Two-way Coupling between the Reactor Dynamics Code DYN3D and the Fuel Performance Code TRANSURANUS at Assembly Level

In the last two decades the reactor dynamics code DYN3D was coupled to thermal hydraulics codes, sub-channel code and CFD codes. These earlier developed code systems allow modelling of the phenomena in higher degree of detail. Nevertheless all of them contain a simplified fuel behaviour model, i.e. without taking into account the fission gas release during normal operation, off-normal conditions and transient. Furthermore, no two-way coupling to a fuel performance code has so far been reported in the open literature for calculating a full core with detailed and well validated fuel behaviour correlations.

A new two-way coupling approach between DYN3D and the fuel performance code TRANSURANUS is presented. In the coupling, DYN3D provides only the time-dependent rod power and thermal hydraulics conditions to TRANSURANUS, which in turn transfers parameters like fuel temperature and cladding temperature back to DYN3D. The main part of the development is a so-called general TRANSURANUS coupling interface that is applicable for other reactor dynamics codes, thermal hydraulics codes and sub-channel codes. Beside its generality, other characteristics of this interface are the calculation at either fuel assembly or fuel rod level, one-way or two-way coupling, automatic switch from steady to transient conditions in TRANSURANUS (including update of the material properties etc.), writing of all TRANSURANUS output files and manual pre- and post-calculations with TRANSURANUS in standalone mode. The TRANSURANUS code can be used in combination with this coupling interface in various scenarios: different fuel compositions in the reactor types BWR, PWR, VVER, HWR and FBR, time scales from milliseconds (i.e. RIA) over seconds/ minutes (i.e. LOCA) to years (i.e. normal operation) and thence different reactor states.

Results of DYN3D-TRANSURANUS are shown for a control rod ejection transient in a UO2 core of a German PWR. In particular it appears that for all burn-up levels DYN3D-TRANSURANUS systematically calculates higher values for the node fuel enthalpy (max. difference of 46 J/g) and node centerline fuel temperature (max. difference of 180 K) compared to DYN3D standalone in best estimate calculations. These differences can be completely explained by the more detailed TRANSURANUS modelling of fuel thermal conductivity, radial heat release profile and heat transfer in the gap. As known from fuel performance codes, the modelling of the heat transfer in the gap is sensitive and causes also larger differences in case of low burn-up.

No convergence problems occurred for DYN3D-TRANSURANUS. The coupled code system can improve the assessment of safety criteria, at a reasonable computational cost with a running time of less than six hours without parallelization.