RELAP5 analysis of pactel primary-to-secondary leakage experiment PSL-07

Primary-to-secondary leaks form a significant class of the loss-of-coolant accidents. In a primary to secondary leak, the damage is at a place where the containment will be bypassed if the safety valve of the broken steam generator opens. Eventually that can lead to a release of radioactive coolant to the atmosphere. Also, the emergency core-cooling water can escape from the system, which can become a serious problem because it prevents ECC water recirculation. Primary-to-secondary leaks can be divided into three categories; small, medium, and large PRISE. Usually a rupture of a single U-tube (small PRISE) causes the incidents, but the possibility of multiple-tube ruptures (medium PRISE) cannot be ignored completely. The VVER-type reactors have horizontal steam generators where the primary collectors are inside the secondary pool. Construction like that makes also large PRISE's possible. Multiple-tube ruptures and collector breaks may cause the core to be uncovered if sufficient ECC water is unavailable. With certain operator intervention, the flow from the primary to the secondary side may reverse. In such a case, a plug of partially diluted or completely unborated water may be formed on the primary side. If the mixing on the primary side is poor the plug can reach the core and lead to a reactivity accident. Data from several experimental and analytical studies exist for both single- and multiple-tube ruptures. However, the published experiments have focused on the facilities modeling western-type reactors. For the VVER reactors the only published experiment is the SPE-3 on the Hungarian PMK-NVH test facility. The first series of steam generator multiple-tube rupture experiments on PACTEL was carried out in June 1994. Those three experiments focused on a possibility of the leak flow reversal occurring as a consequence of operator intervention. The results showed that the reversal is possible under suitable conditions. In the first series, the facility included two steam generators and one accumulator. A new set of experiments was carried out in February 1996 with three steam generators and two accumulators. The main goal was to find the worst possible conditions during which the leak flow reversal can occur. (orig.)