Considering Reactor 3 was the one most devastated following its explosion earlier this week, this could be a truly disturbing development. This is especially true consider Reactor 3 is the only one at Fukushima whose fuel rods contain Plutnoium.
As a reminder:
If the reactor pressure vessel has indeed exploded, this is actually very bad. Fukushima is a light-water, boiling water reactor system, and unless the designers had decided to build these reactors differently, then this means that the reactor vessel is quite probably exposed to the air, albeit buried under debris from the collapsed pressure vessel. Light-water BWR reactors use light (regular) water as a moderator to enable the fission reaction in the core (and is thus exposed to the reactor vessel), of which is brought to a boil by the heat from the reaction, then the steam resulting from this directly drives the turbines to produce electricity. An explosion in this system would mean the release of, at the very least, mildly radioactive steam, with quite probably the majority of the volume of the water escaping. Officials are reporting that the reactor vessel is still intact: If this is true, then at least the fuel and it's by-products are still contained.
What makes it even more disturbing is that Reactor 3 is the one containing traces of Plutonium:
The fuel rods at all six reactors at the stricken Fukushima Dai-ichi complex contain plutonium — better known as fuel for nuclear weapons. While plutonium is more toxic than uranium, other radioactive elements leaking out are likely to be of greater danger to the general public.
Only six percent of the fuel rods at the plant's Unit 3 were a mixture of plutonium-239 and uranium-235 when first put into operation. The fuel in other reactors is only uranium, but even there, plutonium is created during the fission process.
This means the fuel in all of the stricken reactors and spent fuel pools contain plutonium.
Plutonium is indeed nasty stuff, especially damaging to lungs and kidneys. It is also less stable than uranium and can more easily spark a dangerous nuclear chain reaction.