Monte Carlo N-Particle Simulations for Nuclear Detection and Safeguards: An Examples-Based Guide for

Table of contents:

Contents .......................................................................................................................................... 4

Introduction ..................................................................................................................................... 8

1.1 Geometry ............................................................................................................................... 8

1.1.1 Simplest possible input file............................................................................................. 8

1.1.3 Simple Input File .......................................................................................................... 12

1.1.4 Running and plotting MCNP geometries ..................................................................... 14

1.1.5 Surfaces and Complicated Cells: Intersections and Unions ......................................... 17

1.1.6 Duplicate Cells, Compliments and Translations: LIKE BUT and TRCL .................... 22

1.1.7 Filled Cells: Universes.................................................................................................. 26

1.1.8 Lattice Geometries ........................................................................................................ 32

1.1.9 Fully Specified Lattice Geometries .............................................................................. 40

1.2 Materials and Cross Sections .............................................................................................. 45

1.2.1 Specifying Materials ..................................................................................................... 45

1.2.2 Neutron cross sections .................................................................................................. 46

1.2.3 Low-energy neutron problems - thermal free gas treatment ........................................ 49

1.2.4 Low-energy neutron problem data - S( , ) thermal treatment .................................... 50

1.2.5 Photon cross sections .................................................................................................... 53

1.2.6 Electron stopping powers for coupled photon and electron problems ......................... 57

1.2.7 Data and models for Ions and Charged Particles .......................................................... 61

1.2.8 Additional data diagnostics and recommendations ...................................................... 62

1.3 Sources ................................................................................................................................ 63

1.3.1 SDEF Fixed Sources ..................................................................................................... 63

1.3.2 SDEF Source Distributions .......................................................................................... 66

1.3.3 SDEF Dependent Distributions: DS ............................................................................. 69

1.3.4 Criticality Sources ........................................................................................................ 71

1.3.5 Surface Source Read and Write (SSR, SSW) ......................................................... 77

1.3.6 Checking sources .................................................................................................... 87

1.4 Output and Tallies ............................................................................................................... 87

1.4.1 Output Files .................................................................................................................. 87

1.4.2 MCNP Estimators and Tally Types .............................................................................. 93

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1.4.3 Basic Tally Format ....................................................................................................... 95

1.4.4 Special Tally Treatments ............................................................................................ 111

1.4.5 Pulse-Height Tallies ................................................................................................... 128

1.4.6 Point Detectors and Next-Event Estimators ............................................................... 134

1.5 Plotting .............................................................................................................................. 143

1.5.1 Geometry Plotting and Command Files ..................................................................... 143

1.5.2 Cross Section plotting ................................................................................................. 145

1.5.3 Tally Plotting .............................................................................................................. 150

1.5.4 Mesh, Radiography, and Ring Tallies ........................................................................ 167

1.6 Statistics and Convergence ................................................................................................ 186

Section 2: Examples for nuclear safeguards applications ........................................................... 198

2.1 Example 1: Fuel Assembly in Water Tank ....................................................................... 198

2.1.1 Description.................................................................................................................. 198

2.1.2 Geometry description ................................................................................................. 203

2.1.3 Other data: sources, materials, tallies, and more ........................................................ 204

2.1.4 MCNP Output ............................................................................................................. 206

2.2 Example 2: Coincidence Counter with F4 and F8 Tallies for Coincidence and Multiplicity Counting Rates ........................................................................................................................ 207

2.2.1 Description.................................................................................................................. 207

2.2.2 Materials ..................................................................................................................... 212

2.2.3 Source ......................................................................................................................... 212

2.2.4 Tallies ......................................................................................................................... 213

2.2.5 Warning Messages ...................................................................................................... 214

2.2.6 Results ........................................................................................................................ 216

2.2.6A: From the Point Model ............................................................................................. 216

2.2.6B: Rates Calculated without Point Model assumptions............................................... 221

2.3 Gamma pulse height (to be completed) ................................................................................ 227

2.4 Active Neutron Example: Cf Shuffler ............................................................................... 231

2.4.1 Description and input file ........................................................................................... 231

2.4.2 Results ........................................................................................................................ 234

Section 3: Examples of Advanced Concepts .............................................................................. 240

Section 3.1 Variance Reduction .............................................................................................. 240

3.1.1 Introduction ................................................................................................................ 240

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3.1.2 Multigroup Weight Windows and Time Splitting: Lead Slowing Down Spectrometer ............................................................................................................................................. 242

3.1.2.1 Input File Notes ....................................................................................................... 253

3.1.2.2 Variance Reduction Step 1: simplify problem and add weight window generator . 256

3.1.2.3 Iteration 2 ................................................................................................................. 261

3.1.2.4 Additional iterations ................................................................................................ 263

3.1.2.5 Cylindrical Mesh Weight Window Summary ......................................................... 267

3.1.3 Cell-based weight windows for the Lead Slowing Down Spectrometer .................... 271

3.1.4 Time splitting .............................................................................................................. 277

3.1.5 Variance Reduction for the Cf Shuffler ......................................................................... 280

3.1.5.1 Cf Shuffler modified input ...................................................................................... 280

3.1.5.2 Particle production bias, time splitting, and windows ............................................. 285

3.1.5.3 Analysis of Cf Shuffler Variance Reduction ........................................................... 288

3.2 DXTRAN and Other Capabilities for Distributed Source Problems .................................... 288

3.2.1 UF6 Cask Model ............................................................................................................ 290

3.2.2 DXTRAN ....................................................................................................................... 300

3.2.3 Source Position Biasing ................................................................................................. 307

3.2.4 Best Single Detector Solution ........................................................................................ 310

3.3 Neutron Detector Operation in More Detail ......................................................................... 318

3.3.1 Introduction .................................................................................................................... 318

(i) Make reaction products (model, data) and recoil nuclei .................................................... 318

(ii) Track created particles in real gas composition ................................................................ 321

(iii) Tally energy deposition of particles in active volume (F8 CAP EDEP for coincidence/multiplicity counting) .......................................................................................... 322

Examples ................................................................................................................................. 322

3He detector Pulse Height ................................................................................................... 322

3He Detector Coincidence Calculation ................................................................................ 324

10B-lined detectors ............................................................................................................... 325

References ............................................................................................................................... 332

Section 4: Additional Topics ...................................................................................................... 332

4.1 Troubleshooting or "How can I be confident in the results?" ...................................... 332

4.1.1 Geometry and Materials ............................................................................................. 333

4.1.2 Detector modelling ..................................................................................................... 334

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4.1.3 Source modelling ........................................................................................................ 334

4.1.4 Sample modelling ....................................................................................................... 335

4.1.5 Tracking limitations .................................................................................................... 335

4.1.6 Nuclear Data ............................................................................................................... 335

4.1.7 Statistics ...................................................................................................................... 336

4.1.8 User ............................................................................................................................. 337

4.1.9 Summary and Conclusions - What to do?.................................................................. 337

4.2.1 Analysis of Delayed Neutron Production ....................................................................... 338

4.2.2 Comparison of Table Physics vs Model Physics ........................................................... 342

5 References and Bibliography ................................................................................................... 354

6 Table of Figures ....................................................................................................................... 355