Stability of self-pressurized, natural circulation, low thermo-dynamic quality, nuclear reactors: The stability performance of the CAREM-25 reactor

The stability performance of self-pressurized, natural circulation, low-thermodynamic quality, nuclear reactors such as CAREM-25 is very different from existing conventional nuclear reactors. In this work, the linear stability of such a reactor is investigated in depth for both, nominal and low pressure-low power (start-up) conditions. As a result it is found that the flashing effect is crucial to correctly investigate the stability performance of the CAREM-25 reactor. In addition, it is verified that the dominant destabilizing mechanism are density waves travelling through the chimney section corresponding to Type-I instabilities. From the results it is observed that at rated conditions the unstable region is only limited to cases in which the two-phase boundary is located within a region which extends approximately from the core outlet until the middle of the chimney. It is also found that the condensation taking place in the steam dome has a great impact in defining the stability of the reactor and thus can be used to tune the CAREM-25 operational point. It is observed that the reactor shows a better stability performance when increasing the system pressure. From the steady state results it is found that the system could be pressurized without encountering instabilities if a certain minimum condensation power level is guaranteed.