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 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 possible damage".
This pressure drop is consistent with the chamber cracking and releasing steam.
If the chamber is now de-pressurised, it is likely that radioactive material will be ejected at a lower rate.
Improvised solutions If there is a positive side to this, it is that a crack could make the job of cooling the reactor easier.
At times, technicians have struggled to force water into the reactor vessel simply because the pressure inside was too great.
The downside is that the water will turn to steam more easily at a lower pressure.
The government is clearly concerned about the possible health impact of radioactive material escaping from Fukushima, with Chief Cabinet Secretary Yukio Edano recommending: "Please do not go outside. Please stay indoors. Please close windows and make your homes airtight."
Displaced residents of the Fukushima area were taken to evacuation centres There is as yet no comprehensive picture of readings across the wider region - and wind direction is a major factor.
The US National Institutes of Health notes that a dose approaching one sievert (1,000mSv) would certainly induce radiation sickness.
But in the 1986 Chernobyl disaster, some workers who received doses above this level are still alive.
Whether there is a straightforward relationship between exposure and cancer risk remains unclear, with some experts believing low levels do not have an impact.
It also depends which substances are involved. A major risk is when the body absorbs and stores substances such as radioactive iodine, which leads to prolonged, internal exposure.
In the meantime, the key task for workers at the plant remains to get enough water into the reactors - and, now, into the spent fuel pools - with the poor resources at their disposal.
Among all the news coming out of Fukushima from the Tokyo Electric Power Company (Tepco), the nuclear safety agency, and local and national authorities, there has hardly been a mention of new equipment arriving at the site.
That implies that technicians are still working with water pumps designed for putting out fires and an improvised technique based on the turbine hall's fire extinguishing system.
There have also been reports that most of the station's staff have been evacuated, with only about 50 remaining on site - though this remains to be confirmed, like so much else in the constantly changing picture.
http://www.bbc.co.uk/news/science-environment-12745186
Comments