By Toshihiro Okuyama, senior staff writer
From March 16 to March 18 in 2011, only a few days after the accident at the Fukushima Daiichi Nuclear Power Plant of the Tokyo Electric Power Co. (TEPCO), the U.S. government thought that there was no water left in the spent fuel pool of reactor no. 4. The false judgment partly resulted from information that a high level of radiation was recorded at a certain part of the plant compound, which would directly cause the death of a person if exposed. This detail was learned from a record kept by the U.S. government and from conversation with well-informed sources.
In reality, the pool had water. The U.S. government, however, assumed that there was no water in the pool and became concerned that the fire at the plant would spew large amounts of radioactive material into the air to be carried toward the Tokyo Metropolitan area. This led the U.S. government to issue an evacuation advisory to U.S. citizens who were within 80 kilometers of the plant. At the time, there was a serious discrepancy in understanding between Japan and the U.S. on the possibility of a worst-case scenario in which the nation’s capital would be contaminated with radiation.
The structure of reactor no. 4 exploded in the morning of March 15. The spent fuel pool on the building’s top floor had stored 1,331 spent fuel rods. The office of the Fukushima 10-year project (headed by Professor Kazuto Suzuki at the University of Tokyo), of which I was a member, made an inquiry to TEPCO on this point. According to TEPCO, the company estimated at the time of the accident that if the rods were exposed on the surface of the water, the level of radiation would reach 10 sieverts per hour (10,000 millisieverts) in the area surrounding the building.
The actual radiation level was several orders of magnitude lower than 10 sieverts. In mid-March, the highest level recorded by TEPCO outside of the building was 400 millisieverts. No acute radiation injuries were reported. At the time, TEPCO was publishing all radiation data it monitored, and based on those data, both TEPCO and the government were confident that there was water in the pool.
According to a record of the teleconference of the U.S. Nuclear Regulatory Commission (NRC), it received a report from staff members sent to Tokyo stating that the [fuel] pool lost its walls to the explosion and failed to contain water. Based on this report, the commission’s chairman at the time, Gregory Yatsko, decided to call for the evacuation of U.S. citizens within 80 kilometers of the plant. He also testified at a Congressional hearing that “there is no water in the pool.” In his book published in 2019, Yatsko admitted that this testimony of his was wrong.
Charles Casto, who led a group of NRC officials dispatched to Japan, wrote in his book in 2018 that the NRC group judged there was no water in the pool because they heard that the radiation was extremely high outside of the building, suggesting the strong possibility that the radiation came directly from the fuel.
Casto said during a teleconference on March 17, 2011 (morning of March 18, Japan Time), that a radiation level of 6,000 millisieverts was recorded in the space between reactor no. 2 and no. 3. Another participant of the teleconference also reported hearing the number, to which Cato remarked, “That is a lethal dose.” When asked about this by the Fukushima 10-year project, Casto only said that he vaguely remembered hearing about the lethal level of radiation from another NRC official.
The NRC record also revealed that false information such as “five people received a lethal dose of radiation” and “the radiation level at the facilities and the control room is lethal; it will be impossible to do anything” circulated in the NRC from March 15 to March 17.