Operators Not Ready : Equipment, Mistakes Blamed for Accident

Federal investigators blamed faulty equipment and employee error for the nuclear power plant over-cooling accident that has crippled one of the nation’s largest privately owned utilities.

The incident started when low-voltage, direct-current power to a control room panel at the Rancho Seco plant was cut off for a reason unknown at the time and deadened instruments, according to Nuclear Regulatory Commission investigators.

With the “integrated control system” out, Sacramento Municipal Utility District operators were unable to control pumps and valves that send cold water into the reactor and let out hot water and steam, used to generate electricity.

An automatic built-in safety feature sent valves to half-open position and pumps to mid-speed. Reactor temperature climbed for 16 seconds because less water was pumped in.

The heat triggered another safety feature: Control rods automatically plunged around the core to halt fission in the reactor vessel. As water continued to flow into the core, the reactor cooled too quickly.

Workers scurried through the plant to close valves by hand because cooling such a steel vessel too quickly can cause cracks, which could drain the water that acts as a coolant and trigger a meltdown.

The overcooling did not last long enough to threaten a meltdown, but in a 24-minute period, the plant cooled 180 degrees--well beyond the approved 100 degrees an hour.

Human Error Indicated

Four workers independently checked the dead control panel but failed to see that circuit-breaker switches had tripped off. A senior operator, who collapsed later in the incident from hyperventilation, noticed and flipped them back on 26 minutes after the power failure, restoring remote control over water flows.

But a pump, left running after the flow to it was cut off by hand, burned out and spilled 450 gallons of radioactive water inside the reactor containment building. A trace evaporated through vents, carrying too little radiation outside the plant to be a hazard.

Two workers who entered the area were exposed to radiation, but at a level well below the annual amount permitted to nuclear employees.

An NRC investigative team concluded:

- Operators were not trained for that type of electrical outage, which left them surprised when the plant rapidly cooled.

- Poor power supply design left the panel vulnerable to small electrical changes that would cause it to turn off.

- Operators forgot they had an auxiliary panel that would have averted scurrying to turn valves manually.

- A backup system that was to have been installed in 1984 was never added to the plant.

- The overcooling was properly declared an “unusual event”--the lowest emergency category.