A portion of an underground tunnel containing rail cars filled with radioactive waste collapsed Tuesday at a sprawling storage facility in a remote area of Washington state, forcing an evacuation of some workers at the site that made plutonium for nuclear weapons for decades after World War II. Officials detected no release of radiation at the Hanford Nuclear Reservation and no workers were injured, said Randy Bradbury, a spokesman for the Washington state Department of Ecology. No workers were inside the tunnel when it collapsed, causing soil on the surface above to sink up to 1.2 metres over a 38 square metre area, officials said. The tunnels are hundreds of metres long, with about 2.5 metres of soil covering them, the US Department of Energy said. The cause of the collapse was not immediately known. It was discovered as part of a routine inspection and occurred during a massive cleanup that has been under way since the 1980s and costs more than US$2 billion a year. The work is expected to take until 2060 and cost more than US$100 billion. Hanford officials said they were studying the area of the collapse to determine how to create a barrier between the contaminated equipment in the tunnel and the outside air. Workers near the site were evacuated and hundreds of others farther away were told to remain indoors for several hours, the federal agency said. Non-essential workers among the labour force of 9,000 at the site were sent home early along a safe route. “No action is currently required for residents of Benton and Franklin counties,” the Energy Department said, referring to the nearly 300,000 residents near the site about 320km southeast of Seattle. “There is no indication of a release of contamination at this point.” US Energy Secretary Rick Perry was briefed on the incident that Washington Governor Jay Inslee called it a serious situation. “Ensuring the safety of the workers and the community is the top priority,” said Inslee, a Democrat who previously represented the Hanford region in Congress. The anti-nuclear group Beyond Nuclear said the incident helped show “radioactive waste management is out of control.” Democratic US Senator Maria Cantwell of Washington said worker safety must be the priority. “My thoughts are with the first responders who are working to assess the situation on the ground,” she said. The accident Tuesday occurred at a plant known as the Plutonium Uranium Extraction Facility, or PUREX, located in the middle of the 13,000sq km Hanford site. Hanford for decades made plutonium for nuclear weapons and is now the largest US depository of radioactive defence waste, with about 200 million litres of waste, most of it in 177 underground tanks. As part of the huge, ongoing cleanup, rail cars full of radioactive waste were often driven into tunnels and buried. The Hanford site was built during World War II and made plutonium for most of the US nuclear arsenal, including the bomb dropped on Nagasaki, Japan, at the end of the war.