SCONE Guide

❮ Nuclear DataParallel Computing ❯

SCONE Verification Notice

SCONE is a research-oriented code with a smaller user base than MCNP, OpenMC, or SERPENT. Our examples are intended as educational guidance. For authoritative syntax, physics options, and nuclear data requirements, consult the official documentation.

Official SCONE documentationSCONE on GitHubIssues & discussions

Variance Reduction in SCONE

Weight windows and what requires source code modification

New users: get stable results with enough inactive / active / pop (Basics) before exploring weight windows. SCONE does not offer MCNP-style DXTRAN or cell importances in the input deck.

Scope of VR in SCONE

SCONE supports weight windows via neutronCEimp (set weightWindows 1) and a varianceReduction block with type weightWindowsField. It does not support DXTRAN spheres, exponential transform, cell importance maps, or source biasing as input-deck features. See the official Input Manual for weight window syntax.

Why Variance Reduction Matters

Monte Carlo simulations can be computationally expensive, especially when tracking particles through thick materials or measuring rare events. Variance reduction techniques help focus computational effort where it matters most. If you have experience with MCNP or SERPENT, you may be familiar with weight windows, implicit capture, splitting/roulette, and DXTRAN. SCONE supports only a subset of these concepts through its input deck.

Common VR Concepts (for Reference)

  • Implicit capture: Absorb particles with reduced weight instead of terminating them; reduces variance in absorbing regions.
  • Weight windows: Supported in SCONE via neutronCEimp and varianceReduction block.
  • DXTRAN spheres: Force particles toward small detectors; not available in SCONE input.
  • Exponential transform: Bias path lengths in deep-penetration problems; not available in SCONE input.

What SCONE Supports: Collision Operator

SCONE configures collision behavior through the collisionOperator block. The standard neutron continuous-energy operator is neutronCEstd. Analog vs implicit capture behavior is determined by the collision operator implementation, not by a separate variance_reductionblock. There is no variance_reduction { ... }, weight_windows,exponential_transform, or dxtran in SCONE input syntax.

Correct SCONE Input Structure

Excerpt matches tutorial_nuclear_data_basic_testlib lines 8–26 (same lines appear in the full file run in run_all.ps1). Add geometry and nuclearData below this header to form a complete input.

tutorial_nuclear_data_basic_testlib (lines 8–26)
type eigenPhysicsPackage;
pop      10000;
active   50;
inactive 20;
XSdata   ce;
dataType ce;

collisionOperator { neutronCE { type neutronCEstd; } }
transportOperator { type transportOperatorDT; }

inactiveTally {}

activeTally {
  fissionRate {
    type collisionClerk;
    response (fission);
    fission { type macroResponse; MT -6; }
  }
}
caalh/ReactorMCBrowse filesscone-examples/verify/tutorial_nuclear_data_basic_testlib

Verify harness (testLib + run_all.ps1)

The collisionOperator block uses semicolon-terminated key-value pairs and nested curly braces. The collision operator controls how neutrons interact at collision sites; any implicit-capture or weight-control logic is built into the operator type, not specified via separate VR cards. Paste full geometry and nuclearData from Basics or Simple Examples to make a complete input.

What Does Not Exist in SCONE Input

The following constructs are not valid SCONE syntax. Do not use them in SCONE input.

  • variance_reduction { ... }
  • weight_windows (MCNP-style) — SCONE uses varianceReduction { type weightWindowsField; file ... } instead
  • exponential_transform
  • dxtran
  • cell_importances or importance maps
  • source_biasing
  • Any ... end block terminators (SCONE uses })

Improving Efficiency Within SCONE's Limits

Within the constraints of the input deck, you can still improve simulation efficiency:

  • Increase pop and active cycles for better statistics.
  • Use appropriate inactive cycles to allow source convergence.
  • Choose tally responses and maps that match your problem (e.g., fission rate with spatial maps).
  • Ensure nuclear data and geometry are correctly specified to avoid wasted particles.

Advanced VR: Source Code Modification

SCONE is a research/educational code with a modular Fortran architecture. Weight windows are supported via input. Adding DXTRAN, exponential transform, or cell importance maps would require extending the collision operator, transport operator, or tally logic in the source code. There is no input-deck hook for these features. If your application demands advanced variance reduction, consider using MCNP, SERPENT, or OpenMC, which support these techniques natively.

Related Topics

❮ Nuclear DataParallel Computing ❯