Lecture on Applications of the Monte Carlo Adjoint Shielding Methodology

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
• Cant 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 Cant Handle The Shield
  Geometry (Stair Steps ?)
• Monte Carlo Cant 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)
• No Distance 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 Doesnt
  Perturb the Discrete Ordinates Solution
• If Untrue, Add a Dummy Shield
• Rotation of the Shield Before Coupling Doesnt
  Affect the Answer Not True for Big Shields

13
Theory

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

14
Dose Calculation

• Need Flux at Detector or Importance
  at Source

• Or Flux and Importance at a Coupling Surface

15
Definitions

• Neutron Reduction Factor NRF
• Neutron Dose 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
• Neutron Dose Outside (Gray) / N and ? Dose Inside
  Shield Caused by Neutron Source
• Gamma Protection Factor GPF
• Gamma Dose Outside (Gray) / ? Dose Inside Shield
  Caused by ? 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
• Doesnt 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 !
• No Experimental 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