According to BBC News, and countless other media outlets reporting on the emergency currently underway at the Fukushima light water nuclear reactor plant,…
“Japanese officials fear a meltdown at one of the plant’s reactors after radioactive material was detected outside it.”
No they don’t. What officials fear is the venting of significant amounts of radioactive material into the local environment following a major explosion at the plant. A meltdown of the reactor core does not come into it, as that would be physically impossible with a light water reactor of this kind.
A catastrophic failure of the reactor’s cooling system will lead to a rise in temperature, and possibly a rise in temperature sharp enough to cause an explosion that destroys the plant, but the nuclear reaction in the core cannot continue under those circumstances. The core will not melt. It cannot.
What is happening at the Fukushima nuclear plant could be very serious, but it is not the doomsday scenario being painted by science-illiterate reporters and pundits. You do not need a degree in physics to understand what is happening at Fukushima. Even a cursory glance at Wikipedia should be enough for those with a few functioning brain cells.