An aerial view of the Fukushima No. 1 nuclear power plant (Provided by Air Photo Service)
Editor's note: We will update our earthquake news as frequently as possible on AJW's Facebook page: http://www.facebook.com/AJW.Asahi. Please check the latest developments in this disaster. From Toshio Jo, managing editor, International Division, The Asahi Shimbun.
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Japan raised the severity rating of the crisis at the Fukushima No. 1 nuclear power plant on Tuesday to the same level as the Chernobyl disaster, weeks after it was criticized for downplaying the seriousness.
Pointing to the large amount and wide dispersion of radioactive material from the stricken facility, the Nuclear and Industrial Safety Agency (NISA) and the Nuclear Safety Commission of Japan raised their accident assessment to level 7, defined as a "major accident" on the International Nuclear and Radiological Event Scale.
While the quantity of radioactive material leaked so far from Fukushima is only about 10 percent of that released by the 1986 Chernobyl fire, officials are concerned that multiple reactors are still out of control.
Junichi Matsumoto, an official with the plant's operator Tokyo Electric Power Co., said: "Discharge (of radioactive materials) has not been completely stopped and there are concerns that the amount released could equal or exceed that for Chernobyl."
Immediately after the March 11 earthquake, NISA rated the situation at Fukushima as a level 4 accident, meaning that radioactive materials had been released outside of the Fukushima nuclear plant.
On March 18, the assessment was raised to level 5, the same as the 1979 accident at the Three Mile Island nuclear plant in the United States.
One standard used for a level 5 accident is the emission of radiation levels equivalent to several hundreds to several thousands of terabecquerels of radioactive iodine. One terabecquerel is a trillion becquerels.
Subsequent estimates of the total amount of radioactive materials released had levels of iodine between 370,000 and 630,000 terabecquerels. That is above the standard of several tens of thousands of terabecquerels that is used to define a level 7 accident on the International Nuclear and Radiological Event Scale.
The explosions and fire at the Chernobyl plant spread about 5.2 million terabecquerels across several countries.
Keiji Miyazaki, professor emeritus of nuclear reactor engineering at Osaka University, said: "At Chernobyl, (radiation) was spread out over a wide area due to an updraft caused by the fire and high temperatures. While Fukushima has not reached that level, a major contributing factor is the damage to fuel rods at several reactors and in storage pools for spent fuel rods."
At Fukushima, radioactive material has been released into the environment in a series of separate incidents. Radioactive steam was initially deliberately vented into the atmosphere to reduce pressure within the core containment vessels. Subsequent hydrogen explosions at the buildings housing the No. 1 and No. 3 reactors blew away part of the ceilings and dispersed radiation.
An explosion near the suppression pool for the No. 2 reactor and a fire at a storage pool for spent fuel rods at the No. 4 reactor are also believed to have released radioactive material.
The absence of any revision in the accident assessment after March 18 led to criticism abroad that the Japanese government was underestimating the severity of the situation.
The new assessment has been reported to the International Atomic Energy Agency, but it may not be enough to eliminate international concern that the Japanese government has not released all relevant information.
Keiji Kobayashi, a former lecturer of nuclear engineering at Kyoto University's Research Reactor Institute, said: "From the time when a dry boil at the reactor core and hydrogen explosions occurred, it became difficult to stop the discharge of radiation. It was clear the accident was more than a level 6. There are also elements not found in the Chernobyl accident such as the release of radioactive materials into the ocean."
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