Risk Assessment of Nuclear Steam Turbine Destructive Overspeed

In electric power generation facilities, steam turbine disk rupture due to overspeed can produce high-energy projectiles that may penetrate the turbine casing and damage other plant systems. While such projectiles could seriously damage a fossil fired plant, the risks are even greater for nuclear plants where a projectile could strike safety related systems. Plant maintenance personnel routinely test overspeed protection systems according to the manufacturer’s recommended schedule, but such tests require interruptions in normal steady state operation, and sometimes lead to undesirable events such as reactor SCRAMs. This risk evaluation of steam turbine destructive overspeed for nuclear power generation units reexamines the probability of an overspeed protection system failure, with the goal of optimizing maintenance practices and providing a basis for overspeed trip system testing intervals. Industry reliability databases and plant maintenance records are utilized to develop component and system failure probabilities and models that account for the impact of condition monitoring and periodic testing on overspeed protection system availability.