In May 2012, TEPCO reported that at least 900 PBq had been released "into the atmosphere in March last year alone" although it has been said staff may have been told to lie, and give false readings to try and cover up true levels of radiation. Tokyo officials temporarily recommended that tap water should not be used to prepare food for infants. Measurements taken by the Japanese government 30–50 km from the plant showed caesium-137 levels high enough to cause concern, leading the government to ban the sale of food grown in the area. Significant amounts of radioactive material have also been released into ground and ocean waters. The Japanese government estimates the total amount of radioactivity released into the atmosphere was approximately one-tenth as much as was released during the Chernobyl disaster. On 20 March, the Chief Cabinet Secretary Yukio Edano announced that the plant would be decommissioned once the crisis was over. The Japanese government and TEPCO have been criticized in the foreign press for poor communication with the public and improvised cleanup efforts. The level was later raised to 5 and eventually to 7, the maximum scale value. Japanese officials initially assessed the accident as Level 4 on the International Nuclear Event Scale (INES) despite the views of other international agencies that it should be higher. Electrical power was slowly restored for some of the reactors, allowing for automated cooling. The earthquake damage and flooding in the wake of the tsunami hindered external assistance. At the same time, sea water that had been exposed to the melting rods was returned to the sea heated and radioactive in large volumes for several months until recirculating units could be put in place to repeatedly cool and re-use a limited quantity of water for cooling. During the early days of the accident workers were temporarily evacuated at various times for radiation safety reasons. Concerns about the repeated small explosions, the atmospheric venting of radioactive gasses, and the possibility of larger explosions led to a 20 km (12 mi)-radius evacuation around the plant. As workers struggled to cool and shut down the reactors, several hydrogen-air chemical explosions occurred. The heat and pressure of the melting reactors, caused a reaction between the nuclear fuel metal cladding and the remaining water producing explosive hydrogen gas. Because of the delay in this process, meltdown progressed, depositing most of the melted fuel at the bottom of the reactor vessel. In an attempt to halt a meltdown, the government ordered that seawater be used to cool the reactors, as no alternative was available at the time. In the hours and days that followed, Reactors 1, 2 and 3 experienced full meltdown. As the water boiled away in the reactors and the water levels in the fuel rod pools dropped, the reactor fuel rods began to overheat severely. When the pumps stopped, the reactors overheated due to the high radioactive decay heat produced in the first few days after nuclear reactor shutdown. The flooded generators failed, cutting power to the critical pumps that circulate coolant water to keep them from melting down. A tsunami following the earthquake flooded the low-lying rooms where emergency generators were housed. Immediately after the earthquake, the remaining reactors 1-3 shut down automatically, and emergency generators came online to control electronics and coolant systems. At the time of the quake, Reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. The plant comprises six separate boiling water reactors originally designed by General Electric (GE), and maintained by the Tokyo Electric Power Company (TEPCO).
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |