Mechanical Over-Speed Testing of Nuclear Safety-Related Turbines Without Driving Steam
Author: Jack Little and Robert J. Stakenborghs
Steam turbines are used as drivers in several Safety-Related applications in commercial nuclear power. In Boiling Water Reactor units, they are frequently used to drive Reactor Core Isolation Cooling and, in some cases, High Pressure Coolant Injection. In Pressurized Water Reactor units, turbines are commonly used to drive Auxiliary Feedwater Pumps. All such turbines are equipped with a turbine Over-speed Prevention Device that prevents a catastrophic destructive over-speed failure of the turbine or its driven equipment. Due to insurance requirements, regulatory commitments and other considerations, the over-speed trip function must be tested, periodically. This testing has historically been performed in one of two ways: (a) performed during a unit outage, typically as part of the outage critical path or (b) performed off critical path, utilizing some alternate source of driving steam (i.e. with the reactor not critical).
This paper will describe an alternate overspeed testing methodology, which allows testing to be performed either as a non-critical path outage activity, or while the plant is operating at power, during a system maintenance window. Typical testing durations for at-power testing using the alternate method have required less than 25% of the out-of service time allowed by the USNRC. Required equipment, the scope of permanent plant modifications and several case histories for the alternate method will also be presented.