The International Commission on Radiation Protection (ICRP) is an international academic society of experts that makes recommendations about radiation protection. It has been reported that the reference level is set, based on the ICRP’s recommendations and with advice from the Nuclear Safety Commission of Japan.
The ICRP’s 2007 recommendations state that in emergencies radiation control standards different from the ones for normal times should be used. Moreover, it is suggested that emergency situations should be classified into two phases: an emergency phase and a rehabilitation phase, to take the following protection measures as a guideline.
1) Normal time: the dose should be limited to 1 mSv or less in a year.
2) Emergency phase: the exposure dose attributable to an accident should not exceed 20-100 mSv.
3) Rehabilitation phase after the accident settles down: the dose should not exceed 1-20 mSv in a year.
The current situation of the Fukushima Daiichi Nuclear Plant applies to the above-mentioned 2) Emergency Phase.
The Japanese government‘s decision has adopted the lower limit of the range between 20 to 100 mSv for the emergency phase. This means, while responding to the situation to control the total exposure dose of the residents in the plant’s surrounding area under 100 mSv, it takes measures to lower the annual dose back to the level of 1 mSv per year in the future.