Two items from today’s Event Notification Report page on the NRC website:
1. Friday afternoon, the Seismic Monitor at Minnesota’s Prairie Island nuclear power plant indicated an earthquake had occurred. Prairie Island operators determined that the alarm was invalid, based on the lack of readings above the alarm setpoint and no seismic activity felt at the plant, as well as checks with the National Earthquake Information Center and the Monticello nuclear power plant, Minnesota’s other nuclear facility.
Following the determination that no earthquake had occurred, an odd engineering situation took place. In order to reset the Seismic Monitor, the monitor had to be taken out of service. This meant that for the time from the false signal to the reset, the plant had no functioning earthquake monitoring system:
“The monitor was declared non-functional since it would not generate a control board seismic event alarm during an actual event until the invalid event was reset. This monitor has no credited compensatory measure that will allow timely classification of two Emergency Action Levels (EALs), NUE (Notification of Unusual Event) and Alert classifications when out of service. This results in a Loss of Emergency Assessment Capability while the Seismic Monitor is out of service.”
With no compensatory measures in place, I wonder about the Seismic Monitor’s ability to register a series of earthquakes or aftershocks if it needs to be taken out of service whenever it needs to be reset.
2. Saturday morning, workers at the Millstone nuclear power plant in Connecticut removed the upper and lower boots on the main steam safety valves on the Unit 2 reactor as part of a planned outage. Unfortunately, the reactor was still operating at 100 per cent power. Upon discovery, Operations personnel declared the Enclosure Building inoperable. Maintenance re-installed the boots and the integrity of the Enclosure Building was restored and the building returned to service. The reactor is currently ramping down to complete the planned maintenance.
Surely, the operations manual says something about making sure the reactor is shut down before removing pieces of the Enclosure Building.