Unit 1 radiation at highest measured levels so far

Tokyo Electric Power Company (TEPCO) announced on Saturday that it has detected radiation of up to 4,000 millisieverts (mSv) per hour at the building housing the No. 1 reactor at the Fukushima No. 1 nuclear plant.

Japanese radiation limits for nuclear workers are 250 mSv per year. The measurement of 4,000 mSv per hour announced by TEPCO is over 140,000 times the Japanese legal limit for nuclear workers exposure. 

To put these levels in perspective, exposure to 400 mSv will cause radiation poisoning symptoms if received in a short period of time. (6 minutes at current levels.)

2,000 mSv will cause severe radiation poisoning - and in some cases fatal poisoning. 

4,000 mSv will usually result in fatal radiation poisoning, although survival is possible with prompt treatment. 

8,000 mSv is a fatal dose - even with prompt treatment. 

The radiation reading, which was taken when Tokyo Electric Power Co. sent a robot into the No. 1 reactor building on Friday, is believed to be the largest detected in the air at the plant so far.

On Friday, Tepco found that steam was spewing from the reactor floor. Nationally televised news Saturday showed blurry video of steady smoke curling up from an opening in the floor.

Tepco said it took the reading near the floor at the southeast corner of the building, under which runs a pipe emitting steam. No damage to the pipe was found, the utility said.

The pressure suppression containment vessel is located under the building and highly radioactive contaminated water generated by the reactor is believed to have accumulated there, Tepco said, adding the steam is probably coming from the water.

The utility said its workers have no plan to work near that area, but it will carefully monitor developments.

Tepco has said radioactive water could start overflowing from temporary storage areas on June 20.

Update:  Japan said Monday that radioactive emissions from the stricken Fukushima Daiichi nuclear power plant in the early days of the March 11 earthquake and tsunami disaster might have been more than twice as large as a previous estimate, suggesting the accident was more grave than the government had publicly acknowledged.


The agency also said it now estimated that the radioactive release from the plant totaled 770,000 terabecquerels in the first week after March 11. The agency had previously estimated 370,000 terabecquerels released in the first month.