Alternate Failure Modes
I never feared an accident of the same magnitude as Chernobyl, because the Fukushima reactors do not use graphite in their core, and thus they lack the fuel for the massive fire that at Chernobyl carried radioactive particles in the atmosphere.
At Fukushima, limited amounts of radioactive elements were discharged in the air when steam was vented from the reactor(s) to relieve vessel pressure, and some escaped from spent fuel rods in a cooling pool when the water evaporated.
However, Murphy's Law is still valid, and more radioactive elements found a way out. Not to the atmosphere, but instead to the sea. From the April 6 update of IAEA:
TEPCO has identified a possible leakage path from the Turbine building of Unit 2 to the sea via a series of trenches/tunnels used to provide power to the sea water intake pumps and supply of service water to the reactor and turbine buildings. On 4 April, a tracer was used in an attempt to determine where the water was coming from. The tracer was also injected into two new bore holes that had been drilled near the pit. On 5 April it was confirmed that the tracer was seen leaking from the crack into the sea. Coagulation agents (liquid glass) were injected into the holes drilled around the pits to block leakage of water. It was reported that the leakage has currently stopped at 20:38 UTC on 5 April. Work continues to prevent further releases to the sea.So, higly contaminated water was able to seep into the ocean. But thanks to the relentless work of the personnel at the site, the leakage is stopped, at least for now.
How much radioactive materials escaped through this route, is hard to say. A lot of this is the not-so-harmful Iodine-131 which decays rather fast - but there is also cesium-137, which is instead rather persistant.
I am quite certain that this won't be the end of the world, neither of Japan as we know it - the material destruction of earthquake and tsunami are more serious issues, although they seem forgotten by the media.