Restart of natural circulation in a PWR–PKL test results and s-relap5 calculations

Abstract At reduced primary water inventory in a pressurized water reactor (PWR) with heat transfer to the secondary by means of reflux-condensation (i.e. after a small-break loss-of-coolant-accident), condense with low boron concentration may accumulate in the primary system. The restart of natural circulation after the refilling of the primary can then lead to displacement of low borated water into the reactor pressure vessel and finally into the core, which can result in a recriticality of the reactor. The restart of natural circulation was investigated in the test facility PKL, which is operated by Framatome ANP GmbH (former Siemens/KWU) in Erlangen/Germany. The test results as well as the results of a post-test calculation using the computer code s-relap 5, which are both subjects of this presentation, demonstrated that despite symmetrical injection of emergency core coolant (ECC), the restart of natural circulation takes place at different times with varying intensities in the individual loops. In a PWR, this would favor the mixing of water flows with different boron concentrations in the reactor pressure vessel downcomer, which again would reduce the risk of a recriticality transient.