New Explosion at Fukushima Nuclear Power Plant

Fukushima Daiichi: Blast at #2 Reactor

An explosion was heard at the number-two reactor of the Fukushima Daiichi Nuclear Power Plant, which currently the scene of several possible core meltdowns. This is the third explosion reported at Fukushima Daiichi, which is managed by TEPCO (Tokyo Electric Power Co.). The latest explosion was heard at 6:10am on March 15, local time.

• Infographic: 3 Cores in Danger at Fukushima Daiichi
• Second Explosion Rocks Japan Nuclear Power Plant
  

Currently, ocean water is being pumped over the reactor cores, which are vaporizing the water. This method is preventing the cores from overheating further, rather than actively cooling them. The pumping mechanism failed at Reactor #2, causing the explosion. One theory is that the hydrogen explosion that hit Reactor #3 damaged the pumping mechanism f Reactor #2. The fuel rods at Reactor #2 had been exposed; TEPCO admitted that a meltdown was indeed possible. The Japanese government described the potential for a meltdown at Fukushima Daiichi to be "highly likely".

What is a Nuclear Meltdown?

Fukushima Daiichi shut down when the earthquake struck, as per safety precautions. However, the reactor cores were (and still are) incredibly hot, giving off what's called decay heat. The coolant used to mitigate that heat could not be properly applied to the superheated reactor cores. This leads to the risk of a meltdown, which is not necessarily synonymous with "the entire facility explodes".

For greater detail provided by actual nuclear scientists see this info sheet from Union of Concerned Scientists: Explaining the Japanese Nuclear Reactor Disaster (pdf).

Why is the Japanese Government Handing Out Iodine Pills?

One of the radioactive elements given off so far in the Fukushima disaster is Iodine-131. Once absorbed into the human body, it gathers in the thyroid, raising the chances of thyroid cancer later in life. By taking more iodine, the patient loads the thyroid with non-radioactive iodine. While this helps, it is not the same as not being exposed to radioactive iodine isotopes in the first place.

Iodine has a half-life of eight days. Cesium-13, however, has a half-life of 30 years, and is absorbed as potassium. This is far more dangerous to surrounding people and animals than the iodine-131, as it will take over a century to decay.