But last year’s 9.0 earthquake and tsunami made things far worse, further destabilizing the local geology. According to recently revised estimates by the Japanese government, the probability of an earthquake of 7.0 magnitude or greater in the region during the next three years is now 90 percent. The Unit 4 reactor building that was substantially damaged by the tsunami and subsequent explosions will not survive a 7.0+ earthquake.
An earthquake of 7.0 or greater is likely to disrupt cooling water flow and further damage fuel storage pools possibly making them leak. If this happens the fuel rods will be exposed, will get hotter and eventually melt, puddling in the reactor basement and beneath the former storage ponds. This is a nuclear meltdown, which will lead to catastrophic (though non-nuclear) explosions and the release of radioactive gases, especially Cesium 137.
The next Japanese nuclear accident (it’s inevitable) will be even worse