Investigations of Natural Circulation Instabilities in VVER-type Reactors at LOCA Conditions

LOCA experiments are characterized by a more or less rapid primary pressure decrease in the early phase of the transient. After
pump coast down natural circulation becomes the dominant decay heat removal mechanism. After leak initiation boiling in the
reactor core leads to formation of two-phase flow conditions in the primary circuit. At these conditions different types of
two-phase flow instabilities can appear. The instabilities play an important role in the behaviour of the primary circuit, because the
instabilities can disturb the decay heat removal from the reactor core.

At the PMK-2 test facility natural circulation instabilities could be detected in 3 different experiments: a 1% cold leg break, a 1%
cold leg break with primary bleed and a surge line break.

With the help of the presented calculations it could be shown, that ATHLET is able to calculate the accident behaviour in a good
agreement with the experimental data. All significant phenomena, such as hot and cold leg loop seal clearing, stagnation of the
natural circulation, phase and mass separation along the facility and especially the natural circulation instabilities are calculated
very well. In some cases the ATHLET calculation provide the more detailed information required to clarify the complex
processes connected with the different kinds of instabilities.