Radiation risk: reactor's protective containment vessel may have been damaged

WESTERLY WINDS are helping Japan to limit risks of exposure of the public to radiation caused by the ongoing threat to reactors at the Fukushima Daiichi nuclear power plant.

Emissions coming from the site are being blown out to sea, where they can cause little or no harm.

Even so, the Japanese government has maintained its 20km exclusion zone around the plant and has urged those living between 20 and 30km away to remain indoors as much as possible, Dr Claire McMahon of the Radiological Protection Institute of Ireland noted.

The main concern of radiation release currently relates to the site’s reactor 2. An explosion or other incident under the reactor has caused damage of some kind, she said. “They are trying to figure out is everything still intact.”

The International Atomic Energy Agency acknowledged the reactor’s protective containment vessel might have been damaged.

If the containment has failed even partially, then radiation could escape. “There are suggestions that caesium has been detected outside the plant,” Dr McMahon said. There was speculation the caesium might have been present given agency suggestions that up to 5 per cent of the fuel in reactor 2 may have melted.

These possibilities have yet to be confirmed, but earlier yesterday there was an explosion and fire adjacent to reactor 4. The incident occurred at a cooling pond which holds used fuel rods, held under water to prevent them from heating up.

The explosion and fire released high levels of radiation measuring 400 milliSieverts (mSv). Radiation exposure figures from the Japanese government indicated readings at the site’s gates at midnight were 11.9mSv. By 6am these had fallen to 0.6mSv.

A person living in Ireland would receive a radiation dose from natural sources equal to 3 or 4mSv over a full year, according to Dr Luis León Vintró, a senior lecturer in the school of physics at University College Dublin.

Researchers have a good understanding of the risks associated with radiation exposure, having data from Japanese survivors of the Hiroshima and Nagasaki bombs, uranium mine staff and nuclear power plant workers.

An exposure to 400mSv would be “quite high” but not fatal, he said. It would cause a significantly increased risk of developing a cancer within 10 years. Doses of 2,000mSv would cause severe radiation sickness, and 50 per cent of those exposed to 4,500mSv would die within 60 days, he said.

Radiological protection agencies assume a “linear non-threshold” approach that means there was no safe dose of radiation, he said.

Almost no official dose information was available for locations in Japan other than Fukushima, except Tokyo, where “no significant levels have been found”, Dr McMahon said.

Levels outside the 20km evacuation zone should also be low, even more so now that westerlies will move any discharges out over the ocean.

Even so, the risk of a more significant radiation release continues because three reactors remain overheated. Rescue workers would likely limit their exposure to 100mSv while doing their job, she said. “You would start thinking, OK, I am not going to linger in this area. Unless they are actually saving lives, they would consider 100mSv a level they should not stay in for more than an hour.”