A new method to measure the U-235 content in fresh LWR fuel assemblies via fast-neutron passive self-interrogation

Journal Article · Sat Oct 07 00:00:00 EDT 2017 · Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment

DOI:https://doi.org/10.1016/j.nima.2017.09.038· OSTI ID:1402652

Menlove, Howard Olsen ^[1]; Belian, Anthony P. ^[1]; Geist, William H.  ^[1]; Rael, Carlos D. ^[1]

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1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

The purpose of this paper is to provide a solution to a decades old safeguards problem in the verification of the fissile concentration in fresh light water reactor (LWR) fuel assemblies. The problem is that the burnable poison (e.g. Gd₂O₃) addition to the fuel rods decreases the active neutron assay for the fuel assemblies. This paper presents a new innovative method for the verification of the ²³⁵U linear mass density in fresh LEU fuel assemblies that is insensitive to the burnable poison content. The technique makes use of the ²³⁸U atoms in the fuel rods to self-interrogate the ²³⁵U mass. The innovation for the new approach is that the ²³⁸U spontaneous fission (SF) neutrons from the rods induces fission reactions (IF) in the ²³⁵U that are time correlated with the SF source neutrons. Thus, the coincidence gate counting rate benefits from both the nu-bar of the ²³⁸U SF (2.07) and the ²³⁵U IF (2.44) for a fraction of the IF reactions. Whereas, the ²³⁸U SF background has no time-correlation boost. The higher the detection efficiency, the higher the correlated boost because background neutron counts from the SF are being converted to signal doubles. This time-correlation in the IF signal increases signal/background ratio that provides a good precision for the net signal from the ²³⁵U mass. The hard neutron energy spectrum makes the technique insensitive to the burnable poison loading where a Cd or Gd liner on the detector walls is used to prevent thermal-neutron reflection back into the fuel assembly from the detector. Here, we have named the system the fast-neutron passive collar (FNPC).

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC52-06NA25396; NNSA/NA-241
• OSTI ID: 1402652
• Alternate ID(s): OSTI ID: 1495635
• Report Number(s): LA-UR-17-27908; TRN: US1703245
• Journal Information: Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment, Vol. 877, Issue C; ISSN 0168-9002
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
Fast neutrons
Reactor fuel assemblies
Passive neutron assay
Neutron collar