SERPENT Guide
Physics Settings
Understanding Physics Models
Serpent's physics settings control how neutron transport and nuclear interactions are modeled. The code includes sophisticated physics treatments for thermal neutrons, resonance absorption, and nuclear reactions that can be tuned for different applications and accuracy requirements.
Default Philosophy: Serpent is designed with sensible defaults that work well for most reactor physics applications. Advanced settings allow fine-tuning for specific applications or improved accuracy at computational cost.
Basic Physics Configuration
% Standard reactor physics settings
set title "PWR Pin Cell Calculation"
% Basic transport parameters
set bc 1 % Boundary conditions (1=black, 2=reflective, 3=periodic)
set neutron 1 % Enable neutron transport
set photon 0 % Disable photon transport (neutron-only)
% Population control
set pop 10000 500 100 % neutrons/cycle, active cycles, inactive cycles
% Output control
set pcc 0 % Disable probability table cache (save memory)
set his 1 % Print history informationEnergy Cutoffs and Physics Options
Energy cutoffs determine the minimum energy at which particles are tracked. Proper cutoff selection balances computational efficiency with physics accuracy, especially important for thermal reactor calculations.
Neutron Energy Cutoffs
% Neutron energy limits
set ncut 1E-11 % Neutron low energy cutoff (eV)
% Default: 1E-11 eV (good for thermal reactors)
% Alternative cutoffs for different applications:
% set ncut 1E-9 % Fast reactor calculations
% set ncut 1E-8 % Intermediate spectrum systems
% set ncut 1E-10 % Very thermal systems (graphite moderated)
% Upper energy limit (rarely needed)
set nhcut 20.0 % Neutron high energy cutoff (MeV)
% Default: 20 MeV (adequate for most applications)Performance vs. Accuracy: Lower energy cutoffs increase computational time but improve accuracy for thermal flux and reaction rates. For shielding calculations, higher cutoffs (1E-8 to 1E-6 eV) may be acceptable.
Time and Weight Cutoffs
% Time cutoffs (for time-dependent calculations)
set tcut 1E-6 % Time cutoff (seconds)
% Default: 1E-6 s (1 μs)
% Weight cutoffs for variance reduction
set wwmin 1E-12 % Minimum weight before russian roulette
set wwmax 1E3 % Maximum weight before splitting
% Weight window parameters
set wwin 1E-6 1E-4 % Weight window lower and upper boundsThermal Neutron Physics
Thermal neutron treatment is critical for accurate reactor physics calculations. Serpent includes advanced models for thermal scattering, upscattering effects, and temperature dependencies in the thermal energy range.
Thermal Scattering Options
% Thermal scattering physics
set sabthresh 10.0 % S(α,β) energy threshold (eV)
% Use thermal scattering below this energy
% Advanced thermal options
set tms 1 % Temperature-dependent cross sections
% 0=off, 1=on (recommended for accurate calculations)
% Thermal upscattering treatment
set ures 1 % Unresolved resonance treatment
% 0=off, 1=probability tables (recommended)
% Thermal motion effects
set doppler 2 % Doppler broadening method
% 0=off, 1=constant, 2=TMS (recommended)Best Practice: For PWR/BWR calculations, enable TMS, unresolved resonance treatment, and Doppler broadening. These physics improvements are essential for accurate temperature coefficients and spectral calculations.
Resonance Treatment
% Resonance absorption treatment
set ures 1 % Unresolved resonance probability tables
set ureslim 1E-6 0.3 % UR energy range: 1 eV to 300 keV
% Resolved resonance handling
set roulette 1 % Russian roulette for low-weight particles
set split 1E3 % Particle splitting for high-weight particles
% Resonance scattering
set rescut 1E-6 % Resonance elastic scattering cutoff (eV)Advanced Physics Features
Photon Transport Coupling
% Coupled neutron-photon transport
set photon 1 % Enable photon transport
set pcut 0.001 % Photon energy cutoff (MeV)
% Photon physics options
set phcompton 1 % Compton scattering (1=on)
set phphoto 1 % Photoelectric absorption (1=on)
set phpair 1 % Pair production (1=on)
% Gamma production
set ygcut 0.01 % Gamma production threshold (MeV)
set egcut 0.001 % Electron energy cutoff (MeV)
% Bremsstrahlung treatment
set brems 1 % Bremsstrahlung production (1=on)Delayed Neutron Physics
% Delayed neutron modeling
set precursors 1 % Track delayed neutron precursors
set delnu 1 % Use delayed neutron data
% Delayed neutron groups (for kinetics)
set dndata 0 % Use 6-group delayed neutron data
% 0=default groups, 1=custom groups
% Effective delayed neutron fraction
set beta_eff 1 % Calculate effective delayed neutron fraction
% Important for reactor kinetics analysisKinetics Applications: Delayed neutron tracking is essential for reactor kinetics calculations, transient analysis, and accurate prediction of reactor behavior during control rod movements or power changes.
Cross Section Processing
% On-the-fly cross section processing
set opti 1 % Optimize memory usage
set memfrac 0.8 % Use 80% of available memory
% Cross section caching
set xsopt 1 % Cross section optimization
set sfbuf 100 % Sort buffer size (MB)
% Parallel processing options
set omp 4 % OpenMP threads (adjust to your system)
set sharedbuf 1 % Use shared memory buffer
set shbuf 100000000 % Shared buffer size (bytes)Performance Optimization
Memory Management
% Memory optimization settings
set memfrac 0.9 % Use 90% of available memory
set pcc 0 % Disable probability table cache
set xsopt 1 % Enable cross section optimization
% Buffer sizes for large problems
set sfbuf 200 % Sort buffer (MB)
set sfactor 1.5 % Safety factor for memory allocation
% Geometry optimization
set gcut 1E-12 % Geometry tolerance (cm)
set lost 100 % Lost particle limit before errorConvergence and Statistics
% Statistical convergence criteria
set pop 20000 400 100 % Particles, active cycles, inactive cycles
set entropy 1 % Track Shannon entropy for convergence
% Optional: tighten lost-particle limits and geometry tolerances
set lost 50
set gcut 1E-12Production Runs: For final results, use larger populations (20k-50k neutrons/cycle) and more cycles (1000-2000) to achieve statistical uncertainties below 10 pcm for k-effective and 1-2% for reaction rates.
Application-Specific Settings
Reactor Physics Calculations
% Standard PWR/BWR reactor physics
set title "PWR Core Physics Analysis"
set bc 2 % Reflective boundaries (core symmetry)
set neutron 1 % Neutron transport only
set tms 1 % Temperature-dependent cross sections
set doppler 2 % TMS Doppler broadening
set ures 1 % Unresolved resonance treatment
set ncut 1E-11 % Thermal cutoff
set pop 20000 400 100 % Good statistics
% Assembly-level calculations
set microxs 1 % Generate micro cross sections
set b1 1 % Critical spectrum correction
set leakage 1 % Include neutron leakage effectsShielding Calculations
% Neutron-gamma shielding analysis
set title "Spent Fuel Cask Shielding"
set neutron 1 % Enable neutron transport
set photon 1 % Enable photon transport
set ncut 1E-8 % Higher neutron cutoff (faster)
set pcut 0.01 % Photon cutoff (10 keV)
% Deep penetration physics
set ygcut 0.01 % Gamma production threshold
set wwmin 1E-15 % Lower weight window for penetration
set roulette 1 % Russian roulette for efficiency
% Variance reduction
set split 1E3 % Weight splitting for important particlesActivation Calculations
% Neutron activation analysis
set title "Reactor Vessel Activation"
set neutron 1 % Neutron transport
set photon 1 % Include activation photons
set ncut 1E-9 % Epithermal cutoff for activation
% Activation-specific settings
set pcc 0 % No probability table cache
set ires 1 % Implicit resonance capture
set delnu 0 % No delayed neutrons needed
% Burnup preparation
set inventory all % Track all nuclides for activation
set declib "endfb7_decay" % Decay data library