A third explosion at the site appears to have damaged reactor 2's containment Wednesday, 16 March 2011
Reactor breach worsens prospects
A third explosion at the site appears to have damaged reactor 2's containment It appears that for the first time, the containment system around one of the Fukushima Daiichi reactors has been breached.
Officials have referred to a possible crack in the suppression chamber of reactor 2 - a large doughnut-shaped structure, also known as the torus, below the reactor housing.
That would allow steam, containing radioactive substances, to escape continuously.
This is the most likely source of the high radioactivity readings seen near the site.
However, an alternative possible source is the fire in reactor 4 building - believed to have started when a pool storing old fuel rods dried up.
When fuel rods reach the end of their useful life and are taken from the reactor, they still contain a lot of radioactive nuclei, which means they get hot.
Typically they will be placed in an indoor pond looking rather like a swimming pool, and can be kept there for decades before being sent for reprocessing or dry storage.
Without an inflow of water, what is in the pool will begin to evaporate - eventually leaving the rods dry and increasingly hot.
Whichever turns out to be the source, the International Atomic Energy Agency (IAEA) reports: "Dose rates of up to 400 millisievert per hour (mSv/hr) have been reported at the site."
Fuel rods in rapidly-drying pools may be the source of increased radiation levels Acccording to the World Health Organisation (WHO), the average person's exposure is 3mSv in a year.
A key question is whether this is just a transient spike or turns into a sustained release.
If a suppression chamber crack is the source, a spike is possible - and this is hinted at in local news reports, with the national agency Kyodo for example saying: "Radiation of up to nine times the normal level was also briefly detected" in Kanagawa prefecture, south of Tokyo.
Under normal circumstances, the suppression chamber stores a large volume of water that can be used to condense steam produced in the reactor.
The industry newsletter World Nuclear News reports that a "loud noise" came from the reactor chamber, and that "the pressure... was seen to decrease from three atmospheres to one atmosphere after the noise, suggesting p
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