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Fission Stories #126: Draining Cooling Water from Reactor Vessels

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Shoreham’s Cameo Appearance

The Shoreham nuclear plant on Long Island, New York was cancelled after being built and tested at very low power levels. It generated more controversy than electricity.

Shoreham’s woes were 1argely confined to the political variety, but the plant managed to have an incident during its brief life. On July 26, 1985, operators prepared to test the low pressure coolant injection (LPCI) function of the residual heat removal (RHR) system.

Fig. 1 (click to enlarge)

The RHR system had been operating in its “shutdown cooling mode” prior to the test. In this mode, the RHR system takes water from one of the two recirculation loops connected to the reactor vessel, routes the water through a heat exchanger where it is cooled by the reactor building service water system, and returns the cooled water to the recirculation loop. The shutdown cooling mode removes the heat produced by decay of radioactive fission products in the irradiated fuel inside the reactor core.

Fig. 2 (Click to enlarge)

During an accident where large amounts of water have drained from the reactor vessel (such as occurs if a pipe connected to the reactor vessel ruptures), the LPCI mode draws water from the suppression pool inside containment and supplies it to the recirculation loop where it flows into the reactor vessel. Because the suppression pool water is not as clean as water inside the reactor vessel, the test procedure returns the LPCI flow to the suppression pool rather than injecting it to the reactor vessel. The red lines on the figure show the test lineup while the cyan line shows the LPCI path during an actual event.

Fig. 3 (click to enlarge)

The operator opened one of the LPCI valves before closing all of the shutdown cooling valves. As a result of this oversight, 7,500 gallons quickly drained from the reactor vessel to the suppression pool. The reactor vessel water level dropped just over four feet before the valve could be closed to stop the draining. Normally, the reactor core is covered with 16 feet of water inside the reactor vessel. During this incident, about one quarter of the water covering the reactor core was inadvertently drained.

Had the operators not taken steps to close the drain pathway, the water level inside the reactor vessel could have dropped until the top third of the reactor core was uncovered. It would have stopped at that level because it is the elevation of the flow from the reactor vessel into the recirculation loop that provides suction to the RHR pumps.

Peach Bottom Two Too

Two months after the Shoreham event, the staff at Peach Bottom Unit 2 outside Philadelphia, Pennsylvania tried the exact same procedure. The RHR system was in shutdown cooling mode and the operators realigned it for a LPCI test. They followed the Shoreham scenario to the letter. They inadvertently created a drainage path from the reactor vessel to the suppression pool. They managed to drop the reactor vessel water level by just over four feet, tying Shoreham’s mark.

And Dresden Makes Three

The Dresden Unit 2 reactor outside Chicago, Illinois was in a refueling outage on October 19, 1999.

 

Fig. 4 (click to enlarge)

Workers removed shield blocks, the drywell head and the reactor vessel head and then flooded the reactor well with water in preparation for removing old fuel assemblies from the reactor core to the spent fuel pool and replacing them with fresh fuel assemblies. Prior to flooding the reactor vessel and reactor well, workers installed plugs (i.e., nuclear-grade wine corks) in the four main steam lines to prevent water from filling the steam lines.

Fig. 5

After workers removed the plug from one of the main steam lines, the water level inside the reactor well began dropping. Operators in the control room started one of the core spray pumps (i.e., an emergency pump designed to supply about 5,000 gallons per minute to the reactor vessel in event of an accident) to recover the water level. Workers put the plug back into the main steam line.

Subsequent investigation revealed one of the safety relief valves on the main steam line was open. It should have been closed. The open relief valve allowed water to flow from the reactor vessel through the steam line out the relief valve and into the suppression pool.

The water level at Dresden only dropped about six inches, far short of the Shoreham and Peach Bottom marks. But Dresdeners deserve points for innovation – they found a novel way to mistakenly empty the reactor vessel into the suppression pool.

Had workers not replugged the main steam line and recovered the water level using the core spray pump, the water level could only have drain to the elevation of the main steam lines – still more than 20 feet above the top of the reactor core.

Brunswick Boo Boo

On September 24, 1984, operators at the Brunswick Unit 2 reactor south of Wilmington, North Carolina prepared to test the containment for leak tightness. The water level in the suppression pool needed to be lowered for the test. Thinking that the RHR system was running in the suppression pool cooling mode, an operator opened up a valve to send water from the RHR piping to the radwaste system. Plenty of water went to the radwaste system, but not from the suppression pool.

The RHR system was actually running in the shutdown cooling mode at the time. Instead of taking water from the suppression pool, cooling it, and returning it to the suppression pool, the RHR system was taking water from the reactor vessel, cooling it, and returning it to the reactor vessel as described in the Shoreham event above. By opening the RHR system valve, the operator drained water from the reactor vessel to the radwaste system.

The reactor vessel water level dropped to the point that caused an automatic reactor scram and an isolation of the primary containment isolation system. Since the plant was shut down at the time, the reactor scram had no effect other than to get everyone’s undivided attention. The primary containment isolation automatically closed the valve in the pathway to the radwaste system and prevented the reactor core from being uncovered.

Our Takeaway

All four of these events happened more than 20 years ago.

Ancient history?

Maybe, but new reactors are currently being constructed in the United States. These reactors have new features and different systems. If these reactors begin operating, there will likely be a trial and error period as workers find the new ways to replicate old miscues.

Hopefully, reminders about past mistakes will reduce both the frequency and severity of the new miscues. Otherwise, operation of the new reactors will provide fertile ground for growing new Fission Stories.

 

“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.

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About the author: Mr. Lochbaum received a BS in Nuclear Engineering from the University of Tennessee in 1979 and worked as a nuclear engineer in nuclear power plants for 17 years. In 1992, he and a colleague identified a safety problem in a plant where they were working. When their concerns were ignored by the plant manager, the utility, and the Nuclear Regulatory Commission (NRC), they took the issue to Congress. The problem was eventually corrected at the original plant and at plants across the country. Lochbaum joined UCS in 1996 to work on nuclear power safety. He spent a year in 2009-10 working at the NRC Training Center in Tennessee. Areas of expertise: Nuclear power safety, nuclear technology and plant design, regulatory oversight, plant license renewal and decommissioning

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