Reflected Neutron Effects in Multiplicity Measurements of Bare HEU Assemblies

Conference · Sun Jul 11 00:00:00 EDT 2010

OSTI ID:983562

McConchie, Seth M. ^[1]; Hausladen, Paul ^[1]; Mihalczo, John T. ^[1]

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1. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

In a passive multiplicity characterization of highly enriched uranium (HEU) assemblies, fission chains are initiated by the characteristically fast neutrons from spontaneous fission of ²³⁸U and ²³⁵U as well as cosmic-ray spallation neutrons. Active interrogation of HEU uses other physical mechanisms for starting chains by inducing fission from high-energy neutrons, high-energy gamma-rays, delayed neutrons, or thermal neutrons. In all cases a contribution to the initiation of fission chains is the reflection of neutrons that initially escape the assembly and re-enter it after undergoing some scattering. The reflected neutron flux is geometry dependent and a combination of fast and thermal energies. The reflected thermal neutron contribution occurs hundreds of microseconds after the beginning of the fission chain and can be distinguished from the cosmic-ray spallation neutrons unrelated to fission chains, resulting in an HEU detection signature with high signal-to-noise. However, the reflected thermal neutron flux can be eliminated with an efficient thermal neutron absorber to investigate reflected neutron effects. In this paper, active and passive multiplicity measurements with HEU oxide assemblies of up to 16 kg of fuel pins and HEU metal assemblies of up to five 18 kg storage castings are reported. Each case demonstrates the differences in HEU signature when a borated thermal neutron absorber is present and shows the various detectable signatures with 3He proportional counters, the standard detector for differential die-way and neutron multiplicity measurements, and liquid scintillators, a detector capable of operating on the timescale of fission chains.

• Research Organization: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP)
• DOE Contract Number: AC05-00OR22725
• OSTI ID: 983562
• Resource Relation: Conference: INMM 51st Annual Meeting, Baltimore, MD (United States), 11-16 Jul 2010
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
73 NUCLEAR PHYSICS AND RADIATION PHYSICS
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
CASTINGS
DELAYED NEUTRONS
DETECTION
FAST NEUTRONS
FISSION
FUEL PINS
GEOMETRY
HIGHLY ENRICHED URANIUM
LIQUID SCINTILLATORS
MULTIPLICITY
NEUTRON FLUX
NEUTRONS
OXIDES
PROPORTIONAL COUNTERS
REFLECTION
SCATTERING
SPALLATION
SPONTANEOUS FISSION
STORAGE
THERMAL NEUTRONS
Nuclear Criticality Safety Program (NCSP)
Highly Enriched Uranium (HEU) Assemblies
Passive Multiplicity Characterization
Active Multiplicity Characterization
HEU Oxide Assemblies
HEU Signature Differences
3-He Proportional Counters