Heat removal capacity for a local air cooler without an active fan (semi-passive local air cooler) and its impact on a severe accident

Local air coolers (LACs) in CANDU nuclear power plants (NPP) has an electric fan to move containment vapours/gases across a set of water cooled finned tubes. LACs are one of the main heat sinks for a NPP, but a loss of electrical power or elevated containment temperatures would render the electric fans inoperable. However, if cooling water continued to flow, then the LACs can operate in a semi-passive mode (passive on the primary side of the LAC). Canadian Nuclear Laboratories has performed experiments in its Large Scale Containment Facility (LSCF) to study the cooling/condensing capacity for semi-passive Dome LAC operation [1]. The test Dome LAC is loosely based on the geometry of an ACR-1000 Dome® LAC, but is not scaled. The cooling coils are copper tubes with aluminum fins, having a flow cross sectional area of 38.1 cm by 50.8 cm with a depth of 19.8 cm. The tests results demonstrated that the LACs can operate in a semi-passive mode to provide 26% to 60% (depending on LAC orientation) of the cooling capacity of a fully functioning LAC at a bulk vapour temperature of 90°C, 100% relative humidity and atmospheric pressure. With this information in mind, a preliminary assessment was performed to assess the impact of semi-passive LAC operation on a postulated CANDU 6 Station Blackout with no moderator or shield cooling system, and no main or auxiliary feedwater available. With no LACs available in the containment, the severe accident progression would proceed as predicted [2], including calandria vessel boil off, containment pressurization and failure, and radionuclide releases to the environment. Containment temperatures continue to persist above the equipment qualification (EQ) level of 125°C for the LAC fan, meaning that even if Class III power was restored, the fans would not be able to start operating again. During the accident, however, about 51,000 kg of steam is added at an average rate of 2.3 kg/s. Restoring just the cooling water to the LACs, and having them operate at only 25% of the capacity of a fully functioning LAC, still offers the ability to condense steam at a rate of about 2.9 kg/s, which is more than the average steam release rate. Therefore, restoring cooling water can potentially prevent containment pressurization, can maintain containment integrity if started early enough, and can even prevent radionuclide emissions by reducing the steam pressure driving force. (author)