LOS ANGELES (AFP) - Workers on Wednesday (May 10) began to plug a hole inside a collapsed tunnel at a nuclear site in the western US state of Washington where an emergency was declared and thousands of workers placed on lockdown.

Federal officials said approximately 50 truckloads of soil will be used to fill the hole in the tunnel at the Hanford Site, a sprawling complex located about 275km south-east of Seattle.

The tunnel is one of two at the site containing radioactive waste from the time the facility helped build nuclear bombs, including the one that ended World War II.

"Overnight, several employees were on the scene out there," said Destry Henderson, spokesman for the Hanford Site's emergency operations centre.

"What they've done is prepare the area to begin filling the hole inside the Purex tunnel," he added, referring to the Plutonium Uranium Extraction Plant.

"They've laid some gravel down on which heavy equipment will operate today to begin placing the soil, slowly and methodically in the caved-in portion of that tunnel."

The Hanford facility has some 9,000 employees, most of whom were told to stay home on Wednesday, Henderson said.

The hole in the tunnel was discovered on Tuesday during a routine inspection, prompting management to order all employees on lockdown for several hours.

The nuclear site, which is twice the size of Singapore, was used to produce plutonium for the bomb that brought an end to World War II.

Its last reactor closed down in 1987 and since then employees have been working to clean up millions of gallons of leftover waste stored in aging tanks.

The cleanup is expected to cost more than US$100 billion (S$140 billion) by the time it is completed in 2060.