Japan's nuclear regulator is to increase the radiation exposure limit for workers in emergency situations from the current 100 millisieverts (mSv) to 250 mSv. The limit was temporarily raised following the March 2011 accident at the Fukushima Daiichi plant.
The Nuclear Regulation Authority (NRA) said in a statement today that it will increase the exposure limit from April 2016.
Normally nuclear workers are allowed to receive a dose of 20 mSv per year, although in practice they often receive very much less. If that limit is exceeded in any year, the worker cannot undertake nuclear duties for the remainder.
In emergency circumstances, safety regulators allow workers to receive up to 100 mSv with the same conditions applying, that they must leave the site should that limit be reached. The 100 mSv level is roughly the point at which health effects from radiation become more likely. Below this it is statistically difficult to connect radiation dose to cancer rates, but above this the relationship starts to become apparent.
Under a special allowance from the then-regulator, the Nuclear and Industrial Safety Agency, workers at Fukushima Daiichi were permitted doses of up to 250 mSv. That limit was lowered back down to 100 mSv in December 2011.
Six workers at Fukushima Daiichi received total doses of over 250 mSv during their time tackling the emergency, while 170 received doses over 100 mSv.
The NRA has now considered the experience at Fukushima Daiichi, as well as overseas standards and scientific studies, and has concluded that permanently raising the limit in emergency situations to 250 mSv is appropriate.
The International Atomic Energy Agency sets 100 mSv as the allowable short-term dose for emergency workers taking vital remedial actions, and 500 mSv as allowable short-term dose for emergency workers taking life-saving actions.
However, the International Commission on Radiation Protection recommends reference levels of 500 to 1000 mSv "to avoid the occurrence of severe deterministic injuries" for rescue workers involved in an emergency exposure situation.
The NRA has commenced a 30-day consultation of its proposed increase of the exposure limit, which ends on 19 June.
Researched and written
by World Nuclear News