Coupled APROS-CFD simulation of generic VVER-440 loss of feedwater transient

The behavior of a VVER-440 nuclear power plant is simulated in postulated loss-of-feedwater accident with two-way coupled 1D-3D simulation. The pressurizer is simulated with Ansys Fluent 3D code, whereas the rest of the plant is modelled with Apros system code. The plant model includes all the main process components and their control automation devices. The coupling is made using Apros Co-simulation tool. The couplings of the 1D and 3D models include the spray line, control of pressurizer heaters, pressure relief line, surge line and water level measurement. The phase change models for the pressurizer have been implemented with user-defined functions of Fluent. In the beginning of the postulated loss-of-feedwater transient the feedwater pumps are assumed to trip during normal operation of the plant. This leads to pressure rise in the primary circuit, which triggers the pressurizer spray system. During the transient, the control rods start to move to decrease reactor power and later SCRAM occurs. Once the pressure has decreased back to normal levels, the spray valves are closed. As the pressure continues to decrease, the heater element groups at the bottom part of the pressurizer are turned on one-by-one to compensate the pressure drop. The results of the coupled simulation are compared with stand-alone Apros simulation to specify differences and possible benefits of coupled simulation compared to system code simulation only. With the simulation the operation of the Apros Co-simulation tool is also tested and demonstrated with relatively complex transient case with several couplings of different kind.