1. 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 LA-UR-08-05602

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 k eff : sensitivity to fission spectrum  (E n,E’ n ) uncertainties in 239 Pu Original  (E n,E n ’) matrix from ENDF/B-VII evaluation  1  deviations using Kawano’s and Livermore’s evaluated uncertainties PARTISN simulations of k eff 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

5. 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%

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. Slide 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

9. New Experimental Program FIGARO @ LANSCE (Haight, Noda) Neutrons spallation source at WNR/LANSCE FIGARO array of 20 liquid scintillators Incident neutron energies: E n =1 to 200 MeV Slide 9 Precise fission results from LANL-LLNL-CEA program in fission, experiment & theory, expected in 2012-2015 timeframe Preliminary data from LANSCE/WNR

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 – 235 U, 239 Pu, 238 U, 237 Np, 240-244 Pu, etc.

11. Next Viewgraphs: High Emission Energy Tails of Spectra: Fission & Inelas./Preequilibrium Scattering Impacts: Criticality (though the region below 1 MeV ~ more important) (n,2n) detector response – in LANL Crit assemblies, Bethe spheres Neutron leakage, e.g. Livermore pulsed spheres 1 MeV n+ 239 Pu: Fission Spectrum14 MeV n+ 239 Pu: All outgoing neutrons

12. LANL Fast Critical Assemblies, E inc ~ 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 reactions b)Careful inclusion of other uncertainties, eg n2n cross section uncertainties

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 14-MeV-like Fiss-spec -like

15. 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 235 U 14 MeV induced Prompt-spectra and preeq/inelastic scattering MCNP simulation of neutron spec. in LiD-U sphere 14 MeV 235 U(n,xn) spec. in ENDF/B-VII

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 Slide 16 New Work at LANL (Kawano): spectrum shape is modified non-isotropic angular distribution Impacts production of (n,2n) products Schematic n,2n 238 U + n at 14 MeV

17. Current & Future Work: Going beyond the Los Alamos model Monte Carlo simulation of the Fission Fragments Evaporation Stage Detailed view of the process Assessment of specific physical quantities, e.g., P(n), correlations, … Slide 17 Pre-equilibrium Neutrons with McGNASH reaction code

18. 14MeV ENDF/B-VII Modeling of Preequilibrium &Collective Inelastics - 2 Pieces of Information Guided Us “Fundamental” differential data Measured data for 238 U used to calibrate preequilibrium/collective model - then also used for 235 U and 239 Pu “Integra transmission data”, Livermore For 238 U 235 U and 239 Pu, 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 238 U 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. Underestimate high emission energy data - collective states may be present in the target excitations, observed in other targets ( 208 Pb and 90 Zr). 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.