Small-break LOCAs. Analysis, control and experimental results

With respect to small breaks, attention has been focused on two items: sufficient injection rates to assure adequate water inventory for decay heat removal; guaranteeing decay heat energy release through the availability of an additional heat sink. In KWU PWRs control of small breaks is guaranteed by automatic measures in the short term and by manual measures in the longer term. Besides the design of the safety injection pumps a main feature is the immediate shutdown of the secondary side to obtain an additional heat sink. The different energy transport modes from the core to the steam generators have been verified by tests in the PKL test facility. The PKL test facility represents a typical KWU 1300-MW(e) 4-loop PWR at a model scale of 1:134. In view of the importance of the driving gravity forces for natural circulation all heights and elevations correspond to actual reactor dimensions. The simulated loops have active steam generators whose secondary sides can be depressurized. In steady-state tests the behaviour of the system with subcooled and two-phase flow natural circulation as well as the energy transport from the core to the steam generators in the reflux condenser mode was investigated. The results proved that the decay heat could be removed from the primary system via the steam generator secondary side by natural circulation and with even better heat transfer in the reflux condenser mode. The analytical and experimental investigations have shown that in KWU PWRs the measures taken provide assured control of all conceivable break sizes. (author)