information about Fukushima published in English in Japanese media info publiée en anglais dans la presse japonaise
26 Octobre 2012
October 26, 2012
http://www.yomiuri.co.jp/dy/national/T121025004053.htm
Local governments will be urged to make new regional disaster management plans based on results from the Nuclear Regulation Authority's diffusion simulation of radioactive substances in the event of a nuclear accident.
The NRA released the simulation data Wednesday, which are rough indicators of diffusion taking into account only weather conditions, such as estimated wind direction and speed.
However, some experts say the simulation results are not necessarily realistic, as the effects of geographic features, such as mountains or plains in the potentially affected areas, were not taken into consideration.
The diffusion simulation, conducted by the government for the first time, showed that at four of 16 nuclear power plants across the nation, high levels of contamination could spread beyond the 30-kilometer-radius zone.
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.
Although the NRA designated the 30-kilometer-radius zones as priority areas in its draft of disaster management guidelines, the simulation showed accumulated radiation levels could reach 100 millisieverts in areas outside the radius in seven days.
At a press conference after Wednesday's announcement, NRA Chairman Shunichi Tanaka emphasized the data is just for reference.
"The figures [are not intended] to attract too much attention," he said.
The NRA plans to hold explanatory meetings for local governments early next month to advise them to use the data in their efforts to devise regional disaster management plans.
The NRA aims to legislate the new safety guidelines for nuclear power plants and is making utmost efforts to prevent a recurrence of a Fukushima-style nuclear crisis. But to reactivate nuclear power plants, measures based on the assumption that a nuclear accident can occur are deemed necessary.
The diffusion simulation data may be a useful reference for local governments when making their disaster management plans.
Accordingly, the NRA decided to release the data despite the risk of creating some confusion.
The simulation was conducted using a computer system similar to that of the U.S. Nuclear Regulatory Commission, which was used to forecast diffusion of radioactive substances.
As this system could not take geographical landscape features into account in its calculations, the simulation was done purely based on weather conditions.
Specifically, calculations were based on hourly weather data, such as wind direction, wind speed, rainfall and the stability of air observed last year. The data was collected over one year for a total of 8,760 hours--equivalent to 24 hours a day for 365 days.
Data was gathered for sites on equidistant straight lines from power plants in 16 directions, and sites where accumulated radiation levels could reach 100 millisieverts in seven days were highlighted on maps.
Sites on these lines closer to the nuclear power plant would show higher levels of radiation than the highlighted sites.
But some sites did not appear on the map because calculations could not be made there. This was because sufficient weather data was not available, which may have occurred if, for example, no remarkable winds were observed last year.
Among the 16 nuclear power plants, the Kashiwazaki-Kariwa plant has the world's largest output capacity with seven reactors, and highly radioactive substances would travel the farthest during a meltdown. The accumulated radiation level could exceed 100 millisieverts at sites about 40 kilometers from the plant.
Tanaka said even if there are sites where the radiation levels may reach 100 millisieverts outside the 30-kilometer zones, he does not intend to change the designation of 30-kilometer priority zones in the NRA's new disaster-management guidelines.
Prof. Hiromi Yamazawa of Nagoya University's graduate school, an expert on diffusion simulation and environmental radioactivity, said: "The method used in the United States, where many nuclear power plants are located on vast plains, was used for [the calculations on] Japanese nuclear power plants, which are mostly located in mountainous regions. The calculation did not include geographical features, which largely affect diffusion.
"Even if you're in one of the areas with high projected radiation levels, there is no cause for excessive concern. But that does not mean you should be complacent. If a large quantity of radioactive substances is actually discharged, weather conditions will largely affect the diffusion, and at any rate, radioactive substances may go beyond the 30-kilometer radius. Local governments should make evacuation plans with these factors in mind."