Operating Experience With Mercury-Vetted Relays at Ontario Hydro's Nuclear Generating Stations

Abstract

Mercury-Wetted Relays (MWRs) are used extensively in the implementation of control logic for safety and safety-related systems in CANDU nuclear power plants. These relays are used in a range of applications including the two-out-of-three loqic for the shutdown systems (SDSl,SDS2), the emergency coolant injection system (ECIS) and the containment system. In some cases, these relays are also used to directly initiate end device functioning such as shut-off rod release or poison injection valve opening. Features such as speed of operation, high current carrying capacity, high reliability for multiple operations and compactness have led to the selection of MWRs in the above applications. This paner reviews Ontario Hvdro's operating experience, relay failure mechanisms, the issue of obsolescence for MWRs, and the actions taken to improve the reliability of the systems utilizing MWRs.