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.
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.
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.