MCNP Guide
Data Cards in MCNP
Essential physics and calculation parameters
What Data Cards Control
Data cards tell MCNP how to handle the physics of your simulation. They control material properties, particle transport, calculation parameters, and output options. While geometry defines the physical structure, data cards define the behavior. This page covers physics control, execution, and output cards. For details on specific card types, see the dedicated pages on Materials, Tallies, Source Definitions, and Criticality Calculations.
Essential Data Card Categories
Materials (M, MT, TMP)
Composition and thermal properties
Physics (PHYS, CUT, MODE)
Transport and interaction controls
Problem Control (NPS, CTME, PRDMP)
Calculation execution parameters
Output (PRINT, PTRAC, DBCN)
Results and diagnostic options
Physics Control Cards
These cards control how MCNP handles particle transport and interactions. Getting these right is crucial for both accuracy and efficiency.
Basic Physics Setup
c Particle transport modes
MODE n p $ Transport neutrons and photons
c Physics parameters
PHYS:N 100 0 0 -1 -1 $ Neutron physics to 100 MeV
PHYS:P 100 0 1 $ Photon physics to 100 MeV
c Energy cutoffs (J = jump, meaning use default for that entry)
CUT:N J 0.001 $ Default time, 1 keV energy cutoff
CUT:P J 0.01 $ Default time, 10 keV energy cutoffThe MODE card specifies which particles to transport. PHYS cards control detailed physics options - the first parameter sets the maximum energy, while others control specific physics models. CUT cards set energy cutoffs below which particles are terminated, balancing accuracy with computational efficiency.
Specialized Physics Options
c Criticality-specific controls
TOTNU NO $ Use prompt nu-bar only (no delayed neutrons)
ACT FISSION=ALL $ Produce all delayed particles from fission
c Variance reduction
IMP:N 1 1 0 2 4 $ Neutron importance by cell
IMP:P 1 1 0 2 4 $ Photon importance by cell
FCL:N 1 1 1 $ Force collisions in cells 1-3TOTNU controls delayed neutron treatment in criticality calculations. IMP cards set importance values for variance reduction, while FCL forces collisions in specified cells for variance reduction.
Calculation Control
These cards control how long your calculation runs and how it terminates. They're essential for managing computational resources and ensuring reliable results.
Fixed Source Problems
c Basic execution control
NPS 1e6 $ Run 1 million particle histories
CTME 60 $ Time limit: 60 minutes
c Statistical checks
DBCN 17J 1 $ Debug control card
PRDMP J 1e6 1 2 $ Dump RUNTPE every 1e6 histories; write MCTAL
c Random number control
RAND GEN=2 SEED=12345 $ Random number generatorNPS sets the number of particle histories to run. CTME provides a time limit as backup termination. DBCN provides debug diagnostics. PRDMP creates restart files for long calculations. RAND controls random number generation for reproducible results.
Criticality Problems
Hover over each card to see what every parameter controls.
c Criticality calculation setupKCODE 10000 1.0 50 250KSRC 0 0 0 $ Initial source point10 0 0 $ Additional points for coveragec Criticality-specific controlsKOPTS BLOCKSIZE=10 $ Batch size for statisticsCTME 120 $ Time limit: 2 hoursPRDMP J 25 1 2 $ Dump RUNTPE every 25 cycles
Criticality Control Cards
Hover over a card line to see what each parameter controls.
KCODE specifies criticality parameters: particles per cycle, initial k-effective guess, cycles to skip for convergence, and total cycles. KSRC provides initial source locations. KOPTS controls advanced criticality options like batch sizes for improved statistics.
Output and Diagnostics
These cards control what information MCNP provides in its output files. They're essential for understanding your results and diagnosing problems.
Standard Output Control
c Print table control
PRINT 30 40 50 110 126 -85 -86 -87 $ Print tables (negative suppresses)
c Particle tracking
PTRAC BUFFER=1000 FILE=ptrac.txt $ Particle trace file
WRITE=POS,CEL,MAT,NPS $ Track position, cell, material
c Performance monitoring
PRDMP 2J 1 2 $ Write MCTAL; keep 2 RUNTPE dumpsPRINT controls which tables appear in the output file. Positive numbers enable tables, negative numbers suppress them. PTRAC creates detailed particle tracking files for debugging. PRDMP controls restart file frequency.
Advanced Diagnostics
c Detailed physics settings
PHYS:N 100 0 0 -1 -1 J $ J = use default for the next entry
PHYS:P 100 0 1 0 0 $ 3rd entry 1 = coherent scattering off
c Cross-section information
PRINT 104 $ Cross-section tables
c The XSDIR file location is set via the DATAPATH
c environment variable, not via an input card.
c Debug diagnostics
DBCN 17J 1 $ Extended diagnosticsIn MCNP, 'J' means "jump" (use the default value for that parameter position). PRINT 104 shows cross-section information. The cross-section directory location is set via the DATAPATH environment variable.
Material Temperature Effects
Temperature affects neutron cross-sections and scattering behavior. For accurate results, especially with thermal neutrons, you need to specify material temperatures.
c Water at room temperature
m1 1001.80c 2.0 $ Hydrogen
8016.80c 1.0 $ Oxygen
mt1 lwtr.20t $ Thermal scattering data (293.6 K)
c Fuel at operating temperature
m2 92235.80c 0.05 $ U-235
92238.80c 0.95 $ U-238
8016.80c 2.0 $ Oxygen
c Graphite moderator
m3 6000.80c 1.0 $ Carbon
mt3 grph.20t $ Graphite thermal treatment (293.6 K)
c Temperature card: one value per cell (kT in MeV)
c Assumes cells use materials 1, 2, 3 in order
TMP 2.5301e-8 6.6918e-8 8.6173e-8The TMP card specifies cell temperatures in MeV (kT = K × 8.617e-11), listed in cell order (one value per cell). It is a cell property, not a material property. TMP adjusts only the free-gas thermal scattering treatment — full Doppler broadening of resonances requires cross-section libraries evaluated at the right temperature (e.g. .81c/.82c for hot fuel). MT cards provide thermal scattering data for bound atoms in molecules or crystals. These effects are crucial for accurate thermal neutron transport in moderators.
Common Mistakes and Solutions
Avoiding Common Errors
Many MCNP problems stem from incorrect data card usage. Here are the most common issues and how to avoid them:
Missing MODE card
Always specify which particles to transport. Default is neutrons only.
Inappropriate energy cutoffs
Set CUT values based on your problem - too high loses accuracy, too low wastes time.
Insufficient statistics
Check the 10 statistical tests MCNP runs automatically on each tally, and increase particle histories if they fail.
Wrong temperature units
TMP uses MeV, not Kelvin. Convert with: MeV = K × 8.617e-11.
Data Card Best Practices
Start with minimal data cards and add complexity as needed. Always include MODE, NPS, and appropriate physics cards. Use PRINT to control output verbosity - suppress unnecessary tables to keep output files manageable.
For criticality problems, ensure adequate source point coverage and sufficient inactive cycles for convergence. Monitor k-effective convergence and adjust parameters if needed. Document all non-default settings for reproducibility.