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

1. LA-UR-08-05602 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

2. 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

3. 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.

4. 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

5. 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.)

6. 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

7. 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

8. 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

9. New Experimental ProgramFIGARO @ 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

10. 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.

11. 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

12. 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 This analysis needs: Extending to include other dosimetry reactions Careful inclusion of other uncertainties, eg n2n cross section uncertainties Slide 12

13. 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

14. 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

15. Suggests a Deficiency in the ENDF/B-VII Modeled 14 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

16. 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 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

17. 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, … • Pre-equilibrium Neutronswith McGNASH reaction code Slide 17

18. 14MeVENDF/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

19. 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%

20. 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