Capability of the coupled code system ATHLET-QUABOX/CUBBOX for safety analysis

There exists a tendency to perform accident analysis of NPP by best estimate codes, that decreases the conservatism of performing calculations and allows more realistic simulation of transients. A necessary step for this approach is the comprehensive validation of the computer codes and the coupling of thermo-hydraulic plant system codes with 3D neutronics models. The application of coupled code systems is mandatory for the analysis of accident conditions which are determined by a strong coupling between neutronics of the reactor core and thermo-hydraulics of the primary circuit, especially, when asymmetrical processes take place in the core leading to strongly space-dependent power generation. The paper gives an overview on the development of the coupled 3D neutronics and fluid-dynamic system code in GRS, in particular, the work is presented performed on the basis of the system code ATHLET and the 3D reactor core model QUABOX/CUBBOX, both developed in GRS. In addition, the experiences from applications in accident analysis are summarized and further developments are discussed. (author)