Experiments on boron dilution in the integral test facility, PKL

For many years the integral test facility PKL has been used for extensive experimental investigations to study the thermal-hydraulic system behavior of pressurized water reactor plants under accident conditions. PKL, which is operated in the Technical Center of Framatome ANP in Germany, replicates a 1300 MW PWR plant with elevations scaled 1:1 and diameters reduced by a factor of 12. (volume and power scaling 1:145). Since April 2001 the PKL project has been included in the international program SETH set up by the OECD with 15 participating countries. The current test series PKL III E is focused on boron dilution events following Small Break Loss Of Coolant Accidents (SB-LOCAs). For these tests, boric acid in the coolant and advanced instrumentation for the on-line measurement of the boron concentration in the Reactor Coolant System (RCS) is used. The importance of boron dilution scenarios for safety lies in the fact that the transport of water with a low boron concentration into the core could result in local recriticality and hence to power excursions. The assessment of various accident scenarios, which could initiate events involving boron dilution, showed that SB-LOCAs with reflux-condenser mode and subsequent initiation of natural circulation after refilling were of particular interest. This paper will summarize the most important findings from these tests focusing on one experiment that was performed under conservative boundary conditions with respect to the amount of accumulated condensate during the reflux-condenser phase