(Sep 01, 2013)

Explanation regarding the high radiation levels (maximum 1,800 mSv/h) found at tanks in Fukushima Daiichi NPS on August 31, 2013

We deeply apologize for the great anxiety and inconvenience caused by the recent contaminated water issues at the Fukushima Daiichi NPS, which affect residents near the power station and the broader society.

With regard to the high radiation levels (maximum 1,800 mSv/h) found at tanks in Fukushima Daiichi NPS on August 31, some articles reported that "by simple calculation, if a person were exposed to this amount of radiation for four hours continuously, it would lead to death," or "it would take only one minute to reach the annual radiation exposure limit for workers," etc. We would like to explain more about the figure of 1,800 mSv/h.

We used measuring equipment that measures both beta radiation and gamma radiation. The 1,800 mSv/h figure represents the total amount of beta radiation and gamma radiation. Most of the 1,800 mSv/h was beta radiation; gamma radiation measured 1 mSv/h.

Since the control level of the equivalent dose for skin is 500 mSv/year, such radiation level (1800mSv/h) should be carefully controlled. However, since beta radiation travels only a short distance, radiation levels can be reduced considerably by maintaining a distance. Moreover, since beta radiation is weak and can be blocked by a thin sheet of metal, such as aluminum, we believe that we can control radiation exposure by the using proper equipment and clothing.

Additionally, although 1,800 mSv/h was detected at 5cm above the floor, the radiation level at 50cm above the floor was 15 mSv/h. Thus, the figure of 1,800 mSv/h does not represent the radiation level of the whole area.

Some articles reported that "if a person were exposed to this amount of radiation for four hours continuously, it would lead to death," by comparing with the radiation level that would result in death (7,000 mSv), or "it would take only one minute to reach the annual radiation exposure limit for workers," by comparing with the annual radiation exposure limit for workers (50 mSv). However, we believe that simply comparing the 1,800 mSv/h figure with these standard levels is inappropriate, since the standard levels represent the cumulative effective dose (not equivalent dose) upon the whole body.

We will investigate the cause of this issue, taking any appropriate countermeasures immediately, and continue to make every effort to secure the safety of workers.