From the article:
“It could not measure the amount released when an explosion occurred at the No. 4 unit on March 15, where all of the reactor’s nuclear fuel was stored in the spent fuel pool.”
- TEPCO puts radiation release early in Fukushima crisis at 900,000 TBq (Mainichi, May 25, 2012):
TOKYO (Kyodo) — An estimated 900,000 terabecquerels of radioactive substances were released into the atmosphere in March last year alone due to the disaster at the Fukushima Daiichi nuclear power plant, plant operator Tokyo Electric Power Co. said Thursday.
The figure is around 1.8 times higher than the latest estimate issued by the government’s nuclear safety agency but far lower than the 5.2 million TBq believed to have been discharged in the 1986 Chernobyl nuclear disaster.
Japan has already acknowledged that the severity level of the nuclear crisis at the Fukushima plant, triggered by the March 2011 earthquake and tsunami, is maximum 7 on an international scale — equivalent to the Chernobyl disaster — based on the amount of radioactive materials released into the atmosphere.
The utility known as TEPCO said it studied the amount of radioactive substances released between March 12 and March 31, 2011, noting that the release during the following months was less than 1 percent of that in March.
It also said around 18,000 TBq of radioactive substances ended up flowing into the adjacent Pacific Ocean in the form of fallout or by getting mixed with water that leaked from the plant between March and September.
The Fukushima plant’s Nos. 1 to 3 reactors suffered meltdowns after the six-reactor complex lost nearly all of its power sources and consequently the ability to cool the reactors and spent fuel pools after it was hit by huge tsunami waves.
Hydrogen explosions occurred at the Nos. 1, 3 and 4 units in the early days of the nuclear crisis, blowing off the walls and roofs of the buildings housing the reactors.
TEPCO said that 4,600 TBq of radioactive substances were emitted when an explosion occurred at the No. 1 reactor building on March 12, and 1,060 TBq when an explosion occurred at the No. 3 reactor on March 14.
It could not measure the amount released when an explosion occurred at the No. 4 unit on March 15, where all of the reactor’s nuclear fuel was stored in the spent fuel pool.
The release peaked on March 15 and 16, possibly because the primary containment vessels of the Nos. 1 to 3 reactors degraded due to high temperatures and massive radioactive substances were released from the upper part of the containers together with steam.
The Nuclear and Industrial Safety Agency has estimated that the radioactive release was 480,000 TBq, and another oversight body, the Nuclear Safety Commission of Japan, estimated that it was 570,000 TBq.
TEPCO said that the differences in the figures could be attributed to calculation methods and further assessment is needed.