Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Due to the low geodetic height differences and the presence of loop seals in VVER-reactors, the behaviour of natural circulation is an important safety feature. The experiment to the OECD/NEA/CSNI International Standard Problem No. 33 was dedicated to the investigation of this phenomenon. During the test, defined quantities of primary coolant waere drained, while the facility was operated at 10% of nominal power and without pumps. The test was stopped with increasing temperatures of the fuel rod simulator.
In RC Rossendorf pre- and posttest calculations were performed by the code ATHLET, which was developed by the "Gesellschaft für Anlagen- und Reaktorsicherheit".
The main events of the experiment could be predicted, shown and explained by the ATHLET calculations:

• After draining of about 20% of the primary inventory, the mass flow in the loops stagnates. Simultaneously, the primary pressure increases and the pressurizer is partly refilled.
• In later periods the mass flow through the three loops is nonequally distributed, which could be reproduced by the ATHLET calculations at least qualitatively.