Coupling of the thermohydraulic code ATHLET with the 3D neutron kinetic model DYN3D

The coupling of advanced thermohydraulic codes with 3-dimensional neutron kinetic codes corresponds to the effort to replace conservative estimations by best estimate calculations. ATHLET is an advanced thermohydraulic code, developed by the German "Gesellschaft für Anlagen- und Reaktorsicherheit" (GRS). The DYN3D code, developed at Forschungszentrum Rossendorf (FZR) for the simulation of reactivity initiated accidents in nuclear reactors with hexagonal and Cartesian fuel element cross section geometry comprises 3-dimensional neutron kinetics, models for the thermohydraulics of the core and the thermomechanical fuel rod behaviour.

The reactor core model DYN3D was coupled with ATHLET according to two basically different strategies. The first way of coupling uses only the neutron kinetics part of DYN3D (internal coupling). In the second way, the whole core is cut out from the ATHLET plant model and is completely described by DYN3D (external coupling). In this case the values of pressure, mass flow rate, enthalpy and boron acid concentration at the bottom and at the top of the core have to be transferred between the codes. This way of coupling is efficiently supported by the General Control and Simulation Module (GCSM) of ATHLET. Almost no changes of the single programs are necessary.

For plausibility test of the code coupling, comparative calculations between point kinetics, internal and external coupling were performed for different hypothetical accident scenarios. First results of the analysis of a main steam line break scenario for a VVER-440 type reactor by the help of ATHLET-DYN3D are presented. The effect of different assumptions for the coolant mixing in downcomer and lower plenum of the reactor has been investigated.