U.S. Nuclear Fuel Pools

The spent fuel pools at Fukushima are currently the top short-term threat to humanity.

But fuel pools in the United States store an average of ten times more radioactive fuel than stored at Fukushima, have virtually no safety features, and are vulnerable to accidents and terrorist attacks.

If the water drains out for any reason, it will cause a fire in the fuel rods, as the zirconium metal jacket on the outside of the fuel rods could very well catch fire within hours or days after being exposed to air. See this, this, this and this. (Even a large solar flare could knock out the water-circulation systems for the pools.)

http://www.washingtonsblog.com/2012/04/nuclear-plant-35-miles-from-major-american-city-with-high-earthquake-risk-has-more-radioactive-cesium-than-released-by-fukushima-chernobyl-and-all-nuclear-bomb-tests-combined.html