MCNP mesh to weight windows (mesh2ww)

Command line tool to convert MCNP mesh tallies of any type to a mesh-based global weight window using the magic method with configurable de-tuning options.

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

Arguments:
  [meshtal]  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
  -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.

Install

Direct from github:

cargo install --git https://github.com/repositony/mesh2ww.git

All executables are under ~/.cargo/bin/, which should already be in your path after installing Rust.

Click here if you have never used Rust

If you have never used the Rust programming language, the toolchain is easily installed from the official website

Unix (Linux/MacOS)

Run the following to download and run rustup-init.sh, which will install the Rust toolchain for your platform.

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

This should have added source $HOME/.cargo/env to the bash profile, so update your environment with source ~/.bashrc.

Windows

On Windows, download and run rustup-init.exe from the official installs.

Overview

Supported mesh 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
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

Once I get my paws on MCNPv6.3 this will be extended to include the new COLSCI, CFSCI, and XDMF/HDF5 formats.

Supported mesh geometries

All functionality is fully supported for both rectangular and cylindrical meshes.

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

Currently 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.

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/meshtal.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 /path/to/meshtal.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 /path/to/meshtal.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 /path/to/meshtal.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 <meshtal> <number> [options] +      \
        <meshtal> <number> [options] +      \
        <meshtal> <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 /path/to/meshtal.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 /path/to/meshtal.msht 104 --power 0.9 0.7   0.8 0.8   0.6 0.5