Only 1 percent to 2 percent of radioactive materials in the No. 1 through No. 3 nuclear reactors at the Fukushima No. 1 nuclear power plant were emitted into the air after explosions that occurred at the plant as a result of the March 11 earthquake, while large amounts of radioactive materials still remain within the reactors, the Nuclear and Industrial Safety Agency (NISA) said Thursday.
Only 1 percent to 2 percent of radioactive materials in the No. 1 through No. 3 nuclear reactors at the Fukushima No. 1 nuclear power plant were emitted into the air after explosions that occurred at the plant as a result of the March 11 earthquake, while large amounts of radioactive materials still remain within the reactors, the Nuclear and Industrial Safety Agency (NISA) said Thursday.
The estimates are based on the analysis of iodine and cesium, the two most typical radioactive materials. The proportion was about 2 percent for iodine-131 and about 1 percent for cesium-137.
According to the NISA, 6.1 million terabecquerels of iodine-131 and 710,000 terabecquerels of cesium-137 existed in the No. 1 through No. 3 reactors prior to the accidents. A terabecquerel is a trillion times a becquerel, which is a unit of radioactivity.
It is thought that radioactive material leaked out through openings in damaged pipes and valves that are connected to reactor pressure vessels and containment vessels. They were also released when vapor in the reactors was vented outside to lower the pressure in the immediate aftermath of the Great East Japan Earthquake.
The half-life--the time needed for radioactivity to reduce to half--of cesium-137 is about 30 years, but the half-life of iodine-131 is as short as eight days, and the quantity of the latter is thought to have reduced considerably from what it initially was.
Meanwhile, the NISA said Thursday that the temperature is rising in the No. 3 reactor at the Fukushima No. 1 plant. The temperature, which stood at about 170 degrees on Tuesday, rose to about 200 degrees on Wednesday, and to about 250 degrees on Thursday, although the cause remains unknown.
It is also unclear if the readings are accurate because the thermometers may have been damaged by the earthquake, but it is considered certain that the temperature is rising. Efforts will be made to lower the temperature by adjusting the amount of water injected into the reactor.
(This article was written by Tatsuyuki Kobori and Ryoko Takeishi.)