information about Fukushima published in English in Japanese media info publiée en anglais dans la presse japonaise
25 Octobre 2012
October 25, 2012
Highly radioactive substances dispersed in a nuclear accident could spread beyond the 30-kilometer-radius zones used for disaster management planning, according to diffusion simulation results released Wednesday by the Nuclear Regulation Authority.
In worst-case scenarios at four of 16 nuclear plants in Japan, high levels of contamination would be found beyond the 30-kilometer zones set as priority areas in new disaster management guidelines drafted by the NRA.
The simulations showed accumulated radiation readings in areas around the four nuclear plants could reach 100 millisieverts--the level at which the International Atomic Energy Agency considers it necessary for residents to evacuate--in seven days.
The four plants are the Kashiwazaki-Kariwa plant in Niigata Prefecture, the Hamaoka plant in Shizuoka Prefecture, the Oi plant in Fukui Prefecture and the Fukushima No. 2 plant in Fukushima Prefecture.
It is the first time the government has released such simulation data. But the forecasts were based on weather conditions alone, and geographical features were not taken into account.
At the outbreak of the nuclear crisis at the Fukushima No. 1 plant last year, data from the System for Prediction of Environment Emergency Dose Information, called SPEEDI, to predict the diffusion of radioactive substances were not made public. The government thus failed to use the system effectively to evacuate residents.
The NRA said the simulations this time were made to provide reference data for local governments, which are working on their respective disaster management plans, at their request. Targeting all 16 commercial nuclear plants in the nation, except for the Fukushima No. 1 plant, the authority said it applied a method used in the United States for simulations.
The simulations were made for each plant in two different scenarios--one in which radioactive substances leak in the same amount as in the Fukushima No. 1 plant disaster, and the other in which meltdowns occur at all reactors to release radioactive substances. The latter is the worst-case scenario.
On maps, the NRA showed points at which the accumulated exposure dose of radiation could reach 100 millisieverts in seven days. These points are on lines in 16 evenly divided directions from each nuclear plant. The International Commission on Radiological Protection has estimated that exposure to radiation of 100 millisieverts would increase the risk of death from cancer or other causes by 0.5 percent.
Geographical features, such as plains or mountains, are not factored in at all, said the NRA, adding the simulations were based on meteorological data such as direction and speed of the wind as well as precipitation last year.
Simulation results showed, in the worst case scenario, radioactive substances from the Kashiwazaki-Kariwa nuclear plant would reach as far as Uonuma, about 40 kilometers away. Including this case, high simulated diffusions were seen in six cities beyond the 30-kilometer zones around the four plants.
In the scenario in which the amount of radioactive substances was set at the same level as the Fukushima No. 1 plant case, simulated diffusions were all within the 30-kilometer zones.
In areas where simulated diffusions were seen beyond the 30-kilometer priority areas, local governments likely will need to review their evacuation guidelines and other related matters, observers said.
The NRA's Secretariat said the same day it would hold a briefing for local governments to give advice in their disaster management plans.