MDEP Common Position CP-EPRWG-03. Common position on EPR containment mixing

The EPR containment is a new design, different from many typical pressurised water reactor (PWR) containments in that it uses a two room design concept. Equipment rooms immediately surrounding the reactor coolant system (RCS) are isolated from the rest of the containment. Beyond this inner region, personnel access can be provided during certain maintenance tasks. Separation is provided by structures and closed portals to minimise radiation exposure in the accessible space areas. During power operation, the inaccessible areas inside containment experience higher temperatures and radiation than the accessible areas. The EPR design includes a number of features that promotes mixing. Heat transfer to the containment heat sinks is promoted by the CONVECT system. The CONVECT system consists of rupture foils, convection foils, mixing dampers, and related instrumentation and control equipment. Rupture foils and convection foils are placed in the ceiling of each steam generator compartment. More than half of the foils are convection foils. The mixing dampers are located in the lower part of the containment in the in-containment refuelling water storage tank wall above the water level. There are eight of these. Opening of the foils and dampers is designed to set up circulation patterns in both the accessible and inaccessible areas. The rupture foils are passive components which will burst open if the pressure differential on the foils exceeds a predetermined value. The rupture foils burst in either direction. The convection foils which are passive components, are rupture foils placed in a frame. The frame is kept in the closed position by a fusible link. Should temperature rise to a set level, the link will melt with a short delay, and the frame will swing open by gravity. The result is that a convection foil will open on a pressure differential and will also open if the compartment temperature reaches a certain level. The mixing dampers open either on a differential pressure signal between the accessible and the inaccessible areas or on a preset containment pressure signal. The containment pressure signal is set just above atmospheric pressure, assuring fast opening of the mixing dampers for most accident scenarios. A solenoid operates each mixing damper. When closed, the mixing damper is held in position by an electromagnet against a compressed spring. In case of a power failure to the solenoid of the electromagnet, the spring will drive the mixing damper open. When electric power is restored to the solenoid, it is again available for normal operation. The mixing dampers can be operated from the control room. The purpose of this paper is to identify common positions among the regulators reviewing the EPR accidents and transients in order to: 1. Promote understanding of each country's regulatory decisions and basis for the decisions, 2. Enhance communication among the members and with external stakeholders, 3. Identify areas where harmonisation and convergence of regulations, standards, and guidance can be achieved or improved, and 4. Supports standardisation of new reactor designs