Six Japanese nuclear reactors are now facing serious cooling problems in the wake of Friday's earthquake and tsunami, with officials saying that partial meltdowns are presumed to have already occurred at two of them. So how much worse could it get?

If the temperature inside the Fukushima reactor vessel continued to rise even more - to roughly 4,000 degrees Fahrenheit (2,200 Celsius) - then the uranium fuel pellets would start to melt.

According to experts interviewed by The Associated Press, any melted fuel would eat through the bottom of the reactor vessel. Next, it would eat through the floor of the already-damaged containment building. At that point, the uranium and dangerous byproducts would start escaping into the environment.

At some point in the process, the walls of the reactor vessel - 6 inches (15 centimeters) of stainless steel - would melt into a lava-like pile, slump into any remaining water on the floor, and potentially cause an explosion much bigger than the one caused by the hydrogen. Such an explosion would enhance the spread of radioactive contaminants.

Yikes.

Meanwhile, police in tsunami-ravaged Miyagi prefecture now say that the death toll there could surpass 10,000.