Natural circulation experiments at the ISB-VVER integral test facility andcalculations using the thermal-hydraulic code ATHLET

In 1995 at the integral test facility ISB-WER in Elektrogorsk near Moscow n atural circulation experiments were performed that were scientifically supp orted by the Forschungszentrum Rossendorf These experiments were the first of this kind at a test facility that models WER-1000 thermal hydraulics. Us ing the code ATHLET, which is being developed by Gesellschaft fur Anlagen - und Reaktorsicherheit, pre- and posttest calculations were done to determi ne the thermal-hydraulic events to be expected and to define and tune the b oundary conditions of the test. The conditions found for natural circulatio n instabilities and cold-leg loop-seal clearing could be confirmed by the t ests. The main thermal-hydraulic phenomena were identified and compared wit h the results gained during similar experiments on WER-440 test facilities. Besides the thermal-hydraulic standard measuring system, the facility was equipped with needle-shaped conductivity probes for measuring the local voi d fractions.