Training Cited in A-Plant Leak : Rancho Seco Workers Taught Wrong System

A “significant” design difference between the Rancho Seco nuclear plant’s control system and the one used to train its operators may have contributed to last month’s leak of radioactive steam into the atmosphere, a U.S. Nuclear Regulatory Commission official said Saturday.

The design difference may have led the operators to believe they were bringing the Dec. 26 emergency under control when, in fact, their actions led to the rupture of a pump that caused the leak. The NRC has said the amount of radiation involved did not pose a health threat.

But the potential for similar--or more serious--operating mistakes elsewhere was raised by an NRC official, who noted that the training simulator in Lynchburg, Va., used by Rancho Seco workers is also used to train operators of other nuclear power plants also built by Babcock & Wilcox Co.

Power Failure

The Rancho Seco incident began when a power failure disabled the plant’s integrated control system, causing a sudden reduction in the amount of cooling water to the reactor, which then shut down.

As workers attempted to control the reactor’s temperature swings, they took steps that their training in Lynchburg indicated should have allowed them to bring the plant under control, according to Greg Cook, an NRC spokesman.

Instead, those steps actually destroyed seals on a cooling water pump and resulted in the spill of 450 gallons of reportedly mildly radioactive water into an auxiliary building.

Cook, in an interview Saturday, told The Times that the textbook response called for under simulated conditions had failed to take into account a “significant” difference in design.

“On the simulator, when they close a valve to one of those tanks, it automatically opens the valve to the other. . . . At Rancho Seco that doesn’t happen,” Cook said. As a result, the pump overheated and its seals were destroyed.

Contributing Factors

“That probably is one of the contributing factors to damaging that pump,” Cook said.

Asked if the simulator failed to accurately mirror the designs of other Babcock & Wilcox plants, Cook said, “That’s a big question. Probably so.”

He added: “These plants are big, complex. There’s a lot of systems in there, and if they make a change in one system, one of the things that utilities have to doggedly pursue is to make sure that simulators get changed . . . as soon as possible.”

Cook said the simulator is operated by the Babcock & Wilcox Co., which built Rancho Seco and several other nuclear power plants in the United States, including Three Mile Island in Pennsylvania, three reactors at a South Carolina plant and one near Toledo, Ohio. Attempts Saturday to reach company officials were unsuccessful.

Trained at Lynchburg

A spokesman for the Sacramento Municipal Utility District, Rancho Seco’s operator, confirmed Saturday that its workers were trained at the Lynchburg simulator.

But spokesman Brad Thomas said that to “the best of my knowledge” workers had been told of the design difference between the simulator and Rancho Seco before the emergency. He said Rancho Seco is now building its own training simulator, which will be ready within two years.

According to Cook, other actions taken by Rancho Seco engineers during the Dec. 26 emergency--which were unrelated to the design difference--subjected the reactor to sudden swings in temperature that could have weakened the reactor vessel’s eight-inch thick steel walls that contain the core’s lethal radiation. However, there apparently was no major damage to the vessel, according to a preliminary assessment.

Cook said that more than 30 minutes passed before operators at the plant, located 25 miles southeast of here, realized that a power failure that triggered the series of events leading to the leak was caused by two open circuit breakers that cut power to the plant’s integrated control system.

Runs Automatically

Normally, that system runs the power plant automatically, both monitoring operations and making minor adjustments.

But as the reactor warmed up due to an inadequate water supply, it automatically shut down. Then another supply of water quickly began flowing into the reactor and began to cool it too quickly.

With the power to the integrated control system out, workers tried to bring the plant under control by manually manipulating the valves to the various water supplies.

“Instead of trying to send operators out in the plant to manually close valves, which took some time and with which they had some trouble, they could have very quickly reestablished control from the control room (by closing the circuit breakers),” Cook said.

In their haste to bring the plant under control manually, Cook said, two workers entered the auxiliary building, where monitors had detected low levels of escaped radiation, without wearing respirators. The dose received by the workers, however, was well within safety limits, according to the Sacramento Municipal Utility District.