The incident at the Columbia Generating Station late Thursday afternoon resulted in no injuries or equipment damage, they said.
"It was like a little flash, like what you would see when lighting a barbecue," said Rochelle Olson, spokeswoman for the plant which is operated by Energy Northwest.
The nuclear reactor at the plant was shut down at the time for routine maintenance and refueling.
The plant, in Richland, Washington, about 210 miles southeast of Seattle, said the incident occurred in the main turbine building.
Workers were cutting a pipe after closing and purging it, when a small amount of residual trapped hydrogen gas was released and ignited, officials said in a statement, describing the release as a "less-than-one-second puff."
"It's such a minor event that it wouldn't be reported in any other industry," Olson added.