Numerische Untersuchung zur Strömungsentwicklung im passiven Flutsystem des Reaktordesigns Kerena

In case of a loss-of-coolant accident in a nuclear power plant, the passive gravity-driven core flooding system recovers the reactor’s water inventory so that the core is covered at all times. It is essential to estimate, when passive flooding valves open, whether reverse flow occurs and to which extent this may delay flooding or even lead to initial mass displacement from the reactor to the flooding pools. Experimental investigations including passive safety systems of the KERENA-design are performed in a scaled-down facility. Such so-called integral tests are carried out with initial and boundary conditions of different design basis accidents. The behavior of the passive core flooding system is analyzed proofing the influence through other passive safety systems. Evidence is shown that a stable two-phase reverse flow occurs, due to incomplete condensation at the outlet of the emergency condenser and to adiabatic evaporation in the flooding line. The hydrostatic column is reduced during reverse flow but regained in all considered cases. As a result the onset of flooding is delayed only moderately. The influence on the flow development of the containment pressure, of the geometry of the flooding line, and of other peculiar boundary conditions of the considered experiments is discussed. Results from numerical simulations are presented and confirm the experimental results. The considered numerical model of the flooding system can predict fundamental phenomena governing the flow development in the flooding line with satisfactory accuracy.