Main Steam Line Break Analysis of a NPP with VVER by Means of the Coupled Code DYN3D/ATHLET

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.

Both versions of DYN3D for hexagonal and Cartesian fuel element geometry were 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.

The coupled code DYN3D/ATHLET was used to investigate the possibility of recriticality during an asymmetrical overcooling of the reactor core of a NPP with VVER-440 after a main steam line break (MSLB). This MSLB analysis was performed for hot zero power and end of fuel cycle conditions. Different coolant mixing conditions in the lower plenum of the reactor were simulated. The results show the importance of these conditions. In case of a realistic mixing model and without consideration of mixing, i.e. where each loop is connected to a particular 1/6 sector of the core, a recriticality after reactor scram was predicted. For ideal mixing only, recriticality can be avoided.