Gov't assumed massive radiation exposure from Fukushima nuclear plant

TOKYO, May 3 Kyodo - The government assumed a worst-case scenario of ''significant public exposure'' to radiation when workers were struggling to bring a nuclear reactor under control at the Fukushima Daiichi power plant a day after the March 11 earthquake and tsunami, Kyodo News learned Tuesday.
The scenario assumed the container of the No. 1 reactor would be damaged and people around the plant would be exposed to several sieverts of radiation, a potentially lethal level, if the workers failed to reduce pressure within the containment vessel by venting steam, according to sources in the government and the plant operator as well as documents.
Around 10 percent of people exposed to 1 sievert of radiation over a short period of time suffer nausea and tiredness, and half of those exposed to 4 sieverts die within 30 days.
It took five and a half hours to confirm the release of radioactive steam due to trouble with one of the two vents of the containment vessel at the Fukushima nuclear complex.
The Nuclear and Industrial Safety Agency assumed that the failure to vent the steam could increase pressure inside the container to three times the design limit by 11 p.m. on March 12.
The high pressure would break the container and a huge amount of radioactive iodine, cesium and other substances would be released into the atmosphere, posing ''a risk of significant public exposure within 3 to 5 kilometers from the power plant'' depending on weather conditions, the agency assumed.