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Lecture on Applications of the Monte Carlo Adjoint Shielding Methodology

Lecture on Applications of the Monte Carlo Adjoint Shielding Methodology. By Roger A. Rydin , University of Virginia, Consultant U.S. Army Craig R. Heimbach , formerly with Army Pulse Radiation Facility. Personnel. Rydin - University Expert, NGIC, VA

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Lecture on Applications of the Monte Carlo Adjoint Shielding Methodology

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  1. Lecture onApplications of the Monte Carlo Adjoint ShieldingMethodology By Roger A. Rydin, University of Virginia, Consultant U.S. Army Craig R. Heimbach, formerly with Army Pulse Radiation Facility

  2. Personnel • Rydin - University Expert, NGIC, VA Computational Studies of Military Vehicles and Structures • Heimbach – Experimentalist, APG, MD Neutron and Gamma Ray Spectroscopy • APRF, Crane-Mounted Bare Fast Reactor • WWD, Munster, Germany, Movable Fallout Simulator • ETBS, Bourges, France, Fallout Simulator

  3. Order of Talk • Generalities About Shielding Methodology • Available Computer Codes • Statement of Problem • Solution – Hybrid Method Called MASH • Examples Galore

  4. Comments on Mixed FieldNeutron-Gamma Ray Shielding • Shielding is an Art Requires Skilled Modeling • Shielding Requires Transport Theory Highly Anisotropic Cross Sections • Discrete Ordinates Sn Methods Large Distances In Regular Geometry • Monte Carlo Methods Short Distances In Detailed Geometry

  5. General Mixed FieldNeutron-Gamma Ray Shielding • Shield Neutrons With Light Materials Water, Plastic, Boron • Shield Gamma Rays With Heavy Materials Lead, Iron • Beware of Holes and Gaps !

  6. Shielding Codes • ORNL (Shielding) ANISN, DORT, TORT, Discrete Ordinates MORSE, Multi-group Monte Carlo • LANL (Weapons Design) TRIDENT, etc, Discrete Ordinates MCNP, Continuous Energy Monte Carlo • Cross Section Libraries, Quadratures Incompatible! (2 l +1) / 2 Factor

  7. Monte Carlo Codes • MORSE Volumetric Primitives - SPH, RPP, ARB, ARS, TRC, BOX, ELL, etc Boulean Combinatorial Geometry • MCNP Define Surfaces, Make Volumes Easy Replication, Restart Can’t Do Adjoint Problem

  8. Basic Question • How Do You Accurately Calculate the Dose Inside a Geometrically Complicated Shield a Large Distance from a Mixed Source of Neutrons and Gamma Rays ? • Discrete Ordinates Can’t Handle The Shield Geometry (Stair Steps ?) • Monte Carlo Can’t Handle the Distance or a Small Size Dose Receiver

  9. Air-Over Ground Problem • 2D Problem Covers 2+ Kilometers Large, Geometrically Increasing, Mesh Spaces in Air, Small Mesh in Ground • 42 Neutron, 17 Gamma Ray Groups Cover Inelastic Scattering • P6 Cross Sections Compton Scattering Anisotropy • S16 Forward – Biased Quadrature Set

  10. Adjoint Problem • Every Integro – Differential Equation Has a Dual, Adjoint or Importance Counterpart • Equations Are Connected Through an Integral Variational Principle Functional • They Have the Same Boundary Conditions • The Operators Are Obtainable By Transpositions, Role Reversals, and Energy Direction Reversal

  11. Solution - MASH Methodology • Transport from Source = Discrete Sn Calculation with DORT (2D) or TORT (3D) NoDistance and Geometry Limitations to Vicinity of Shield • Dose in Complicated Shield = Stochastic Calculation with MORSE in Adjoint Mode Shield Geometry Complexity, Orientation, and All Particles Start from Detector Volume • Couple Over a Surface Around Shield

  12. MASH Methodology • Implied – The Presence of the Shield Doesn’t Perturb the Discrete Ordinates Solution • If Untrue, Add a Dummy Shield • Rotation of the Shield Before Coupling Doesn’t Affect the Answer – Not True for Big Shields

  13. Theory • FLUX From Source Distribution • IMPORTANCE From Detector Response • L-Terms Cancel

  14. Need Flux at Detector or Importance at Source Or Flux and Importance at a Coupling Surface Dose Calculation

  15. Definitions • Neutron Reduction Factor NRF NeutronDose Outside (Gray) / Dose Inside Shield • Gamma Reduction Factor GRF Gamma Dose Outside (Gray) / Dose Inside Shield • Fallout Protection Factor FPF Fallout Gamma Dose Outside (Gray) / Dose Inside Shield

  16. Further Definitions • Neutron Protection Factor NPF NeutronDose Outside (Gray) / N and γ Dose Inside Shield Caused by Neutron Source • Gamma Protection Factor GPF Gamma Dose Outside (Gray) / γ Dose Inside Shield Caused by γ Source

  17. Applications • Boxes Near a Prompt Source • Vehicles Near a Prompt Source • BNCT Medical Therapy Room Design • Tank on a Fallout Field • Small Concrete Building • Foxhole • Buildings in an Urban Environment

  18. Verification of Methodology for Simple Geometries • 1 Meter Box, Rotated, With Holes and Gaps • 2 Meter Box ORNL Calculation • RTK Angled Box From WWD

  19. Detectors • ROSPEC – 4 Spherical Proportional Counters, Unfolding • DOSPEC – Dose – Calibrated NaI • Calibrated GM Tubes • TE Ion Chambers International Intercalibration Effort – US, UK, Germany, France, Canada

  20. Small Lined Iron Box

  21. Small Lined Iron Box • Unlined, Polyethylene Liner, Boron Polyethylene Liner • 200 Meters From APRF • Calibrated GM Tubes, Tissue Equivalent Dosimeters Learned The Value of Source Energy Biasing Start More Particles That Give High Dose

  22. Medical Therapy Room

  23. Medical Therapy Room • Dummy Head in DORT Problem Gives Scattering Source to Walls • Conclusions • Doesn’t Make Much Difference If Patient Is Prone In Beam, Seated Out Of Beam, Or Shadow Shielded • Dose To Rest Of Body Comes Through the Neck !

  24. T72 Russian Tank Model >10000 Primitive Bodies: ARS Arbitrary Surfaces; ARB Arbitrary Polyhedrons; etc. >6000 Material Regions by Combinatorial Geometry

  25. T72 Russian Tank Model • The Model Came From BRL CAD – CAM • Required Graphical Debugging – ORGBUG • Required Tolerance Debugging Lost Particles ! • Required a MORSE Modification !

  26. Fallout Field at Bourges, FranceUsing La-140 • 80 by 80 Meter Dirt Field • At Corner, Rotated ~ 160 by 160 Meters • 30 by 30 Meter Concrete Pad • At Corner, Rotated ~ 60 by 60 Meters

  27. Experiment vs. Calculation • Fallout simulated with Fission Products • Fallout Simulated with La-140 • Comparison to ORNL Calculations

  28. FPF Comparisons

  29. Observations • Strong Variation, Seat to Head • Concrete FPF >Dirt , in General • Conc. vs. Dirt Difference, Probably Real • Calculation ~in Middle • Agreement Generally Within Error Bars • Fallout Protection is Significant

  30. FPF Comparison, ORNL

  31. General Conclusions for T 72 • Fallout Protection Factor ~ 40 • Driver Less Well Protected ~ 15 • Some Differences for Source Type • Some Differences for Model Maker • Typical Accuracy, ~ 15 – 20 %

  32. Concrete Building Photo

  33. Concrete Building Model

  34. Concrete Building, Neutrons

  35. Concrete Building, Gammas

  36. Concrete Building Conclusions • Reasonably Good Neutron Protection ~ 3 • Fair Prompt Gamma Protection ~ 3.5 • Good Fallout Protection ~ 9 Stay Away From Doors and Windows

  37. Foxhole Model

  38. Foxhole Protection Factors

  39. Foxhole Conclusions • Reasonably Good Neutron Protection ~ 3 • Fair Prompt Gamma Protection ~ 2 • Good Fallout Protection ~ 12 Keep Head Down and Stay Inside

  40. Tall Buildings

  41. Buildings in an Urban Environment

  42. Large Buildings • We Can Make a Geometry Model • But - New Problem, Not Yet Solved ! • NoExperimental Data ! • TORT Had Computational Limits for 10 Story Building! • MASH Coupling Over Large Surface ?

  43. Large Buildings, cont. • Alternate Method, QAD Point Kernel Gamma Code • QAD Uses MASH Model • Chinese Building Study near Reactor • QAD Point Kernel Buildup Factors ? • Effect of Extended Shadowed Source ?

  44. Conclusions • MASH Works Very Well for Small Shields • C/E Typically 10 – 20 % • Large Buildings Represent an Unsolved Problem • More Research Needed

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