because they were not supposed to happen because accidents in which the Containment Vessels may be damaged were not supposed to happen and the Agency didn't even plan for such accidents.
What may be the common sense among the general population is not at all common sense among politicians and bureaucrats: that Japan is prone to big earthquakes which could trigger big tsunamis (as it did on March 11), and it may not be a good idea to have an aging nuke plant right by the ocean facing the deep, subduction zone called the Japan Trench.
Yomiuri Shinbun (10:27PM JST 4/8/2011) reports:
It was revealed by METI's Nuclear and Industrial Safety Agency on April 8 evening that the hydrogen explosions that took place in the Reactor 1 and the Reactor 3 of TEPCO's Fukushima I Nuclear Power Plant were not what the Agency had anticipated.
Hydrogen gas is generated when the water level within the Reactor Pressure Vessel goes down, exposing the nuclear fuel rods which then heat up.
However, according to the Agency, [the Agency didn't think there would be hydrogen explosions because] the Reactor Containment Vessels which surrounds the RPVs are normally filled with nitrogen gas and there is hardly any oxygen. However, the explosions occurred in the Reactor 1 on March 12, and in the Reactor 3 on March 14, in the Reactor buildings that house the Containment Vessels.
The Agency admitted, "By design, the hydrogen gas is not supposed to leak from the Containment Vessel. In the national safety evaluation, no design specs are called for to deal with a leak should the leak happen."
In other words, according to the Agency, there will be no leak because they are not supposed to leak, end of story. If they leak, well it is not the Agency's problem. Earthquake, tsunami, so what? As long as it is not on their books that Containment Vessels may leak, they won't leak.
Something like that.
What else they didn't plan for? That the pipes may be leaky? That the power will go out for an extended period of time? That concrete trenches may crack? That Spent Fuel Pools may lose water? That the radiation monitoring systems stop working without power? That tsunami sweeps away dosimeters (and possibly anti-radiation suits)? That people may become upset if the contaminated water is released without notice and consultation?
This Yomiuri reporting itself is not entirely accurate. The hydrogen explosions occurred in the Reactors 1 and 3, AND the Reactor 4, except in the case of the Reactor 4 it happened in the Spent Fuel Pool.
Look at any photo of the Reactor 4 after March 15, and consider the fact that there was no fuel in the Reactor Pressure Vessel (RPV) of the Reactor 4. Yet the top of the building blew out. Where did the hydrogen come from, that blew out the building? The Spent Fuel Pool. How was the hydrogen created? The only way would be for the water in the Spent Fuel Pool was spilled or the the Pool was cracked and water escaped (that's what AREVA suspects), and the fuel rods in the Pool were exposed enough for the zirconium cladding to burst and/or melt.