Enhanced safety margins under beyond-design-basis accident conditions for PWRs with eight accumulators. Transient studies in the PKL-test facility

The integral test facility PKL at Siemens/KWU in Erlangen, Germany, simulates a 1300 MWe Siemens-type PWR with four symmetrically arranged primary loops. It is equipped with eight accumulators. In the test PKL III D6.l a station blackout transient was simulated, which means loss of auxiliary power and simultaneous failure of the D1- and D2-emergency power systems. The loss of the heat sink led to primary heat-up, depletion of coolant inventory by discharge at the pressurizer safety valve and finally to core uncovery. When the core outlet temperatures (fluid) were 50 K above saturation, an accident management procedure was performed by a primary bleed-and-feed. For the bleed the pressurizer valve station was opened completely (corresponding to 100 cm2 in the PWR). The pressure decreased. At 26 bar a passive feed from the eight accumulators started. The injected water reestablished core cooling and accelerated the depressurization. The pressure decreased down to 10 bar. In this paper the test results are described and analyzed. They are compared to a PKL-test with only four accumulators at the hot legs assumed to be available, that was performed earlier. The comparison demonstrates an enhanced inherent safety of PWRs with eight accumulators as compared to those with four. (author)