State emergency officials said there was no release of radiation and no danger to the public. It was not immediately known when the Columbia Generating Station reactor would be restarted.

The failure triggered an alert in which state agencies prepared to respond if needed to help Benton and Franklin counties near the Hanford nuclear reservation.

But Brad Peck, spokesman for the reactor's operator, Energy Northwest, said the reactor was stable and the alert was canceled at 11:57 a.m., about two hours after it was declared.

The reactor, which produces power for the Northwest electricity grid, would remain out of service until crews determine what caused the problem, he said.

Energy Northwest spokeswoman Heather McMurdo said lights on a control panel showed that two of 185 control rods did not fully insert into the reactor during the shutdown.

The rods, which control the reactor's operation, were inserted manually about 10 a.m., she said.

Backup systems operated correctly and the alert could have been canceled when the control rods were manually inserted, but plant operators wanted to err on the side of caution, McMurdo said.

"It was conservative for us to have remained in an alert status," she said.

Rob Harper, spokesman for the Washington state Emergency Operations Center, said that although there was no threat to the public, the center at the National Guard's Camp Murray was activated, as called for under the plant's emergency plan. The center deactivated shortly before 1 p.m., he said.

The state Department of Health dispatched a field team to take air samples and soil readings as a precaution, he said.

State officials originally said the shutdown occurred during a test, but Nuclear Regulatory Commission officials later said it occurred during normal operations.

NRC spokesman Ken Clark in Atlanta said the reactor automatically shut down after a high-pressure indication about 9:25 a.m. It was then that equipment indicated some control rods were not fully inserted, he said.

Plant operators will try to determine what caused the high pressure indication and whether the control rods were slow to drive fully into the reactor core, or there were problems with indicator lights, Clark said.

Columbia Generating Station is a boiling water reactor that produces 1,150 megawatts of electricity, which is sold to the Bonneville Power Administration.

Formerly known as the Washington Public Power Supply System No. 2 reactor, it is the only one of five reactors started in the late 1970s to be completed before construction was halted in 1982-83.

Facilities licensed by the NRC have four classes of emergencies in order of increasing severity.

An alert is the second level. When an alert is declared, events are in process or have occurred which involve an actual or potential substantial degradation in the level of safety of the plant, according to an NRC Web site.