Officials of the Nuclear and Industrial Safety Agency warned of possible deadly levels of radiation at the Fukushima No. 1 nuclear power plant if the venting of pressure at a reactor was not conducted successfully.
Officials of the Nuclear and Industrial Safety Agency warned of possible deadly levels of radiation at the Fukushima No. 1 nuclear power plant if the venting of pressure at a reactor was not conducted successfully.
The information was revealed in NISA internal documents obtained by The Asahi Shimbun through the information disclosure law.
One document was compiled by NISA officials at about 1 p.m. on March 12, one day after the Great East Japan Earthquake, and concerned possible outcomes if pressure was not released from the containment vessel of the No. 1 reactor. The document said failure to vent the reactor could lead to radiation levels spiking to several sieverts within the Fukushima plant site. An individual whose entire body has been exposed to seven sieverts of radiation would likely die.
The NISA document indicates that government officials were considering the possibility that no one would be able to enter the plant grounds if venting was not conducted.
At the time the document was compiled, pressure within the containment vessel of the No. 1 reactor reached 750 kilopascals, which greatly exceeded the pressure level the vessel was designed to handle.
To prevent destruction of the containment vessel, TEPCO workers tried to vent gas from the morning of March 12, but they were not immediately successful in opening the valve.
The NISA document said, "If a situation should continue in which venting is not possible, a large volume of radioactive materials will be released about 10 hours later (at 11 p.m.)." The document added, "Depending on weather conditions, there is the possibility of extreme radiation exposure within a range of three to five kilometers from the plant."
The document was faxed to the Nuclear Safety Commission of Japan at 2:02 p.m. on March 12.
However, from about 2:30 p.m., pressure within the containment vessel began to decrease, and the specter of a large volume of radioactive materials being released was avoided.
TEPCO subsequently announced that it had successfully carried out the venting.
Radiation measurements near the main gate to the Fukushima plant did not approach the extreme figures included in the NISA document. The highest radiation level recorded was 11.93 millisieverts per hour, recorded at 9 a.m. on March 15. That is the equivalent of about 0.01 sieverts.