On March 11, 2011, the northeast coast of Japan was shaken by a massive undersea earthquake rated 9.0 on the Richter scale and the resulting tsunami. This was naturally the most intense earthquake in modern history that Japan has faced.
This mega undersea earthquake caused a huge tsunami, which hit the shore some 46 minutes later, causing ample flooding along the coastline. Some towns and market complexes were hit with this natural 'twin' disaster (earthquake plus tsunami), also affecting some nuclear plant sites, namely Tokai, Higashi Dori, Onagawa, Fukushima Dai-ichi and Fukushima Dai-ni. Among the nuclear power plant sites that experienced this natural disaster, the Fukushima Dai-ichi nuclear power plant site - a nuclear involved of six reactors - was the most severely affected, with more than 14-meter-high tsunami waves lashing this complex.
Nuclear Reactor
Actually, even this massive earthquake did not damage the Fukushima nuclear plant. After all, nuclear plants are built to withstand severe earthquakes - and that is exactly what happened here, too. The nuclear plant did not suffer structural damage because of the earthquake. Nevertheless, the huge earthquake caused loss of electricity from the grid to the plant. At the time of the natural disaster, there were three reactors under doing at the plant site, while three were under maintenance. Immediately, upon loss of off-site power, the reactors were automatically brought to a safe shutdown state, as designed, and core reactor cooling was established with power from onsite emergency diesel generators. But the tsunami that followed soon flooded the plant premises, knocking out the onsite emergency diesel generators, too. It was only after this second blow that the things turned tough.
The decay heat emitted by the fission products is a very small fraction of the reactor full power, but this is still a essential number of heat, which needs to be dissipated away. Efforts were initiated for implementing alternative means to achieve cooling for the reactor cores as well as for spent-fuel warehouse pools. The residual heat in the reactor core during the intervening duration caused elevated temperatures. At higher temperatures, there was a essential build-up of steam pressure. To relax inordinate pressure, some controlled venting was resorted to. The higher temperatures also caused metal-water reaction, important to the formation of hydrogen gas, which exploded in the prevailing heat to cause the damage to the roof of the outer concrete containment structure, while inadequate cooling also lead to a partial melting of the fuel rods. This resulted in some publish of radioactivity, which is more of a diffused nature - but there has not been an "en masse" publish of radioactivity.
The hydrogen-gas explosion was a 'chemical' reaction, and Not a 'nuclear' explosion as misunderstood by many. Also, it should be noted that the fabricate of nuclear reactors is such that they cannot explode like nuclear bombs. In order to understand nuclear power better, it is essential to know the facts.
One more distinguished fact is that even this nuclear plant of old vintage (almost 40-year-old) handled this unprecedented earthquake perfectly well until the huge tsunami arrived. That means if this natural disaster had only been a land-based earthquake, with no together with tsunami, then the onsite emergency power generators would not have suffered and this Fukushima disaster would not have occurred at all!
A uncomplicated (and Brief) Explanation of What Happened at Fukushima
