OpenMC Data

Aims to facilitate the use of different nuclear data libraries with OpenMC by providing command line tools that process and download nuclear data automatically.

Prerequisites

You should have already installed OpenMC, see the docs for installation instructions.

Installation

The package is distributed on PYPI and can be installed with pip.

pip install openmc_data

Usage

Once installed several scripts are available in your terminal that are able to download and process nuclear data.

The scripts accept input arguments, to find out the input arguments available for a particular script run the script name with --help after the name. For example:

convert_endf --help

Some scripts (mainly the generate scripts) require NJOY to be installed and added to your path.

A few categories of scripts are available:

• Scripts that produce h5 cross section files:
• Convert ACE files to h5 files
• Generate h5 file from ENDF files
• Download h5 files
• Scripts that produce xml chain files:
• Generate xml chain files from ENDF files:
• Download xml chain files:
• Other scripts that don't fall into either category.

Produce Cross Section Files

Convert cross sections

Generate cross sections

Download cross sections

Produce Depletion Chain files

Generate chain files

Branching ratios for metastable states

Reactions can produce either the ground state or a metastable state of a nuclide and the split between them is dependent on the neutron spectrum. The generate_branching_ratios script finds this split from the isomeric production data in the ENDF neutron files (MF=8 identifies the isomeric states and whether the production data is in MF=9 or MF=10) and collapses it with a multigroup neutron flux that you provide. The one group branching ratio is reaction rate weighted, which is the weighting that preserves the production rate of each isomer in a single energy group chain.

The JSON file produced is in the same format as the branching_ratios_pwr.json and branching_ratios_sfr.json files, so it can be applied to a chain file with add_branching_ratios.

# downloads the ENDF files and makes a chain file from them
generate_tendl_chain -r 2017 --lib endf80

# collapses the isomeric production data with a fusion neutron source spectrum
# taken from the IAEA CoNDERC FNS benchmark
generate_branching_ratios \
    --neutron-dir tendl-2017-endf/neutron \
    --fispact-fluxes fns/Ag/2000exp_5min_fluxes \
    --chain chain_tendl_2017_endf80.xml \
    --jobs 20 \
    -o branching_ratios_tendl_2017_fns.json

# adds the branching ratios to the chain file
add_branching_ratios \
    -i chain_tendl_2017_endf80.xml \
    -b branching_ratios_tendl_2017_fns.json \
    -o chain_tendl_2017_endf80_fns.xml

Most reactions store their isomeric production as cross sections in MF=10 and need no other data. The remainder store multiplicities in MF=9 which have to be weighted by the reaction cross section, and as the MF=3 cross section is only the background in the resolved resonance range these are reconstructed with NJOY. Passing --cross-sections will instead read them from an existing HDF5 library, which avoids the NJOY runs if you have already made one.

Only the shape of the cross section is used, as a weight, so the NJOY modules that do not affect it are skipped and the reconstruction tolerance is loosened. This is what makes the actinides tractable, as the unresolved resonance probability tables take over two hours per nuclide, and it changes the branching ratios by less than 0.2%.

Use --jobs to process files in parallel. A whole TENDL library is about 2800 evaluations and takes roughly half an hour with --jobs 20, against many hours in series.

Download chain files

All the TENDL chains are made with generate_tendl_chain using decay data and neutron induced fission yields from ENDF/B-VIII.0, so the releases can be compared against one another.

FNS (fusion neutron source) adds branching ratios for the production of metastable states, which makes these chains suitable for activation calculations of fusion neutron spectra. They are produced with generate_branching_ratios from the isomeric production data in the TENDL files themselves, collapsed with a spectrum from the IAEA CoNDERC FNS benchmark.

The branching options differ in which reactions they cover, not only in the values. Counts below are for TENDL 2017.

The reactor options only ever split capture. For a fusion spectrum the threshold reactions matter most, and in the FNS chains (n,gamma) is only the fourth largest group behind (n,na), (n,3He) and (n,2n), so SFR and PWR leave out most of what an activation calculation needs.

FNS-ORIGEN is an earlier TENDL 2017 and 2019 chain hosted on the openmc_activator repository, whose branching ratios are derived from ORIGEN data rather than from TENDL. It is kept available for comparison, but the FNS chains are the TENDL only option and the two can differ substantially. For Ag107 capture to Ag108_m1 under a fusion spectrum, for example, FNS gives 0.056 against FNS-ORIGEN's 0.348, because the FNS value is reaction rate weighted and the majority of the capture rate sits in the resolved resonance range where the isomeric ratio is small.

Other scripts