Simulation of the primary circuit breaks in the training simulator

The pressurized water reactors (PWR) have been designed to operate in the way that normally there is only one-phase liquid in the primary circuit. In the primary break accidents, however, the primary pressure decreases as a consequence of the coolant discharge and a two-phase condition may occur. Large computer codes like RELAP5 and TRAC, which consume lots of computer time, are used for the simulation of reactor systems in small break loss of coolant accidents (SBLOCA). The full-scope training simulator of the Loviisa pressurized water reactor has been improved by backfitting a two-phase model SMABRE to the simulator. The two-phase model is necessarily needed when the primary circuit breaks are simulated. The basic physical models in the SMABRE are similar to those in the large simulation codes, but making some simplifications the real time simulation on a PDP-11/70 computer is possible. Two nodalization models, one with 26 volumes and the other with 61 volumes, have been applied in the training simulator. The main reason for the use of the more detailed model is the need to take into account the variations of primary loop elevations.