# Conversion script for homogenized cross sections

In order to be used with deterministic core simulators, the group constant output produced by Serpent has to be converted into appropriate format. Work on an automated conversion script has been started, and the latest version of the SXSFit scirpt is available [on-line]. Currently only the Serpent-ARES code sequence is fully supported, but support for other codes, including PARCS, will be added later on.

The script reads the homogenized group constant data from the `[input].coe` output files produced using the Automated burnup sequence, and takes advantage of a number of predetermined variables to pass information into the output. The format conversion depends on the code sequence, as described below.

## Serpent-ARES

The cross section model in ARES is based on the combination
of tabular interpolation and polynomial expansions.^{[1]} Tabulated data is used for burnup points,
control rod insertion branches and moderator temperature (PWR) or void
(BWR) histories. The remaining variations are accounted for by polynomial
interpolation of the form:

where is the value at the nominal state, are the polynomial coefficients and:

is the difference in relative coolant density compared to nominal value | |

is the difference in the square root of fuel temperature compared to nominal value | |

is difference in moderator temperature compared to nominal value | |

is the control rod history (BWR) | |

is the boron concentration (PWR) | |

is the boron history (PWR) |

Changes in the density of coolant in liquid state is included in the coolant temperature parameter. Separate parametrization for coolant density is to account for the effects of sub-cooled boiling. In practice this means running additional branches with small but non-zero void fraction.

The first 5 parameter variations are handled as branch variations. The procedure is repeated for differnt Moderator temperature (PWR) or coolant void (BWR) histories. Boron histories are accounted for by separate burnup calculations without branches. All branch calculations are repeated for different control rod types.

### Serpent variables

The state point information is passed into the conversions script using the following variables:

VHI |
: moderator temperature (PWR) or coolant void history (BWR) |

BHI |
: boron history |

BOR |
: boron concentration |

CR |
: insertion of contro rods (0 = no rods, n = rod type n inserted) |

TFU |
: fuel temperature |

VOI |
: coolant void fraction |

TMO |
: moderator temperature |

### SXSFit input

## References

- ^ Mattila, R.
*"Kiehutusvesireaktorin vaikutusalamalli CROSS."*YE-PR-5/02, VTT Technical Research Centre of Finland, 2002 (in Finnish)