A Stanford University nuclear-security researcher wonders why officials left radioactive waste in a tunnel at the Hanford Nuclear Reservation.

Rod Ewing also told the Spokane Spokesman-Review it was "surprising" that mounds of dirt and pressure-treated timber were used to address the problem.

“How can waste be left in a tunnel? Whose idea was that?” Ewing said in an interview with the newspaper. “I’ve been to Hanford many, many times for conferences and things like that, and I don’t recall anyone saying that there was waste in tunnels underground. I can’t imagine why that would be the case.”

On May 9, part of a railcar tunnel collapsed near the Plutonium Uranium Extraction Plant. U.S. Department of Energy officials have claimed there was no release of radioactive materials.

The Hanford Nuclear Reservation is 400 kilometres southeast of Vancouver, B.C. 

On a site the size of the City of Seattle, it has 56 million gallons of untreated nuclear waste left over from the U.S. nuclear-weapons program.

The video below explains the scope of the problem and why it should be of concern.

"The current unfolding crisis at Hanford, the bursting barrel at Waste Isolation Pilot Plant  (WIPP) in New Mexico in 2014, and the exploding radioactive waste dump in Beatty, Nevada in 2015, show that radioactive waste management is out of control," Kevin Kamps, radioactive waste watchdog at Beyond Nuclear, said in a news release earlier this week.

Beyond Nuclear notes that the Hanford site was part of the Manhattan Project and was a "major supplier of military plutonium".

"It houses 177 storage tanks containing liquid radioactive sludges, some of which have been leaking radioactive effluent that could eventually threaten the Columbia River," the group states on its website. "Cleanup at the site did not begin until 1989."

According to Beyond Nuclear, the Hanford tunnel collapse may have been caused by vibrations from nearby road works.

The Centre for Public Integrity pointed out on its website that a 2015 report noted that this tunnel "had been seriously weakened and that a 'partial or complete failure' could expose individuals even 380 feet away to dancerous levels of radiation".

"No action was taken by the department in response, and earlier this month—the precise date remains uncertain because conditions at the site were not closely monitored—a portion of the roof collapsed at the tunnel, creating a 20-foot square hole," wrote Peter Cary and Patrick Malone. "Afterwards, the managers of the Hanford site were forced on May 9 to order 3,000 workers to shelter indoors. But instead of shoring up the beams inside the tunnel in question, they poured in 54 new truckloads of dirt."

The U.S. government is spending US$2 billion per year on a clean-up operation that's not expected to be finished for another 75 years.