One thing to keep in mind is that Unit 4 was de-fueled at the time of the accident. The explosion was caused by hydrogen leaking over from Unit 3. (See http://

One thing to keep in mind is that Unit 4 was de-fueled at the time of the accident. The explosion was caused by hydrogen leaking over from Unit 3. (See http://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disaster http://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disas... and look for "Reactor 4").