Hot leg break tests at the ISB-VVER integral test facility

Existing thermal hydraulic best-estimate codes have to undergo a procedure of extended verification in respect to the special qualities of VVERs. Concerning then VVER-1000 the Russian test facility ISB-VVER only is available, which is located in Elekrtogorsk near Moscow. It was put into operation in October 1992 and was used as the source of experimental data for the first Russian Standard Problem, a 2.4 % small break KOCA test. The present paper deals with a series of tests dedicated ro an 11 % break at the upper plenum of the reactor vessel. ISB-VVER is a two-loop model of the VVER-1000. The elevations are kept 1:1, while the volume and power scales are 1:3000. The four loops of the original plant are modelled as one single loop with one and a triple loop with three parallel steam generators. During the experiments needle shaped conductivity probes of the Recearch Center Rossendorf were applied for void measurements. The tests have shown that in case of a leak in the upper plenum, the cooling capability of the high-pressure injection system strongly depends on the location of the injection. The most favourable condictions for the core cooling are achieved by the injection in both the cold and the hot leg simultaneousely. In this case, an 11 % break in the upper plenum can be safely ruled over without fuel rod overheating by operating the HPIS only. In some cases, oscillations of void fractions and water levels were found.