Even as thousands of residents pondered the implications of the nuclear disaster in Fukushima Prefecture last year, Japanese government officials took little notice of up-to-the-minute high radiation measurements provided by the U.S. Energy Department.

The Energy Department used its Aerial Measuring System (AMS) between March 17 and 19, 2011, and compiled a detailed map of radiation levels on the basis of 40 hours of flight time over Fukushima Prefecture.

The data was provided to Japanese government officials, but not released to the public. The map clearly shows an area of high radiation levels extending in a northwesterly direction from the crippled Fukushima plant.

Thousands of Fukushima residents living near the plant, unaware of the danger they faced, evacuated in the direction of those high radiation areas.

This is not the first time the Japanese government has been shown to be slow to respond to the unfolding disaster at the Fukushima facility following last year's Great East Japan Earthquake and tsunami.

The central government also failed to quickly release forecasts of radiation spreading in Fukushima and beyond made by its System for Prediction of Environmental Emergency Dose Information (SPEEDI).

A major difference between the SPEEDI forecast and the Energy Department observations is that the U.S. data concerns actual radiation measurements taken over an area with a radius of about 45 kilometers from the Fukushima No. 1 plant.

The monitoring showed that communities in a northwestern direction from the plant, including Namie and Iitate, had radiation levels exceeding 125 microsieverts per hour over an area as wide as 30 kilometers.

Exposure to that level of radiation for eight hours would exceed what is deemed by the government to be safe over the course of a year.

According to Foreign Ministry officials, the U.S. Embassy in Tokyo twice provided data through e-mail messages. The radiation maps based on the results of the AMS were provided on March 18 and 20.

The Foreign Ministry immediately forwarded the data to the Nuclear and Industrial Safety Agency (NISA) and the science ministry, which is in charge of carrying out radiation measurements.

According to several science ministry officials, including Itaru Watanabe, the deputy director-general of the Science and Technology Policy Bureau, the science ministry and NISA not only failed to publicize the data, but neglected to pass it on to the prime minister's office or the Nuclear Safety Commission (NSC).

The U.S. Energy Department released the monitoring data on March 23. A department official noted that the information had been shared with the Japanese government.

When asked by The Asahi Shimbun why the information was not used to implement evacuation plans, Watanabe said: "While I now feel that the information should have been released immediately, at that time there was no thought given to using the provided data for the benefit of evacuating residents. We should have also passed on the information to the NSC."

Even though 15 months have passed since the data was passed on by the U.S. government, officials of NISA's Nuclear Safety Public Relations and Training Division said they were still looking into whether they obtained the information in response to repeated requests for interviews from The Asahi Shimbun.

The central government designated five municipalities in Fukushima Prefecture for evacuation on April 22. That decision was made after science ministry officials measured radiation levels on the ground and carried out joint airborne monitoring with the Energy Department.

In this way, they confirmed that areas of high radiation existed to the northwest of the plant.

Tokushi Shibata, professor emeritus of radiation management at the University of Tokyo, said: "It was a fatal error in judgment. If the data had been released immediately, the situation of residents evacuating in the wrong direction and becoming exposed to radiation could have been avoided."

The first hint that the science ministry and NISA had obtained the radiation maps from the Energy Department came in late October. However, uncovering what actually transpired ran into months of bureaucratic stonewalling.

In explaining why existence of the radiation map was not passed on to not only the prime minister's office and NSC but even the top officials of the science ministry, Watanabe said, "At that time, we did not even know how accurate the measurement data was."

Uncertainty about the accuracy of the SPEEDI forecast was also a key reason its existence was not made known to Naoto Kan, prime minister at the time, and his Chief Cabinet Secretary Yukio Edano.

When Watanabe was further pressed on why the actual measurement data was not released, he said, "At that time, the biggest issue we faced was to conduct radiation monitoring ourselves."

Although NISA officials will not publicly admit it, several government sources said the radiation map information was passed on to NISA.

One former high-ranking NISA official recalled that a large map of radiation levels was posted on a whiteboard in a NISA office used at the time as the central government's emergency response center.

While admitting to being impressed by the high technological level of the radiation map, the former official said, "At that time, we were doing everything we could to control the reactor cores at the Fukushima No. 1 nuclear plant. There was the thinking that monitoring was the responsibility of the science ministry."

(By Hiroo Sunaoshi and Kazuyuki Kanai, Asahi Shimbun, 18/06/2012)