During the scheduled shutdown to commence the Peach Bottom Unit 3 refueling outage, Unit 3 was manually shutdown using the mode switch in accordance with GP-3, 'Normal Plant Shutdown,' when reactor period lowered below 50 seconds as indicated on the WRNM [Wide Range Nuclear Monitoring] system. The
Feedwater Startup Level Controller was in automatic set at 23" when a small addition of cold water added enough positive reactivity to cause reactor period to be less than 50 seconds. The shortest period observed was 44 seconds. The WRNM system
RPS [Reactor Protective System]
automatic SCRAM setpoint is 19 seconds.
All control rods inserted into the core. Decay heat is being removed by shutdown cooling. The plant is continuing into its scheduled refueling outage.
The licensee notified the NRC Resident Inspector.
