies which have helpedguide the present prototypical instrument design. The MonteCarlo N-Particles eXtended (MCNPX) radiation transport code(v27c) was used to calculate the interrogation and detector response.This paper quantifies the capability of a new design usingan array of 3He detectors to reasonably optimize both DN andPN detection. This new design was created to minimize the (DT)direct fission in 238U. Attention was given to determining suitabletime-interrogation patterns for both DN and PN, and the newdesign also achieved near homogeneous spatial responses in thefuel for both delayed and prompt neutron assays, essential forthe detection of pin diversion in spent nuclear fuel. All was donewithin the constraints of a single practical instrument to reducethe instrument cost.