Main Steam Line Break Analysis of a VVER-440 Reactor Using the Coupled Thermohydraulics System/3D-Neutron Kinetics Code DYN3D/ATHLET in Combination with the CFD Code CFX-4

The coupled thermohydraulic system/3D-neutron kinetic code complex DYN3D/ATHLET was applied to an asymmetric main steam line break (MSLB) analysis for the Russian VVER-440 type reactor. Such type of MSLB accidents cause an asymmetrical overcooling of the reactor. In this case the coolant mixing inside the reactor pressure vessel (RPV) has an important influence on the behaviour of the reactor. The code DYN3D includes a special model for the mixing of coolant from different primary loops in the lower plenum of VVER-440 type reactors which can be used in the coupled code, too. This model is based on the analytical solution of the Navier-Stokes equations in the potential flow
approximation in 2-dimensional cylindrical geometry and the diffusion equation for heat transport or soluble poison. The model is validated against experimental results from a 1:5 VVER-440 flow model with air and experimental data from VVERs with all main coolant pumps (MCP) working. Using this model for the coolant mixing in the MSLB analysis, recriticality of the scramed reactor was predicted. If homogeneous coolant mixing is assumed, no recriticality will be obtained.
The stationary three-dimensional
flow distribution in the downcomer and the lower plenum of a VVER-440/V-230 reactor was calculated with a CFD code (CFX-4). For this calculation the RPV from the cold legs inlets, the downcomer, the lower plenum and the lower core support plate was nodalized in detail. The comparison with experimental data and the above mentioned analytical mixing model showed a good agreement for near-nominal conditions (all MCPs are running). However, the comparison between the CFD-results and the analytical model revealed differences for MSLB conditions. After shutdown of the MCPs, natural convection established in the primary circuit. The mass flow rate of the affected by the MSLB loop is approximately twice the value of one of the other loops, a redistribution of the flow below the inlet nozzle of the affected loop is observed. For this reason the temperature field at the core entry cross section has two equal minima next to the position of the concerned inlet nozzle. The temperature distribution obtained by the analytical model has one minimum, just near to the position of this inlet nozzle. The shape of the temperature distribution for MSLB conditions is practically the same like in nominal conditions. The extension of this sector due to the increased mass flow is properly considered by the model.
The core inlet temperature distribution obtained by means of CFX-4 was used to estimate the reactivity effect in the MSLB analysis.