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Lecture on Applications of the Monte Carlo Adjoint Shielding Methodology

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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

- 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

- Generalities About Shielding Methodology
- Available Computer Codes
- Statement of Problem
- Solution – Hybrid Method Called MASH
- Examples Galore

- 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

- Shield Neutrons With Light Materials
Water, Plastic, Boron

- Shield Gamma Rays With Heavy Materials
Lead, Iron

- Beware of
Holes and Gaps !

- 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

- 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

- 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

- 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

- 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

- 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

- 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

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

Need Flux at Detector or Importance at Source

Or Flux and Importance at a Coupling Surface

- 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

- 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

- 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

- 1 Meter Box, Rotated, With Holes and Gaps
- 2 Meter Box ORNL Calculation
- RTK Angled Box From WWD

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

- 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

- 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 !

>10000 Primitive Bodies:

ARS Arbitrary Surfaces;

ARB Arbitrary Polyhedrons; etc.

>6000 Material Regions by Combinatorial Geometry

- The Model Came From BRL CAD – CAM
- Required Graphical Debugging – ORGBUG
- Required Tolerance Debugging
Lost Particles !

- Required a MORSE Modification !

- 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

- Fallout simulated with Fission Products
- Fallout Simulated with La-140
- Comparison to ORNL Calculations

- 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

- Fallout Protection Factor ~ 40
- Driver Less Well Protected ~ 15
- Some Differences for Source Type
- Some Differences for Model Maker
- Typical Accuracy, ~ 15 – 20 %

- Reasonably Good Neutron Protection ~ 3
- Fair Prompt Gamma Protection ~ 3.5
- Good Fallout Protection ~ 9
Stay Away From Doors and Windows

- Reasonably Good Neutron Protection ~ 3
- Fair Prompt Gamma Protection ~ 2
- Good Fallout Protection ~ 12
Keep Head Down and Stay Inside

- 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 ?

- 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 ?

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