Fission Stories #113: Lake LaSalle: A Leaky Joint with Big Consequences

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On May 31, 1985, an operator ventured out to the pump house at the LaSalle nuclear plant near Seneca, Illinois to investigate why one of the circulating water pumps for Unit 1 had unexpectedly stopped running. The pump house is located by the lake. The circulating water pumps supply lake water to the main condenser to cool the steam exiting the turbine.

The operator found the pump house’s basement filling with water. He traced the leakage to a broken rubber expansion joint between the circulating water pump and its discharge valve. The leak could not be isolated, so the pump house flooded until its water level reached about 15 feet, matching the lake’s level.

Water intake at LaSalle nuclear plant

The flooding disabled all the circulating water and service water pumps for both reactors at LaSalle.

The loss of the service water pumps impaired the instrument air system, the primary containment cooling system, and the turbine lubricating oil cooling system. The instrument air system provides compressed air to plant equipment like the air-operated valves for the rapid insertion of control rods into the reactor core. The primary containment cooling system removes heat from the drywell, protecting electrical equipment and other components from damage caused by excessive temperature. The turbine lubricating oil cooling system removes heat from the oil supplied to the main turbine’s bearings.

The NRC estimated that the equipment disabled and impaired by this flooding event increased the chances of reactor core meltdown by about a factor of 10.

Our Takeaway

Nuclear plants are supposed to be single failure proof. In other words, a single component can fail or an operator can make a single mistake without affecting the safety of the plant. The single expansion joint failure at LaSalle disabled cooling water systems for both units. So much for single failure proof.

The flooding at LaSalle occurred 96 years to the day after a more tragic flood. On May 31, 1989, the Connemaugh River Dam burst, sending a 75-feet high wall of water down the valley into the city of Johnstown, Pennsylvania. Nearly 2,300 people died. Prior to this disaster, an engineer named John Fulton had examined the damn and reported safety defects to the dam’s owners. Unfortunately, his warning went unheeded.


“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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  • Richard

    David McCullough wrote a book about the Johnstown flood. He noted that Fulton was not the first, let alone the only, person to have warned people about the dangers of this happening. The first warnings were made more than 20 years before the dam broke and the flood happened.

    Then, as now, people ignored such information because it meant ‘inconvenience’ to their daily lives. Or because it would disrupt the lives of the rich and famous steel mill owners and railroad men of the era who benefited from having a lake to fish on. The townspeople paid a very dear price for this. The powerful men got away pretty much unscathed from it all because the Courts did little about it.

  • Eric

    Correction: That should be 1889, not 1989 for the date of the flood.

  • pat b

    it seems many of these nuclear plants either have single fault failure modes all over them, or they have
    correlated risk all over.