Event-by-Event Evaluation of the Prompt Fission Neutron Spectrum From ²³⁹Pu(n,f)

Conference · Mon Oct 17 04:00:00 EDT 2011

OSTI ID:1033732

Vogt, R. ^[1]; Randrup, J. ^[2]; Brown, D. A. ^[1]; Descalle, M. A. ^[1]; Ormand, W. E. ^[1]

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1. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
2. Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)

We have developed an improved evaluation method for the spectrum of neutrons emitted in fission of ²³⁹Pu induced by incident neutrons with energies up to 20 MeV. The <v> covariance data, including incident energy correlations introduced by the <v> evaluation method, were used to fix the input parameters in our event-by-event model of fission, FREYA, by applying formal statistical methods. Formal estimates of uncertainties in the evaluation were developed by randomly sampling model inputs and calculating likelihood functions based on agreement with the evaluated <v>. Our approach is able to employ a greater variety of fission measurements than the relatively coarse spectral data alone. It also allows the study of numerous fission observables for more accurate model validation. The combination of an event-by-event Monte Carlo fission model with a statistical-likelihood analysis is thus a powerful tool for evaluation of fission-neutron data. Our empirical model FREYA follows the complete fission event from birth of the excited fragments through their decay via neutron emission until the fragment excitation energy is below the neutron separation energy when neutron emission can no longer occur. The most recent version of FREYA incorporates pre-equilibrium neutron emission, the emission of the first neutron before equilibrium is reached in the compound nucleus, and multi-chance fission, neutron evaporation prior to fission when the incident neutron energy is above the neutron separation energy. Energy, momentum, charge and mass number are conserved throughout the fission process. The best available values of fragment masses and total kinetic energies are used as inputs to FREYA. We fit three parameters that are not well under control from previous measurements: the shift in the total fragment kinetic energy; the energy scale of the asymptotic level density parameter, controlling the fragment 'temperature' for neutron evaporation; and the relative excitation of the light and heavy fragments, governing the number and energy of neutrons emitted from each fragment. The latter two parameters are assumed to be independent of the incident neutron energy while the first varies with incident energy. We describe our method and the subsequent spectral evaluation and present the results of several standard validation calculations that test our new evaluation. These benchmarks include critical assemblies, sensitive to criticality in fast systems; pulsed sphere measurements testing the spectra at incident neutron energies of 14 MeV; and other tests.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP); National Science Foundation (NSF)
• DOE Contract Number: AC52-07NA27344; AC02-05CH11231
• OSTI ID: 1033732
• Report Number(s): LLNL--CONF-517471
• Country of Publication: United States
• Language: English

Related Subjects

22 GENERAL STUDIES OF NUCLEAR REACTORS
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
73 NUCLEAR PHYSICS AND RADIATION PHYSICS
BENCHMARKS
CRITICALITY
DECAY
ENERGY-LEVEL DENSITY
EVALUATION
EXCITATION
FISSION
FISSION NEUTRONS
KINETIC ENERGY
KINETICS
MASS NUMBER
NEUTRON EMISSION
NEUTRON SEPARATION ENERGY
NEUTRONS
Nuclear Criticality Safety Program (NCSP)
SAMPLING
SPECTRA
TESTING
VALIDATION
ZERO POWER REACTORS