Distinguishing fissile uranium isotopes using an active well neutron coincidence counter

Journal Article · Wed Jan 08 00:00:00 EST 2025 · Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment

DOI:https://doi.org/10.1016/j.nima.2024.170176· OSTI ID:2538396

Reed, Richard L. ^[1]; Evans, Louise G. ^[1]; McElroy Jr., Robert D. ^[1]; Lockhart, Madeline L. ^[2]

---

1. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
2. Los Alamos National Laboratory (LANL), Los Alamos, NM (United States); North Carolina State University, Raleigh, NC (United States)

Proposed thorium-based nuclear fuel cycles are likely to require quantification and verification of ²³³U within nuclear material. Because of their similar fission cross sections, active neutron nondestructive assay (NDA) systems may respond similarly to ²³³U and ²³⁵U. Traditional safeguards equipment has been optimized for ²³⁵U and ²³⁸U quantification associated with conventional uranium/plutonium fuel cycles and may not be directly applicable to ²³³U quantification when mixed with other actinides. This work used models of the large volume active well coincidence counter (LV-AWCC) at Oak Ridge National Laboratory to evaluate the performance of this neutron NDA system to differentiate fissile uranium isotopes. The models were developed to simulate NDA system performance in response to a number of triangular radiation signature training device sources within the central cavity or well. This work predicted that the LV-AWCC can effectively differentiate ²³³U from ²³⁵U in certain modes of operation. In active mode, the LV-AWCC with the cadmium liner results in different doubles count rates between the fissile isotopes for a given fissile uranium mass. Without the cadmium liner, the uranium isotopes provide a statistically indistinguishable doubles count rate response for the fissile masses considered in this work (up to approximately 150 g). The cadmium liner serves to harden the neutron interrogation spectrum, which better exploits the notable difference in the ²³³U and ²³⁵U fission cross sections at approximately 1 eV. In passive mode, the two fissile isotopes exhibit different doubles and singles count rates regardless of liner presence because the passive source strength of ²³³U is approximately 2 orders of magnitude stronger than that of ²³⁵U due to the shorter half-life and correspondingly higher (α, n) yield. We conclude that using neutron interrogation in the LV-AWCC, two measurements are needed to quantify ²³³U content in mixed uranium items. The first measurement is used to determine the total fissile uranium mass using a mode that cannot distinguish fissile isotopes (i.e., where a similar response is observed for both fissile uranium isotopes such as active doubles without cadmium or using a thermal neutron interrogation source). In conclusion, the second measurement is used to determine the ²³³U content by using a differentiating technique (e.g., passive doubles, passive doubles to singles ratio, active doubles with cadmium).

• Research Organization: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA), Office of Defense Nuclear Nonproliferation
• Grant/Contract Number: AC05-00OR22725
• OSTI ID: 2538396
• Journal Information: Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment, Journal Name: Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment Vol. 1072; ISSN 0168-9002
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
Active well coincidence counter
Coincidence Counting
Nondestructive assay
Radiation signature training device
Uranium-233