MCNP mesh to weight windows (mesh2ww)

Command line tool to convert MCNP mesh tallies of any type to a mesh-based global weight window.

This uses the Method of Automatic Generation of Importances by Calculation (a.k.a MAGIC) with configurable de-tuning options.

Usage: mesh2ww <file> <number> [options] [+]

Arguments:
  <file>                  Path to meshtal file
  <number>                Mesh tally identifier

Weight options:
  -p, --power <num>...    Set the softening/de-tuning factor
  -e, --error <num>...    Maximum rel. error, use analogue above
  -t, --total             Weights from 'Total' groups only
  -s, --scale <num>       Multiply all weights by a constant

Global file options:
  -o, --output <path>     Name of output file ('wwinp' default)
      --trim              Exclude unused particles from wwinp header

Global VTK options:
      --vtk               Write VTK files for plotting weights
  -f, --format <fmt>      Set the VTK file format
  -r, --resolution <cst>  Cylindrical mesh resolution
      --endian <end>      Byte ordering/endian
      --compressor <cmp>  Compression method for XML

Flags:
  -v, --verbose...        Verbose logging (-v, -vv)
  -q, --quiet             Supress all log output (overrules --verbose)
      --help              Print help info (see more with '--help')

See --help for detail and examples

Help is printed with the -h flag, and --help will show default values, examples, and any important behaviour.

Overview

Supported output formats

For more detail, see the OUT keyword for the FMESH card definition in the MCNPv6.2 or MCNPv6.3 user manuals.

Output format	Supported?	Description
XDMF/HDF5	Yes	MCNP >6.3 runtape HDF5 format
COL	Yes	Column data (MCNP default)
CF	Yes	Column data including voxel volume
IJ	Yes	2D matrix of I (col) and J (row) data, grouped by K
IK	Yes	2D matrix of I (col) and K (row) data, grouped by J
JK	Yes	2D matrix of J (col) and K (row) data, grouped by I
CUV (UKAEA)	Yes	UKAEA Cell-under-Voxel column data
NONE	N/A	NONE or unknown output format

Supported mesh geometries

All functionality is fully supported for rectangular and cylindrical meshes.

Mesh geometry	Supported?	MCNP designators
Rectangular	Yes	rec, xyz
Cylindrical	Yes	cyl, rzt
Spherical	No	sph, rpt

Spherical meshes are not supported because barely anyone knows about them, let alone uses them. They are therefore a low priority, but raise an issue if anyone needs it.

Install

Download and unpack the latest binary executable release here for running in a terminal/powershell.

Linux/MacOS

Unpack the relevant executable from the latest release.

# Linux
tar -xjf mesh2ww-x86_64-unknown-linux-gnu.tar.xz  # Generic linux
tar -xjf mesh2ww-aarch64-unknown-linux-gnu.tar.xz # ARM64 Linux

# MacOS
tar -xjf mesh2ww-x86_64-apple-darwin.tar.xz       # Intel macOS
tar -xjf mesh2ww-aarch64-apple-darwin.tar.xz      # Apple Silicon macOS

And either run from there or add the executable to your $PATH.

./mesh2ww -h

Windows

Extract mesh2ww-x86_64-pc-windows-msvc.zip from the latest release.

Navigate to this folder and run from powershell.

.\mesh2ww.exe -h

This may be set as an alias for convenience.

Set-Alias -Name mesh2ww -Value C:\Path\To\Folder\mesh2ww.exe

Examples

Tuning weights

Typical usage will generally define a de-tuning factor (-p/--power) and possibly a relative error cutoff (-e/--error) for generating weights.

# Chenge the de-tuning factor and relative error cut
mesh2ww /path/to/file.msht 104 --power 0.70 --error 0.25

The --power value modifies calculated weights by w => w^(power), which helps with softening extreme values. Any voxels with errors above --error (>25% in this case) continue to use analogue transport until the uncertainty imporves.

Renaming output files

The weight window file may be renamed as needed.

# Chenge output file name to "mywwmesh.wwinp"
mesh2ww file.msht 104 --output mywwmesh.wwinp

Simplified weight window

It is often fine to simply generate a global weight window using the 'Total' group rather than every explicit energy/time group.

# Only use the Total energy/time groups
mesh2ww file.msht 104 --total

This is probably the recommended use case for any finely binned groups, as nobody should really be trying to optimise for every energy in a 175-group mesh anyway.

Re-scale weights

Generated weights are typically normalised to the total flux for each group. These may be rescaled by a constant multiplier.

# Multiply all normalised weights by x2.5
mesh2ww file.msht 104 --scale 2.5

Multi-particle weight windows

Multiple tallies may be combined for weight windows covering multiple particle types. This is achieved using the + operator.

The usage is as simple as combining multiple argument sets with + as the delimiter.

# Syntax for combining weights for multiple particle types
mesh2ww <file> <number> [options] +      \
        <file> <number> [options] +      \
        <file> <number> [options]

For example:

• NP_tallies.msht contains neutron (FMESH14:n) and photon (FMESH24:p) tallies
• E_tallies.msht contains an electron (FMESH34:e) tally

If all of these are the same geometry, they may be combined with all the usual options applied to each tally individually:

# Make weight windows for neutrons, photons, and electrons
mesh2ww NP_tallies.msht 14                   +      \
        NP_tallies.msht 24 -p 0.8 -e 0.15    +      \
        E_tallies.msht  34 --total                  \

Here the neutron tally uses defaults, the photon tally de-tunes weights, and the electron tally only uses the total energy/time bins.

Writing weights to VTK

A Visual Toolkit file can be generated for every weight window set using the --vtk flag.

# Write to VTK for plotting
mesh2ww file.msht 14 --vtk

Of course all the usual options are available, such as increasing the resolution of cylindrical meshes with few theta bins.

# Change cylindrical mesh resolution
mesh2ww file.msht 14 --vtk --resolution 2

Advanced options include changing the file format, byte ordering of binary outputs, and which compressor to use for XML.

# Change VTK file format options
mesh2ww file.msht 14 --vtk          \
            --format legacy-ascii   \
            --compressor lzma       \
            --endian big-endian

Advanced de-tuning

For fine control, the --power and --error parameters may be set explicitly for every unique group.

For example, if a mesh has 3 energy groups at 1.0 MeV, 10.0 MeV, and 100.0 MeV, the power factor for each may be set to 0.8, 0.7, and 0.65 respectively.

# Set energy group power factors individually
mesh2ww file.msht 104 --power 0.8 0.7 0.65

This also applies to time bins. To set values for all unique groups, the values must be given in the order expected by MCNP.

For example, a mesh with 3x energy groups and 2x time groups:

Energy 1.0        Power
    Time 1e10      0.9
    Time 1e20      0.7
Energy 10.0
    Time 1e10      0.8
    Time 1e20      0.8
Energy 100.0
    Time 1e10      0.6
    Time 1e20      0.5

# Set energy and time group power factors individually
mesh2ww file.msht 104 --power 0.9 0.7   0.8 0.8   0.6 0.5