Derivation of the Two-Exponential Probability Density Function for Rossi-Alpha Measurements of Reflected Assemblies and Validation for the Special Case of Shielded Measurements

Hua, Michael Y.; Hutchinson, Jesson D.; McKenzie, George; Shin, Tony H.; Clarke, Shaun D.; Pozzi, Sara A.

doi:10.1080/00295639.2019.1654327

Cited by 14 publications

(6 citation statements)

References 8 publications

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“…Traditionally, the histogram is fit with a single-exponential-plus-constant model [4]. Recent work has developed a double-exponential-plus-constant-model [5] that is more suitable than the single exponential model for measurements of reflected assemblies. The two-exponential model estimates a second parameter: ctd , which describes the mean time a neutron spends in the reflector prior to detection.…”

Section: The Rossi-alpha Methodsmentioning

confidence: 99%

Rossi-alpha measurements of fast plutonium metal assemblies using organic scintillators

Hua

Bravo

MacDonald

et al. 2020

Nuclear Instruments and Methods in Physics Research Section A:

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In this work, Rossi-alpha measurements were simultaneously performed with a 3 He-based detection system and an organic scintillator-based detection system. The assembly is 15 kg of plutonium (93 wt% 239 Pu) reflected by copper and moderated by lead. The goal of Rossi-alpha measurements is to estimate the prompt neutron decay constant, alpha. Simulations estimate k eff = 0.624 and α = 52.3 ± 2.5 ns for the measured assembly. The organic scintillator system estimated α = 47.4 ± 2.0 ns, having a 9.37% error (though the 1.09 standard deviation confidence intervals overlapped). The 3 He system estimated α = 37 µs. The known slowing down time of the 3 He system is 35-40 µs, which means the slowing down time dominates and obscures the prompt neutron decay constant. Subsequently, the organic scintillator system should be used for assemblies with alpha much less than 35 µs.

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Section: The Rossi-alpha Methodsmentioning

confidence: 99%

Rossi-alpha measurements of fast plutonium metal assemblies using organic scintillators

Hua

Bravo

MacDonald

et al. 2020

Nuclear Instruments and Methods in Physics Research Section A:

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“…The method is based on the nonrandom distribution between neutron detection times: the population of correlated prompt neutrons (those from the same fission chain seed) decays exponentially as fission chains extinguish [15]. Experimental results showed that the decay is modeled better by a two-exponential function than a one-exponential function when the assembly of fissile material is reflected [16,6], the model was theoretically derived from two-region point kinetics [17,18,11], and validated with a combination of experiments and simulations [12,19]. The two-exponential Rossi-alpha probability density function for reflected assemblies p R (τ) is given by…”

Section: Rossi-alpha Methodsmentioning

confidence: 99%

“…The first order uncertainty in α is determined by propagating the uncertainty in the fit parameters through Eqn. (11) via…”

Section: Uncertainty Propagation From Fit Parameters To αmentioning

confidence: 99%

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On the Feynman-alpha Method for Reflected Fissile Assemblies

Hua¹,

Hutchinson²,

McKenzie³

et al. 2020

Preprint

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The Feynman-alpha method is a neutron noise technique that is used to estimate the prompt neutron period of fissile assemblies. The method and quantity are of widespread interest including in applications such as nuclear criticality safety, safeguards and nonproliferation, and stockpile stewardship; the prompt neutron period may also be used to infer the k eff multiplication factor. The Feynman-alpha method is predicated on time-correlated neutron detections that deviate from a Poisson random variable due to multiplication. Traditionally, such measurements are diagnosed with one-region point kinetics, but two-region models are required when the fissile assembly is reflected. This paper presents a derivation of the two-region point kinetics Feynman-alpha equations based on a double integration of the Rossi-alpha equations, develops novel propagation of measurement uncertainty, and validates the theory. Validation is achieved with organic scintillator measurements of weapons-grade plutonium reflected by various amounts of copper to achieve k eff values of 0.83-0.94 and prompt periods of 5-75 ns. The results demonstrate that Feynman-alpha measurements should use the two-region model instead of the one-region model. The simplified one-region model deviates from the validated two-region models by as much as 10% in the estimate of the prompt neutron period, and the two-region model reduces to the one-region model for small amounts of reflector. The Feynman-alpha estimates of the prompt neutron period are compared to those of the Rossi-alpha approach. The comparative results demonstrate that the Feynman-alpha method is more precise than the Rossi-alpha method and more accurate for k eff < 0.92, whereas the Rossi-alpha method is generally more accurate for higher multiplications. The uncertainty propagation developed in this work should be used for all Feynman-alpha measurements and will therein improve fitting accuracy and appropriate precision estimates.

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“…The scintillator systems are primarily used for gamma-ray die-off analysis 107 while the neutron detectors utilize neutron noise methods. 108,109 As shown in Fig. 19c, some measurements were performed outside the DAF.…”

Section: Ivd Neutron Diagnosed Subcritical Experimentsmentioning

confidence: 99%

A New Era of Nuclear Criticality Experiments: The First 10 Years of Radiation Test Object Operations at NCERC

Hutchinson

Bounds

Cutler

et al. 2021

Nuclear Science and Engineering

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The work presented in this paper focuses on the first 10 years (2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020) of radiation test object (RTO) operations at the National Criticality Experiments Research Center. RTOs are subcritical configurations of special nuclear material that are built by hand. These configurations are utilized for benchmark experiments, detector testing/characterization, and training. An overview of the types of measurements used in RTO operations is given as well as a history of RTO operations at Los Alamos National Laboratory from 1944-2011.

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Derivation of the Two-Exponential Probability Density Function for Rossi-Alpha Measurements of Reflected Assemblies and Validation for the Special Case of Shielded Measurements

Cited by 14 publications

References 8 publications

Rossi-alpha measurements of fast plutonium metal assemblies using organic scintillators

Rossi-alpha measurements of fast plutonium metal assemblies using organic scintillators

On the Feynman-alpha Method for Reflected Fissile Assemblies

A New Era of Nuclear Criticality Experiments: The First 10 Years of Radiation Test Object Operations at NCERC

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