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Saturday, September 22, 2012

Three Mile Island Unit 1 shutdown: Details

There is a fair amount of press this morning being given to the September 20th shutdown at Three Mile Island Unit 1, so I'll cover the known details here.

Briefly, there's no risk to the public - or anyone at the plant, for that matter.  The shutdown was automatic, due to an equipment failure.  Let's look at the NRC Event Notification:



"On September 20th at 1416 EDT, Three Mile Island automatically tripped due to a flux to flow imbalance as a result of a trip of the 'C' reactor coolant pump. The cause of the trip of the 'C' reactor coolant pump is still under investigation.

"The electrical grid is stable and unit 1 is being supplied by offsite power. All control rods have fully inserted. Decay heat is being removed by main feedwater flow to both steam generators that are exhausting via the normal main condenser cooling loop under manual control. Preliminary evaluation indicates that all plant systems functioned normally following the reactor trip, except for automatic operation of turbine bypass valve control due to failure of the automatic control function to control precisely at setpoint. Three Mile Island remains stable in hot shutdown mode while conducting the post trip review. No radioactive releases were experienced as a result of this event.

"This event is reportable under 10 CFR 50.72(b)(2)(iv)(B), Reactor Protection System (RPS) actuation, and under 10 CFR 50.72 (b)(2)(xi) due to an information release to local officials. Both are four hour reports.

"The licensee notified the NRC Resident Inspector."

The licensee has notified the state and local governments, and will be making a media release.


As promised, the licensee, Exelon Nuclear, did make a press release, the content of which is below.



Londonderry Township, PA– Three Mile Island Generating Station automatically shut down at 2:20 p.m. EDT today. The plant responded as designed and remains in normal shutdown condition while operators investigate the cause of the shutdown.

During the shutdown, steam was released into the atmosphere, creating a loud noise heard by nearby residents.

TMI uses state-of-the-art equipment to continuously monitor plant systems and conditions. The plant is designed to automatically shut down when necessary. The automatic shutdown presents no risks to public health or safety.

Three Mile Island is located about 12 miles south of Harrisburg, Pa. The plant can generate 852 megawatts of carbon free power - enough electricity for about 800,000 homes. Electric customers will not be affected.


Looking at the reports, it's clear that the following events happened; the plant was on the grid at 100% power when one of the four reactor coolant pumps tripped, for reasons not yet known or explained.  When this happened, the automatic protective action of a reactor scram resulted; at this time, the turbine also tripped.  Because of a failure in the steam plant (outside the reactor building, not a part of the reactor plant itself) which caused the bypasses around the now-isolated turbine generator not to work properly, the steam generators (part of the reactor plant, inside the reactor building) built up pressure and the safety (or, relief) valves lifted to vent off the pressure.  This was the source of the "loud noise" reported by Exelon's press release.  The exact failure is more detailed in the NRC Event Notification, which does not mention lifting of the secondary reliefs.

Equipment failures like this do happen from time to time with all kinds of equipment, of course - not just nuclear power plants.  It's just far less often with nuclear plants because of the rigid scrutiny given to everything involved.  We're now waiting to find out why the coolant pump tripped, and that could be any number of things (mechanical failure of the pump, shorts in the wiring, a failed breaker or controller) so speculation at this point is rather a simplistic paper troubleshooting exercise for us on the outside until Exelon or the NRC release further details.  A second problem, which I was discussing with @Atomikrabbit yesterday, is that secondary plant issue that led to lifting of the reliefs.

Again, the only reason I'm reporting on this at Atomic Power Review is the fact that this morning I can see that there are almost 200 stories on this event, and it's important to get details out with credible speculation -- and ONLY credible speculation.

8:20 AM Eastern 9/22/2012

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