On April 19, 1984, eight workers entered the seal table room at the Sequoyah Unit 1 facility in Tennessee to clean the incore probe thimble guide tubes.

The incore probe is a neutron detector on the end of a long, flexible cable that is normally stored outside the reactor core. Periodically, the probe is inserted through a number of thimble guide tubes located within the reactor core to determine the power distribution.

The workers disconnected the thimble tubes and inserted a cleaning brush. While the brush was being hand-cranked up one of the thimble tubes, workers noticed water leaking around the fitting holding the guide tube.

The eight workers immediately evacuated the seal table room. Seconds later, the fitting broke loose, ejecting the entire thimble guide tube and cleaning assembly from the reactor core. An unisolatable reactor coolant leak ensued. The hot reactor water flashed to steam as it shot into the room. The leakage, initially about 30 gallons per minute, continued for approximately eleven hours until the reactor was shut down and the reactor water level was reduced to below the seal table. Approximately 16,000 gallons of reactor coolant leaked into the containment during this period.

Radiation surveys conducted the following day indicated two to three rem per hour at the entrance to the seal table room, 200 to 300 rem at the end of the thimble tube near the seal table and greater than 1,000 rem in the center of the ejected tube. The tip of the thimble tube was reading approximately 4,000 rem. [27] A lethal radiation dose is 450 to 600 rem.

The investigation of this incident revealed how close it came to being a disaster. While the workers were inside the seal table room, another group of workers arrived to perform maintenance on an airlock door, which happened to be the outer door on the airlock into the seal table room. They disabled the outer door for nearly 30 minutes while doing some welding. With the outer airlock door disabled in the open position, the inner door was interlocked closed. The workers inside the room wouldn't have been able to leave the room quickly during this time. The steam conditions and the radiation levels inside the room would have threatened their lives.

When the workers got into the airlock, one of the workers picked up the telephone to inform the control room operators about the leaking reactor water. The phone line was dead. [28]

Luckily, only the phone line was dead this time.

Explosive Subject

On Halloween, 1996, the NRC reported the following event for the Davis-Besse plant in Ohio:

"On October 30, 1996, plant personnel discovered what appeared to be undetonated explosive ordnance in the owner controlled area. The ordnance involved 11 shells/projectiles that were found in a marsh area near the edge of Lake Erie, the closest shell being approximately 500 yards from the plant's [allegedly] protected area. The ordnance was identified during an emergency preparedness drill when a radiation monitoring team (RMT) entered the marsh area to collect samples and noted the shells/projectiles. The licensee postulates that the ordnance was from nearby Camp Perry's artillery test range. Since World War II, artillery has been fired from Camp Perry to a target area in Lake Erie a short distance from Davis-Besse. As a result, shrapnel and unexploded ordnance occupy the lake bottom near the plant site. Because of lake currents, etc., some of the submerged ordnance has gradually shifted towards the shoreline. In addition, due to high wind conditions at the time of discovery, Lake Erie water level had receded sufficiently to expose the subject ordnance. Similar findings along the lakeshore or in the nearby Toussaint River have been made in the last several years.

"Following discovery of the shells/projectiles, plant security cordoned off the areas, conducted additional inspections of the shoreline, and restricted access to the marsh. The explosive ordnance disposal (EOD) unit at Wright-Patterson Air Force Base was contacted and a group of explosives experts were dispatched to disposition the discovered items. Upon arrival, the EOD unit was able to characterize the 11 rounds as follows: 2-106mm, 2-155mm, and 7 bazooka (rocket) type rounds. The ordnance was subsequently detonated in 5 separate explosions, indicating that at least several of the rounds were live." [29]

Maybe nuclear plant owners should bring in those metal detector guys from the beaches to check around their sites. It also seems reasonable to ask that proposed sites for nuclear plants be checked for buried ordnance before construction begins.

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