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Actinide ENDF/B-VII Cross Section Evaluations & Validation Testing : Precise Fission SpectraPowerPoint Presentation

Actinide ENDF/B-VII Cross Section Evaluations & Validation Testing : Precise Fission Spectra

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### Actinide ENDF/B-VII Cross Section Evaluations & Validation Testing :Precise Fission Spectra

Mark Chadwick, T. Kawano, P. Talou

Deputy Theoretical Division Leader, LANL

Talk at IAEA Consultant’s Meeting on

Prompt Fission Spectra,

November 22, 2008

Overview

- 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)

Also, rigorous chi covariance work done by Ishikawa et al. and ANL/Idaho groups, & Capote, Trkov, etc

- 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

Initial uncertainties in fission spectra (propagated relative to ENDF/B-VII)

~7%

~20-30%

LANL has nearly completed a new estimate of these uncertainties (Talou et al.)

Possible Scope of a Future Int. Collaboration relative to ENDF/B-VII)

- 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 T-16 Nuclear Reaction Codes Underpin our ENDF/B Evaluations

- GNASH - reactions on actinides & medium mass nuclides
- Hauser-Feshbach, preequilibrium, & fission modeling
- McGNASH = modern version

- CoH suite of codes
- Another Hauser-Feshbach code, with width fluctuation treatment, and gamma-ray capture formalism using direct-semidirect theory
- Implements Kerman’s KKM theory for deformed nuclei

- Moller structure codes: fiss. barriers, g.s. masses, Q-values, & deformations.
- Preequilibrium codes
- FKK, NWY quantum treatments with RPA collectivity
- Semiclassical HMS and exciton options

- EDA for light nucleus reactions (R-matrix theory)
- NJOY - data processing for application codes (+kerma, damage, scattering kernels, doppler, covariances in the ERRORJ module)

Then LANL transport codes MCNP & PARTISN use the ENDF/B data

New Work on Predicting Prompt Fission Neutrons Spectrum & Uncertainties with Kalman Code

We have written a modern code implementing the Los Alamos model (Madland & Nix, basis for ENDF/B-VII evaluations)

We’re refining the physics

- Uncertainty Quantification:
PFNS Model constraints + Experiment + KALMAN

Slide 8

New Experimental Program Uncertainties with Kalman CodeFIGARO @ LANSCE (Haight, Noda)

Neutrons spallation source at WNR/LANSCE

FIGARO array of 20 liquid scintillators

Incident neutron energies:En=1 to 200 MeV

Preliminary data from LANSCE/WNR

Precise fission results from LANL-LLNL-CEA program in fission,

experiment & theory, expected in 2012-2015 timeframe

Slide 9

Program of fission neutron output measurements continues – Haight presentation at APS/DN ’08

- Reduce background from accidental coincidences
- Came from neutron scattering on backing foils – 0.12 mm Pt
- Presently we are using a much better chamber

- Measure fission neutrons below 1 MeV
- Need better n-gamma discrimination
- A 0.5 MeV n+235U expt planned, but requires much beam time

- Measure fission neutrons better above 8 MeV
- Better timing on fission chamber (LLNL-LANL collaboration)
- More efficient neutron detectors (larger solid angle for detection)

- Quantify uncertainties better ENDF
- More isotopes – 235U, 239Pu, 238U, 237Np, 240-244Pu, etc.

Next Viewgraphs: High Emission Energy Tails of Spectra: Fission & Inelas./Preequilibrium Scattering

1 MeV n+ 239Pu: Fission Spectrum

14 MeV n+ 239Pu: All outgoing neutrons

- Impacts:
- Criticality (though the region below 1 MeV ~ more important)
- (n,2n) detector response – in LANLCrit assemblies, Bethe spheres
- Neutron leakage, e.g. Livermore pulsed spheres

LANL Fast Critical Assemblies, E Fission & Inelas./Preequilibrium Scatteringinc~ 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

This analysis needs:

Extending to include other dosimetry reactions

Careful inclusion of other uncertainties, eg n2n cross section uncertainties

Slide 12

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

Fiss-spec

-like

14-MeV-like

Suggests a Deficiency in the ENDF/B-VII Modeled But problems with assemblies involving Uranium14 MeV Inelastic Scattering (Fission and/or Preeq)

MCNP simulation of neutron spec. in LiD-U sphere

- n2n product = fluence * n,2n cross section.
- Possible problem with 9-12 MeV fluence:
- prompt fiss spec?
- Preeq/inelastic?

14 MeV 235U(n,xn) spec. in ENDF/B-VII

Motivates future work on 235U 14 MeV induced

Prompt-spectra and preeq/inelastic scattering

Assessment of ENDF/B-VII.0 Library But problems with assemblies involving [email protected] 14 MeV. Preequilibrium Needs Including in Fission Spectrum.

To date, preequilibrium neutron scattering component of fission spectrum has been ignored in US

New Work at LANL (Kawano):

238U + n at 14 MeV

Schematic

n,2n

- spectrum shape is modified
- non-isotropic angular distribution
- Impacts production of (n,2n) products

Slide 16

Current & Future Work: But problems with assemblies involving UraniumGoing 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, …

- Pre-equilibrium Neutronswith McGNASH reaction code

Slide 17

14MeV But problems with assemblies involving UraniumENDF/B-VII Modeling of Preequilibrium &Collective Inelastics - 2 Pieces of Information Guided Us

“Fundamental” differential data

Measured data for 238U used to calibrate preequilibrium/collective model - then also used for 235U and 239Pu

“Integra transmission data”, Livermore

For 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%

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.

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

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