What a double whammy? One of the three least affected Fukushima unit 3 succumbed to Hydrogen bubble induced blast, blowing out the concrete building today. If hear I it right someone failed to notice the fuel gauge and DG sets ran out of fuel. In two hours time due to lack of forced circulation of water in the reactor vessel, fuel rods became so hot that steam must have formed around the top side of these enriched Uranium , Zirconium claded fuel assemblies. Once temperature goes above 1204 Degrees C, the water vapour and Zirconium will violently react and generate Hydrogen. It is explosive gas and once mixed with Oxygen, there will be explosion. Oxygen will be formed as a result of Zr + water reaction itself along with Hydrogen. 4% Hydrogen is good enough to explode ferociously. That could be the cause for blast in second occasion.
In each of these blasts a large amount of radioactive material will escape into human environment. Iodine can reach Thyroid, Cesium in food plates. That will be awful for the population.
Second explosion at Fukushima proves a great error is design of the safety systems , back up DG failure and assessment of multiple failures of hardware , all in tandem.
The Big Lesson is that when there is Nuclear disaster, the common human errors, which otherwise would have no consequence can be devastating.