Safety improvement of MAPS containment isolation system based on reliability analysis

The containment isolation at MAPS is provided by two dampers in series in the supply and exhaust of RB ventilation system ducts. The pneumatically operated isolation dampers are activated by Boiler room pressure high signal or containment high activity signal through respective relay logics and solenoid valves. Scenario of one of the exhaust dampers closing spuriously during reactor operation and during the period of corrective action, the requirement of damper to be kept open was taken up for study. To review the Containment Isolating System (CIS) reliability Risk Spectrum Fault Tree (FT) methodology was used to determine the Probability of Failure on Demand(PFD) and spurious failure rates of CIS dampers. Suggested improvements in the logic design to reduce the PFD and spurious failure rate are discussed. (author)