As the book, Heal and Protect: A Nuclear World, discusses often, the greatest threat is the spent-fuel stored onsite of nuclear reactors. From the book:

The spent-fuels resulting from nuclear energy production are one of humanity’s great threats and challenges. Once nuclear fuel rods have been used to create energy, they will still burn and have lasting, residual heat after they have served their primary purpose. Once irradiated, they are extremely radioactive and must be stored, cooled, and protected in large pools of water adjacent to nuclear reactors, often on the roof. These pools are constantly refreshed with water to keep the spent nuclear fuel from overheating and melting. If the cooling water boils off or leaks out, hundreds and thousands of spent fuel rods could melt into an unimaginable pile of smoldering nuclear fuel that could not be contained or extinguished. It would release more radiation than all the world’s past nuclear testing by far, and it would destroy and degrade life across the globe.

Here is an article from Science Magazine discusses this scenario in the United States. It takes a theoretical spent-fuel fire at Peach Bottom Nuclear Generating Station and shows what areas of the continental United States could be impacted based on time and weather patterns. It is sobering to say the least, and a reminder that we need to focus on securing all this spent-fuel that was never meant to be piling up at reactor sites with no long-term strategy for storage, re-use, or disposal.

Science Magazine, May 2016

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.

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.

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.