A fire from spent fuel stored at a U.S. nuclear power plant could have catastrophic consequences, according to new simulations of such an event.

A major fire “could dwarf the horrific consequences of the Fukushima accident,” says Edwin Lyman, a physicist at the Union of Concerned Scientists, a nonprofit in Washington, D.C. “We’re talking about trillion-dollar consequences,” says Frank von Hippel, a nuclear security expert at Princeton University, who teamed with Princeton’s Michael Schoeppner on the modeling exercise.

The revelations come on the heels of a report last week from the U.S. National Academies of Sciences, Engineering, and Medicine on the aftermath of the 11 March 2011 earthquake and tsunami in northern Japan. The report details how a spent fuel fire at the Fukushima Daiichi Nuclear Power Plant that was crippled by the twin disasters could have released far more radioactivity into the environment.

The nuclear fuel in three of the plant’s six reactors melted down and released radioactive plumes that contaminated land downwind. Japan declared 1100 square kilometers uninhabitable and relocated 88,000 people. (Almost as many left voluntarily.) After the meltdowns, officials feared that spent fuel stored in pools in the reactor halls would catch fire and send radioactive smoke across a much wider swath of eastern Japan, including Tokyo. By a stroke of luck, that did not happen.

But the national academies’s report warns that spent fuel accumulating at U.S. nuclear plants is also vulnerable. After fuel is removed from a reactor core, the radioactive fission products continue to decay, generating heat. All nuclear power plants store the fuel onsite at the bottom of deep pools for at least 4 years while it slowly cools. To keep it safe, the academies report recommends that the U.S. Nuclear Regulatory Commission (NRC) and nuclear plant operators beef up systems for monitoring the pools and topping up water levels in case a facility is damaged. The panel also says plants should be ready to tighten security after a disaster.

At most U.S. nuclear plants, spent fuel is densely packed in pools, heightening the fire risk. NRC has estimated that a major fire at the spent fuel pool at the Peach Bottom nuclear power plant in Pennsylvania would displace an estimated 3.46 million people from 31,000 square kilometers of contaminated land, an area larger than New Jersey. But Von Hippel and Schoeppner think that NRC has grossly underestimated the scale and societal costs of such a fire.

NRC used a program called MACCS2 for modeling the dispersal and deposition of the radioactivity from a Peach Bottom fire. Schoeppner and Von Hippel instead used HYSPLIT, a program able to craft more sophisticated scenarios based on historical weather data for the whole region.

Nightmare scenarios

A simulated spent fuel fire at the Peach Bottom nuclear power plant in Pennsylvania had a devastating impact on the mid-Atlantic region. Click on the dates to see the extent of contamination, which depended on weather patterns. Courtesy of F. Von Hippel and M. Schoeppner

1 Jan. 2015 1 April 2015 1 July 2015 1 Oct. 2015

In their simulations, the Princeton duo focused on Cs-137, a radioisotope with a 30-year half-life that has made large tracts around Chernobyl and Fukushima uninhabitable. They assumed a release of 1600 petabecquerels, which is the average amount of Cs-137 that NRC estimates would be released from a fire at a densely packed pool. It’s also approximately 100 times the amount of Cs-137 spewed at Fukushima. They simulated such a release on the first day of each month in 2015.

The contamination from such a fire on U.S. soil “would be an unprecedented peacetime catastrophe,” the Princeton researchers conclude in a paper to be submitted to the journal Science & Global Security. In a fire on 1 January 2015, with the winds blowing due east, the radioactive plume would sweep over Philadelphia, Pennsylvania, and nearby cities. Shifting winds on 1 July 2015 would disperse Cs-137 in all directions, blanketing much of the heavily populated mid-Atlantic region. Averaged over 12 monthly calculations, the area exposed to more than 1 megabecquerel per square meter -- a level that would trigger a relocation order -- is 101,000 square kilometers. That’s more than three times NRC’s estimate, and the relocation of 18.1 million people is about five times NRC’s estimates.

NRC has long mulled whether to compel the nuclear industry to move most of the cooled spent fuel now held in densely packed pools to concrete containers called dry casks. Such a move would reduce the consequences and likelihood of a spent fuel pool fire. As recently as 2013, NRC concluded that the projected benefits do not justify the roughly $4 billion cost of a wholesale transfer. But the national academies’s study concludes that the benefits of expedited transfer to dry casks are fivefold greater than NRC has calculated.

“NRC’s policies have underplayed the risk of a spent fuel fire,” Lyman says. The academies panel recommends that NRC “assess the risks and potential benefits of expedited transfer.” NRC spokesperson Scott Burnell in Washington, D.C., says that the commission’s technical staff “will take an in-depth look” at the issue and report to NRC commissioners later this year.