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Monte Carlo methods in ADS experiments

Monte Carlo methods in ADS experiments. Study for state exam 2008 Mitja Majerle. “Phasotron” and “Energy Plus Transmutation” setups (schematic drawings). What are Accelerator Driven Systems ?. Accelerator Driven Systems. Subcritical reactor

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Monte Carlo methods in ADS experiments

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  1. Monte Carlo methods in ADS experiments Study for state exam 2008 Mitja Majerle “Phasotron” and “Energy Plus Transmutation” setups (schematic drawings)

  2. What are Accelerator Driven Systems ?

  3. Accelerator Driven Systems • Subcritical reactor • Wider choice for reactor fuel (238U, 232Th) • Transmutation of nuclear waste • Increased safety • Accelerator • Protons or light ions of energy around 1 GeV • Very high powers (hundreds of MW) • Stable beam • Spallation reaction

  4. Spallation(process in which hadrons or light ions are ejected from the nucleus due to impact of relativistic particle) • In textbooks divided to two phases: • Intra Nuclear Cascade • De-excitation (evaporation, fission) • INC phase has to be corrected at lower energies to fit the experiments – preequilibrium phase

  5. Calculations • Monte Carlo method: • Event by event simulation on nuclear scale • Accuracy a sqrt(N) a sqrt(calculation time), • Parallelization is possible (MPI, PBS) • Used codes: MCNPX, FLUKA • Resources: • CESNET Meta Centrum • 56 processors on OJS (MPI and PBS)

  6. How MC code works • Pseudorandom numbers - x • Example - photon source in matter: • Determine track (x for direction) • Determine where the reaction will happen - l=1/S ln(x) • Determine which interaction will happen (photoeffect, Compton, pair production …) - x • Determine angles – x, energies of generated particles (physics) • Loop until particles escape from the phase space of interest • Tallying: • Count the particles that passed through selected phase space (surfaces, volumes, deposited energy …) • Precision and accuracy: • Wrong physics => wrong results • Central limit theorem – for large N, results distribution approach normal distribution

  7. Phasotron experiment • Simple, lead target (diameter 10 cm, length 0.5 m) • Intensive protons 660 MeV, 10 min • Activation detectors, iodine samples (129I)

  8. Simulated neutron spectrum

  9. NAA experimental data

  10. Exp/sim comparisons MCNPX – CEM03 MCNPX - INCL4/ABLA FLUKA

  11. Proton influence Proton fluence in the target central plane – around 30th cm protons are scattered out of the target Contribution of proton reactions to total reaction rate

  12. Iodine samples Sample 1 – 9th cm Sample 2 – 21st cm

  13. Systematic uncertainties • Simulations with changed parameters are compared • Beam parameters (3 mm ~ 15% uncertainty) • Detector displacement (1 mm ~ <5%) • Worse situation: • Around 30th cm, protons exit the target, beam parameters have bigger influence • Iodine samples – not precise placement

  14. Energy Plus Transmutation

  15. SABRINA plot from MCNPX input file, provided by J. Šolc.

  16. MC analysis • Influence of setup parts • polyethylene box • iron and detectors have negligible influence • Systematic uncertainties – displacements: • incident beam : 3mm = 15-20% uncertainty • detectors displacement: 5mm = 20% • The calculations apply only to threshold activation detectors.

  17. Polyethylene box –biological shielding Neutron spectra emitted to environment Neutron spectra inside box

  18. Neutron production, keff • At 1.5 GeV experiment, 50 neutrons were produced per proton • Maximum production (proton-1 GeV-1) in 1-1.5 GeV range • keff=0.202 • flooded with water keff rises to 0.41 (and heavy water 0.26) Total neutron production with the EPT setup.

  19. MC vs. experiment • Good agreement below 1.25 AGeV • Wrong predictions at 1.5 and 2 GeV • Similar behavior reported for thin target experiments – could there be any connection ? FLUKA calculations of reaction rates in Au detectors placed in the first gap

  20. Conclusion • Phasotron experiment • Completely analyzed • Good agreement with MC codes • Uncertainties ca. 15% • EPT • All performed experiments (4x protons, 2x deuterons) were simulated • Significant disagreement with MC codes at higher energies • Uncertainties 30% • Outlooks • MC in spectrometry – revision of spectrometry methods used at our work (precision of different corrections: geometrical, COI …) • Is EPT disagreement connected with thin target experiments ? Acknowledgments to META Centrum, where most of presented calculations were performed.

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