Nature has a great look at what’s going on inside the Japanese nuclear reactor damaged by the tsunami:
So, without emergency cooling, the temperature at the core of both reactors began to rise. As it did, what water that remained began to boil off, increasing the pressure inside the pellet-shape pod.
When temperatures reached around a thousand degrees Celsius, the zirconium alloy holding the fuel pellets probably began to melt or split apart. As it did, it reacted with the steam and created hydrogen gas, which is highly volatile.
Operators may or may not have known what was happening when they decided to release some of the pressure from Unit 1 on Saturday. The hydrogen apparently caused a massive explosion which blew the roof off of the fuel hall, though the reactor’s primary containment vessel appears to have remained intact….
If, as it appears, the zirconium came apart, then some of the uranium and plutonium pellets in the fuel rods may have become loose or melted and sunk to the bottom of the pressure vessel. In that case, the cores of units 1 and 3 are now a volatile test tube filled with radioactive fuel, melted zirconium and water.
The real danger is the fuel. If enough fuel gathers at the bottom of the reactor, it could burn through the concrete containment vessel.
Diagrams at the link.
Also, another story on what they’re not telling us about radiation. The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) is keeping radioactive data under wraps because of bureaucracy, essentially. No one has told them to make the information public, so they aren’t.