Radiation levels high enough to harm human health have been reported at the Fukushima Daiichi nuclear power station. Levels are falling, but it appears that there has been a major release of radiation, and residents within 30 kilometers of the plant have been advised to take shelter. The Japanese broadcaster NHK reports that radiation is spreading south: Tochigi prefecture is reporting radiation levels 30 times the background, and radioactive cesium and iodine has reached Tokyo, though at extremely low levels. It remains extremely unlikely that any harmful levels of radiation will reach the capital.
NHK also reports levels of 5 mSv/hour around Tokai village, 10 km south of the plant. That is roughly 100 times the normal rate.
Around 6:00am Japan time, unit 2 at the Fukushima Daiichi nuclear power plant was rocked by a massive explosion. Reports remain sketchy, but it appears to be a hydrogen blast. Hydrogen is generated by a chemical reaction between steam in the reactor core and heated fuel rods. If it is exposed to air, the highly volatile gas will explode. Similar blasts have damaged the unit 1 and unit 3 reactors in recent days (see here for further explanation).
According to NHK, a fire was reported at 9:38 am on the fourth floor of reactor #4. Unit 4 has been shutdown since before the accident, so the cause of the fire was not the core. Workers who went to extinguish the blaze found that old fuel rods, which are kept on site, had overheated and were partially exposed. Spent fuel rods generate heat through decay, and although they are unlikely to meltdown, they can still generate hydrogen—the likely cause of the blast at unit 4.
Even as the morning’s events unfolded radiation levels were on the rise. According to Chief Cabinet Secretary Yukio Edano, rates of 400 mSv per hour have been detected around unit #3 and 100 mSv/hour have been seen around unit 4. 100 mSv is the threshold at which serious health effects such as cancer, radiation sickness, and temporary sterilization come into play. A dose of 10,000 mSv is generally fatal (more on exposure later).
Previous peak radiation levels have been in the 1000 μSv/hr (1 mSv/hour) range, so this marks a serious increase in radiation levels. Edano says that radiation levels at the front gate of the plant were:
8:31 AM 8217 μSv/hr
9:00 AM 11,930 μSv/hr
12:30 PM 1362. μSv/hr
3:30 PM 596.4 μSv/hr (possibly 496.4, NHK translators gave two values).
The situation remains extremely precarious. According to the latest information from the Japan Atomic Industrial Forum, reactor #2’s hardened containment vessel may have been damaged by this morning’s blast. That would mean that there is a serious threat of radiological release—indeed it may explain the levels we have seen now.
Cooling systems are also offline at units 4, 5, and 6, and according to Edano, the spent nuclear fuel in these reactors is beginning to heat up. That raises the prospect of further fires or releases.
For full coverage of the Fukushima disaster, go to Nature’s news special.