This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
• 4 Summary viewgraphs I presented at the IAEA/INDC meeting, on motivation for an international coordinated research effort on fission neutron spectra
• Aspects of fission neutron evaluations– Theory & Uncertainty Quantification– Experiments at LANL– Preequilibrium processes – (n,2n) dosimetry detectors - Bethe sphere testing &
critical assemblies
Evaluation of Precise Fission Neutron Spectra for Actinides
Motivation:• Accurate predictions of criticality are central to many applications - reactors and waste
transmutation technologies, nonproliferation, etc• Many labs have determined that current uncertainties in the fission spectrum
represent one of the biggest sources of uncertainty in k-eff predictions (esp. recent work at LANL, Argonne/INL, Japan, Europe …)
• A new collaboration could largely reduce the spectrum uncertainties.Other background information:• Dosimetry benchmark testing suggests ENDF/B-VII (Madland’s work) high-energy
spectrum too hard for 235U,239Pu in fast neutron energy region; too soft for thermal? (Mannhart) - see later viewgraphs
• ENDF community rejected Madland’s newest 235U at thermal because of poor performance in thermal benchmarks. We need to resolve this.
• For 239Pu and 235,8U, few precise measurements exist below ~ 1 MeV emission energy, and above ~ 7 MeV emission energy. We presently rely on old models that are calibrated to the few measured data.
Motivation: Jezebel keff: sensitivity to fission
spectrum (En,E’n) uncertainties in 239Pu
• Original (En,En’) matrix from ENDF/B-VII evaluation
1 deviations using Kawano’s and Livermore’s evaluated uncertainties
• PARTISN simulations of keff in Jezebel critical assembly, using all five files (including nominal)
Significant impact:
(-0.3%,+0.4%) using Kawano’s evaluated uncertainties, and (-0.35%,+0.5%) using LLNL’s.
Very big! This is one of our biggest
“levers” in predicting criticality
Also, rigorous chi covariance work done by Ishikawa et al. and ANL/Idaho groups, & Capote, Trkov, etc
Initial uncertainties in fission spectra (propagated relative to ENDF/B-VII)
LANL has nearly completed a new estimate
of these uncertainties (Talou et al.)
~7%
~20-30%
Possible Scope of a Future Int. Collaboration
• Goal - new evaluated fission spectrum data for major actinides, with reduce uncertainties, available for possible adoption by worldwide evaluated data projects
• Develop new theoretical methods for fission spectra. Build on historic approach, to use insights from recent advances in many Laboratories
• Utilzie new covariance data capabilities being established to best evaluate the new fission spectra and their uncertainties
• Utilize new measurements that may become available. e.g. Hampsch, Kornilov etc, and FIGARO/LANSCE
• Include significant validation benchmark testing– Criticality– Reaction rate dosimetry for (n,2n) and (n,p) detectors– Pulsed sphere transmission
LANL Fast Critical Assemblies, Einc~ 0.5-2 MeV(n,2n) Dosimetry Activations for Testing High Energy Tail of Fission Neutrons
• LANL’s fast critical assembly data (Einc ~ 1 MeV) analogous to Mannhart’s work for thermal 235U fission spectrum testing
• Data available for fast systems: Godiva (235U), Jezebel (239Pu) assemblies, + others (Flattops, etc)
• Measurements available for various (n,2n) dosimetry reactions, on: 89Y, 169Tm, 191Ir, 197Au, 90Zr, 238U, 58Ni, …
• Some preliminary testing of ENDF/B-VII (see Nucl.Data. Sheets 108, No 12, 2716 (2007) )- 239Pu (Jezebel): Tm and Ir suggest spectrum > 8 MeV ~ 10-33% too high
- 235U (Flattop-25): Tm and Ir suggest spectrum > 8 MeV ~ 4-25% too high
Slide 12
This analysis needs:a) Extending to include other dosimetry reactionsb) Careful inclusion of other uncertainties, eg n2n
cross section uncertainties
Assemblies with 14 MeV ( and fission multiplied ~ 1 MeV) Neutrons: Bethe Spheres - Reaction rates for (n,2n) Y, Tm, Ir, … dosimeters measured in the neutron flux
1970s experiments, that we simulate with MCNP using ENDF data• 14 MeV source surrounded by Li-D and by uranium• Complements our critical assembly data testing
Dosimetry n,2n testing: 14 MeV dominated fluences look good; But problems with assemblies involving Uranium
Results for cases with uranium appear too high
No uranium: look good to ~ 6%
Measure of the neutron energy spectrum
hardness at a particular location 14-MeV-like Fiss-spec-like
Suggests a Deficiency in the ENDF/B-VII Modeled 14 MeV Inelastic Scattering (Fission and/or Preeq)
n2n product = fluence * n,2n cross section.
Possible problem with 9-12 MeV fluence:-prompt fiss spec?-Preeq/inelastic?
Motivates future work on 235U 14 MeV induced Prompt-spectra and preeq/inelastic scattering
MCNP simulation of neutron spec. in LiD-U sphere
14 MeV 235U(n,xn) spec. in ENDF/B-VII
Assessment of ENDF/B-VII.0 Library @ 14 MeV. Preequilibrium Needs Including in Fission Spectrum.
• To date, preequilibrium neutron scattering component of fission spectrum has been ignored in US
Slide 16
New Work at LANL (Kawano):
• spectrum shape is modified• non-isotropic angular distribution• Impacts production of (n,2n) products
Schematicn,2n
238U + n at 14 MeV
Current & Future Work:Going beyond the Los Alamos model
• Monte Carlo simulation of the Fission Fragments Evaporation StageDetailed view of the processAssessment of specific physical quantities, e.g., P(n), correlations, …
14MeV ENDF/B-VII Modeling of Preequilibrium &Collective Inelastics - 2 Pieces of Information Guided Us
“Fundamental” differential dataMeasured data for 238U used to calibrate preequilibrium/collective model - then also used for 235U and 239Pu
“Integra transmission data”, LivermoreFor 238U 235U and 239Pu, used to validate preequilibrium/collective modeling
Preequilibrium & Inelastic Scattering: For ENDF/B-VII, Young & Chadwick Implemented a Phenomenological Model
Continuum preequilibrium model based on DWBA scattering to collective states in the continuum, inferred from 238U data of Baba. New spectrum measurements from LANSCE/GEANIE needed
Resulting spectrum is much harder than previous evaluations.Although the agreement with pulsed sphere data is “good”, present accuracy is possible no better than +/- 25%
Underestimate high emission energy data - collective states may be present in the target excitations, observed in other targets (208Pb and 90Zr). We are developing an RPA capability to model collective states
New Microscopic Calculation of Direct and Pre-equilibrium Neutron Emission: M. Dupuis, T. Kawano, L. Bonneau: More Work Needed
No “ad-hoc” adjustment Structure from HF calculations with a Skyrme effective interaction (reproduces g.s. properties of major stable nuclei). Effective interaction between the projectile and target nucleons: in-medium 2-body force from bare nucleon-nucleon interaction, reproduce scattering data.