For the time being, the spent fuel is overtaking the the reactors themselves as the big radiation threat at the Fukushima Daiichi nuclear power station. This morning Japan time, a second fire broke out at the unit 4 reactor building. Meanwhile, a white plume of smoke or steam has been seen coming from one of the reactors. Tokyo Electric Power Company (TEPCO) is reported as saying they believe it may be coming from the Unit 3 storage pool. It is likely to be radioactive. The government has also admitted that the containment vessel for the Unit 3 reactor was likely damaged in a massive explosion on 14 March.
In a dramatic effort to stem further damage, the Japan Defense Force deployed a CH-47 Chinook helicopter to airdrop water onto units 3 and possibly 4, but it was forced to turn back because of radiation risks, according to NHK TV. Airdropping could be an indication of very high radiation at the site, as it would reduce the threat to ground staff. Earlier today TEPCO’s staff had to temporarily evacuate the site because of high radiation levels, according to a statement.
This morning, radiation rates at the plant’s gate remain 10 millisievert-per-hour range, but have since fallen to around 1.5 mSv/hr according to chief cabinet secretary Yukio Edano. NHK reports that the government has increased the permissible dose for workers at the plant from 100 mSv (presumed per year) to 250 mSv (presumed per year) in order to allow them to continue working on the site. In many countries, the limit for radiation exposure of nuclear workers is 50 mSv per year.
Radiation monitoring beyond the 20km exclusion zone shows “no immediate threat to human health,” according to Edano.
Update 10:15 UTC, 16 March
NHK TV reports the following rates from the ministry of science:
80 μSv/hr at 25 kilometers west-northwest of the plant.
58.5 μSv/hr at 30 km northwest of the plant.
6.7 μSv/hr at 25 km southwest of the plant.
Yuhei Sato, the governor of Fukushima prefecture, is reporting food and gasoline shortages because people are afraid to enter the region. He warns that there is not currently enough fuel to evacuate refugees from the exclusion area around the plant. He implored the government and TEPCO to “please give out correct, accurate information quickly.”
(You can read more about what these numbers mean here).
Boric acid
It appears that Japan may be short of boric acid, a critical chemical in suppressing nuclear reactions. Japan had been using the acid immediately after the quake to avert a meltdown at units 1 and 3, but I have not seen a release from either TEPCO or elsewhere that says it was injected into the deeply troubled unit 2 reactor or the spent fuel pools. To help, South Korea announced it will ship its reserves of boric acid to Japan over the coming days. (To learn more about how boric acid works, read ‘emergency procedures’ on this post.)
TEPCO still isn’t providing many details about the situation at Fukushima Daiichi, but in its statement today the company said, “We are aware of and sincerely apologize for the great distress and inconvenience this incident has caused to not just those inhabitants residing in the immediate vicinity but also society at large.”
For full coverage of the Fukushima disaster, go to Nature’s news special.
Credit: Digital Globe