1 Septembre 2013
We deeply apologize for the great anxiety and inconvenience caused by the recent contaminated water issues at the Fukushima Daiichi NPS, which affects the residents near the power station and the broader society.
Regarding the high radiation (maximum 1,800 mSv) found at tanks in Fukushima Daiichi NPS on August 31, some articles reported that "by simple calculation, if a person is exposed this much radiation amount for four hours continuously, that would lead to death" or "it takes only one minute to reach the annual radiation exposure limit for workers," etc. We would like to explain more about the 1,800 mSv.
We used measuring equipment that measures both beta radiation and gamma radiation. The 1,800 mSv is the total amount of beta radiation and gamma radiation. Gamma radiation was 1 mSv and most of the 1,800 mSv was beta radiation.
Since 1,800 mSv is approximately 3.5 times higher than the control level of equivalent dose for skin which is 500 mSv/year, we should carefully control radiation exposure. Since beta radiation travels only a short distance, radiation level decreases considerably if we keep a distance. Moreover, since beta radiation is weak and can be blocked by a thin metal sheet such as aluminum, we think that we can control radiation exposure by using proper equipments and cloths.
Additionally, although 1,800 mSv was detected at 5 cm above the floor, the radiation level of 50 cm above the floor was 15 mSv. Thus, 1,800 mSv does not mean the radiation level of the whole nearby place.
Some articles reported that "if a person is exposed this much radiation amount for four hours continuously, that would lead to death" comparing with the radiation level that would result in death (7,000 mSv), or "it takes only one minute to reach the annual radiation exposure limit for workers" comparing with the annual radiation exposure limit for workers (50 mSv). However, we believe that simply comparing the 1,800 mSv with those standard levels is not proper, since the standard levels are accumulation of effective dose (not equivalent dose) that express effects for whole body.
We will find out the cause of this issue and make proper counter measures immediately, and continue to make every effort to secure safety of workers.