12 March 2011
Satilite image of Fukushima Dai-ichi nuclear facility
Copyright DigitalGlobe
We remain extremely concerned for the safety of workers at and the community surrounding damaged nuclear plants in Fukushima, Japan.
Here's what we know abut the situation at the two most affected plants:
Fukushima I (Daiichi)
This morning, the water level in unit 3 sank three meters below the top of the fuel rods, exposing them almost fully. The fuel elements overheated and are damaged, but, according to Japanese authorities, full meltdown has been avoided for now.
There is, however, hydrogen buildup and a risk of hydrogen explosion, which might damage the containment and release radioactivity.
The badly damaged unit 1 is being cooled and radioactive gases are vented out in intervals.
Water levels in unit 2 were reported to be abnormally low and cooling is still not working. On Saturday, the unit was reported to be leaking, but no further information has been made available.
Risk differences between unit 1 (which was our main concern yesterday) and unit 3 (which is our main concern as of this update):
- Unit 3 is about two thirds larger, which means that the operators have correspondingly more residual heat to deal with.
- Unit 3 contains mixed plutonium-uranium fuel (MOX), which is hotter than “conventional” uranium fuel.
The MOX fuel in unit 3 can fail more easily when heated up because the melting point is lower and there is more gas buildup with the fuel elements.
There is much less experimental data and practical experience of the behavior of MOX fuel under accident conditions compared with conventional uranium fuel.
If the fuel melts, the risk of “recriticality” or a spontaneous nuclear reaction, is higher with MOX fuel. This is the absolute worst case scenario. In accident conditions, borated cooling water is used to prevent criticality, but the presence of larger amounts of fissible plutonium reduces the effectiveness of boron. (Further reading)
When the core is damaged, the greatest threat are the radioactive gases that accumulate between grains within the fuel. The amount of these gases is roughly double as large in uranium-plutonium fuel as in “conventional” uranium fuel, implying the potential for twice as large radiological releases. (Further reading)
If the reactor is heavily damaged, the plutonium itself can be dispersed into the environment, causing very long term contamination. MOX fuel contains about 5 times as much plutonium as irradiated uranium fuel. (Further reading)
Fukushima II (Daini)
According to latest available information, units 1, 2 and 4 are still without cooling and radioactive gases are vented periodically.
Radioactivity levels around the power plants are significantly above normal, which indicates continued radiological release from the plants. There are reports of high readings up to 100 km away near another Onagawa nuclear power plant, but the cause of these has not been identified – could be wind blown contamination from Fukushima, or a local leak.
Reportedly over 80,000 people are being (or have been) evacuated from within a 20 km radius from Fukushima I and 10km radius from Fukushima II.
At least 19 people are reported to have been exposed to harmful levels of radiation.
Please see this page for future updates, and follow us on Twitter for breaking news.