Icone23-2212 heat transfer tests for passive water-cooling system : (1) test plans using a tube bundle

We are developing the passive decay heat removal system for boiling water reactors (BWRs) for long term station blackout (SBO) accidents based on the lessons learned from the Fukushima Daiichi Nuclear Power Plant accident. Our objective is development of a new passive water-cooling system. The system is operated with the active safety systems already installed on exiting BWRs. The passive water-cooling system is capable of running without electricity for 10 days after a SBO to remove a relatively large amount of early stage decay heat from the reactor core. Steam from the reactor pressure vessel (RPV) is condensed in the condensation tubes of the condenser, and condensed water flows out into the suppression pool (S/P) in the primary containment vessel (PCV). The condensed water is pumped up and supplied from the S/P to the RPV by the reactor core isolation cooling system (RCIC) with a steam-turbine-driven pump. Water temperature at the condensation tube outlet is decreased to less than the saturated temperature of the maximum PCV design pressure under the originally filled nitrogen atmosphere to prevent the PCV from being damaged. The condenser is located at a low level, e.g., underground, for easy access and supply of cooling water to the condenser pool without electricity during a SBO. The low level condenser pool has an advantage of being seismically well-designed. We have already conducted heat transfer tests to obtain basic heat transfer data for the design of the condenser in the water-cooling system. In the tests, full-scale U-shaped single tubes with three kinds of diameters were used under a wide range of pressure and inlet steam velocity conditions. Furthermore, we plan to conduct additional heat transfer tests using a tube bundle with three rows and four levels of tubes to investigate particular phenomena such as the steam flow rate distribution among the tubes, enhancement of boiling heat transfer on the tubes due to convection flow and the dryout characteristic on the upper level tubes in a real condenser with tube bundles. Differential pressure transducers are installed to measure flow rate at representative tubes. Voids are injected from beneath the tube bundle to induce convection flow in the tube bundle and to simulate void fraction for several tube levels in a real condenser to confirm there is no dryout even under a high void fraction. The temperatures at the inner and outer tube walls are measured to estimate heat transfer for the tube bundle. In this paper, we introduce the test plans using the tube bundle for the passive water-cooling system which will clarify the tube bundle effects.