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More than a year after the triple meltdown at the Fukushima Daiichi power plant, the Japanese government, Tokyo Electric Power Company (Tepco) and the U.S. Nuclear Regulatory Commission (NRC) present similar assurances of the site’s current state: challenges remain but everything is under control. The worst is over.

But nuclear waste experts say the Japanese are literally playing with fire in the way nuclear spent fuel continues to be stored onsite, especially in reactor 4, which contains the most irradiated fuel — 10 times the deadly cesium-137 released during the 1986 Chernobyl nuclear accident. These experts also charge that the NRC is letting this threat fester because acknowledging it would call into question safety at dozens of identically designed nuclear power plants around the U.S., which contain exceedingly higher volumes of spent fuel in similar elevated pools outside of reinforced containment.

Reactor 4: The Most Imminent Threat

The spent fuel in the hobbled unit 4 at Fukushima Daiichi not only sits in an elevated pool outside the reactor core’s reinforced containment, in a high-consequence earthquake zone adjacent to the ocean — just as nearly all the spent fuel at the nuclear site is stored — but it’s also open to the elements because a hydrogen explosion blew off the roof during the early days of the accident and sent the building into a list.

Alarmed by the precarious nature of spent fuel storage during his recent tour of the Fukushima Daiichi site, Sen. Ron Wyden, D-Oregon, subsequently fired off letters to Secretary of State Hillary Clinton, Secretary of Energy Steven Chu, NRC Chairman Gregory Jaczko and Japanese ambassador to the U.S. Ichiro Fujisaki. He implored all parties to work together and with the international community to address this situation as swiftly as possible.