Spectra conversion

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The nuclear data provided in the FISPACT-II releases includes multi-group cross sections which are processed in complex procedures for use in the code. While it is possible to have multi-group data in other group structures (contact the UKAEA nuclear data manager if required), the provided nuclear data are multi-purpose and more than cover nearly any application. To simulate with a user-supplied particle spectrum, the energy discretisation must be identical, so a non-compliant particle spectrum must be converted into a suitable group structure.

This is a common issue for simulations performed using codes with coarse energy treatment or legacy results which are only available in some (typically low-density) alternative group structure. Users are strongly advised to calculate particle spectra in a compatible group if possible, however the built-in group conversion tools may be used if no alternative is available.

Legacy EAF data was provided in several multi-group formats including micro-flux weightings which varied depending on the group and application. Besides the benefits of using ENDF-format data and the TENDL data in particular, the use of coarse groups and application-specific group weightings is a serious handicap for many simulations.

Example flux convert

To modify a flux spectrum, FISPACT-II has a GRPCONVERT keyword which can be used to re-bin an artibrary flux spectrum. The flux_convert/ folder within getting started has an example using a spectrum from a hafnium foil irradiation at FNG. For this conversion only these three files are required: arb_flux provides a list of energy boundaries for the original group structure, convert.i contains the simple input

<< convert flux to 709 group structure>>

and the files file only must contain ind_nuc, a pointer to the arb_flux and an output fluxes file - for example:

# Nuclide index file

# arb_flux location
arb_flux /path/to/my/arb_flux

# ouptut fluxes file
fluxes /path/to/my/output/fluxes

This fluxes file can then be used in simulations which collapse with the provided TENDL libraries, for example. Please note that the 175 group structure contains one group from 1.0E-5 to 1.0E-1 eV. Converting this massive bin will result in an extrapolated, flat spectrum to 1.0E-5 eV in the 709 group which will be collapsed with a 1/v cross section and yield unphysical results. In general, groups with such coarse structure cannot be reasonably used to model multiple regions where the specific flux weighting used to process the data will not be valid - and a flat weighted coarse structure will result in useless simulations. The user must at minimum clip the groups to physically realistic values or, if accurate results are desired (particularly if the reaction rate is non-negligible in the energy range in question), calculate the spectrum in a group structure which captures the physics of the system.

The getting_started example can be executed with

fispact convert files.convert

where the specific files file has been specified. FISPACT-II will generate a detailed output which summarises the input spectrum, distribution of flux by groups in the new structure, and the converted particle spectrum. There will also be a new spectrum file, fluxes, which is in the correct format for subsequent simulations. The equal-lethargy spectra before and after conversion for the example case is shown below. Note that this should be nearly identical to the original spectrum, with some differences where resonance-related discontinuities are present in the original spectrum.