The passive residual heat removal system (PRHRS) is designed to remove the core decay heat during the accident conditions when the active systems are not available. In the case of a normal shutdown of the SMART, the residual heat is removed through the steam generators by a turbine bypass system. During accident conditions, the coolant temperature of the primary system goes down to a certain lower level due to the heat transfer through steam generators that is attained by the natural circulation flow paths established in the primary and the secondary systems of the SMART. The PRHRS consists of four independent trains with 50% of the heat removal capacity for each train. Two trains are sufficient to remove the decay heat generated in the primary system after the reactor trip. Each train is composed of an emergency cool down tank (ECT), a condensation heat exchanger, a compensating tank (CT), and several valves, pipes, and instrumentations as shown in the Figure XX-1. The condensation heat exchanger consists of inlet and outlet headers connected with several straight tubes for the heat exchange with the inner diameter of 13 mm. The compensating tank is filled with the water and pressurized nitrogen gas, which can be used to make up the losses of initial inventory in the PRHRS. The system is designed to prevent core damage for 72 hours after the postulated design basis accidents without any corrective actions by operators.

Three natural circulation circuits are involved in the operation of the PRHRS. In case of design basis events, the main steam isolation valve (MSIV) and the main feedwater isolation valve (MFIV) are closed automatically according to the reactor trip signal. After the automatic opening of the cut-off valves (V1 and V2), a natural circulation path is established between the heat exchanger in ECT and the steam generator due to the density difference of the two elevations. The ECT is located high enough relative to the steam generator in order to retain the heat removal capability during the events by supplying sufficient driving forces to the natural circulation flow. In the primary system, after the RCP trip, a natural circulation path is established between the reactor core and the steam generators. The decay heat generated in the reactor core is transported to the steam generators by the natural circulation flow. The third natural circulation path is established around the heat exchanger inside the ECT. The heat carried by the natural circulation flow in the primary and secondary systems is transferred to the ultimate heat sink through the natural convection at the vicinity of the heat exchanger.

XIX — 3. Conclusions

The PRHRS provides an ultimate heat sink when the off-site power is not available during the design basis events. The reliability of the PRHRS is being examined at KAERI through a high temperature and high pressure thermal-hydraulic test facility, named VISTA (experimental Verification by Integral Simulation of Transients and Accidents). The VISTA is an integral test facility simulating the primary and secondary systems as well as the major safety-related systems of the SMART-P. The scale ratios of the VISTA relative to the PRHRS of the SMART-P are 1/1 by the height and 1/96 by the volume. The primary system of the VISTA consists of the reactor vessel with electrical heaters, the main coolant pump, the pressurizer, and the helical coil steam generator. They are connected with pipes for easy installation of the instrumentation and simple maintenance. The secondary system is designed to remove the primary heat source by employing a single train of the PRHRS. Preliminary investigations have been conducted on the natural circulation performance of the PRHRS and the primary system as well as the heat transfer characteristics of the heat exchanger in the ECT, by employing the VISTA facility.

[1] Accumulators constitute the design practices of the operating water cooled reactors.

[2] See Section 4 for characterization of the phenomena influencing natural circulation.

[3] See Section 4 for characterization of the phenomena influencing natural circulation

[4] The starting point for identification of phenomena associated with natural circulation was the OECD-CSNI report ‘Relevant Thermal Hydraulic Aspects of Advanced Reactor Design’ [26].