In which Hank explains nuclear power and the situation at the Fukushima Daiichi power plant in Japan.
In short, fission products continually decaying in both active and spent fuel rods create enough heat to melt themselves. The coolant systems that would prevent this from happening were disabled by the tsunami. Cracks in the fuel rods created an environment allowing hydrogen to form, which concentrated and then exploded several times destroying both the primary and secondary containment structures at at least one reactor.
The result is that the reactor needs to remain cooled or else radiation from the meltdown will be released to the environment in significant quantities. The people working to keep the reactor from overheating are risking their lives to do so, and we all hope they will be able to stabilize the reactor without harm.