Fukushima Daiichi suffered three core meltdowns and one reported spent fuel pool fire (unit 4).
Nuclear fallout from the explosions and fires significantly increased the world’s “background” level of radionuclides: “During the passage of contaminated air masses from Fukushima, airborne 137Cs levels were globally enhanced by 2 to 3 orders of magnitude.”[i]
Fukushima precipitation in March 2011 immediately after the Fukushima accident was more than 3 orders of magnitude higher than the background level of this region [ii]
Newer research has documented that 75 percent of the radiation released by the plant into the atmosphere occurred after March 15 because of ongoing melting of hot fuel in the cracked spent fuel pools,[iii] with subsequent episodes of increased atmospheric levels of emissions from Fukushima in November and December of 2011, April of 2012, and September of 2012, after which the Japanese researchers discontinued sampling.[iv]
The severe dangers of spent fuel pool fires in the US documented in a 2005 report by the National Academy of Sciences:
Committee on the Safety and Security of Commercial Spent Nuclear Fuel Storage, National Research Council, “Safety and Security of Commercial Spent Nuclear Fuel Storage,” (2006). Washington, DC: National Academies Press.A new study by the Union of Concerned Scientists indicates that a fire at a US nuclear spent fuel pool could dwarf Fukushima because of the amount of fuel stored in US pools:
Richard Stone. May 24, 2016. Spent fuel fire on U.S. soil could dwarf impact of Fukushima. Science Magazine, http://www.sciencemag.org/news/2016/05/spent-fuel-fire-us-soil-could-dwarf-impact-fukushima
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.
Here is a blast from the past: Blue Glow and Fire(?) at Fukushima: