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On March 11, a magnitude 9 earthquake occurred 112 miles off the east coast of Japan. Ten nuclear power plants that were operating in the area shut down safely as designed, including the Fukushima Daiichi nuclear facility. About an hour later, a series of massive tsunamis— estimated to be at least 45 feet high—struck the east coast. This tragedy led to more than 15,000 people dead and more than 8,000 people missing. The tsunami was one of the largest in the history of Japan.
The earthquake struck an area where multiple smaller individual earthquakes had previously occurred. The interaction over a large area contributed to the earthquake being the largest Japan has ever experienced and the fourth largest recorded earthquake in the world. The earthquake lasted approximately three minutes and resulted in the Japanese coastline subsiding an average of 2.6 feet.
Japan’s Tokai and Onagawa nuclear facilities both withstood the earthquake and tsunami without significant damage. However, the earthquake caused some water to leak out of the used fuel pools at the Onagawa nuclear plant, but there was no change in the radiation levels outside the plant. The Onagawa nuclear energy facility also served as a refuge for hundreds of evacuees following the natural disasters, providing shelter, electricity, food and water to nearby residents.
The Fukushima Daini facility went into a state of emergency due to damage to its cooling water pumps, which cools equipment and systems that are used to cool the uranium fuel, at reactors 1, 2 and 4. By March 14, workers were able to re-establish cooling capability in reactors 1 and 2 and safely shutdown all the reactors on the site.
The earthquake damaged breakers and distribution towers, causing a loss of all off-site electrical power sources to the Fukushima Daiichi nuclear facility. The emergency diesel generators automatically started and provided AC power to emergency systems. However, the facility, including the emergency diesel generators, suffered major damage from the tsunami causing a complete loss of electricity on the site. Electricity is needed to maintain the key cooling functions within the reactor core. At the time of the earthquake, reactors 1, 2 and 3 at the plant were operating, and reactors 4, 5 and 6 were shut down for routine maintenance and refueling. The reactors are designed so that they share common facilities and structures, such as a shared control room and turbine building. The nuclear power facility also has a shared used fuel pool and dry cask storage facility.
Though reactors 1-3 safely shut down from the emergency, they still continued to produce “residual heat,” or heat that is generated from the decay of radioactive fission products while the nuclear chain reaction slowly reduces in magnitude. Because of the damage to the facility’s cooling water, plant electrical and emergency diesel generation systems, there was no electricity available to power the emergency reactor cooling pumps. These pumps cool both the reactor core and the primary containment. Turbine-driven pumps operated for a time using the suppression pool as a source of water and a discharge source for the water that had passed through the reactor. The suppression pool eventually heats up to a point where it cannot absorb any additional heat. Then, the suppression pool begins to boil, decreasing the turbine-driven pumping efficiency and flow rate and increasing pressure in the primary containment.
Tokyo Electric Power Co. (TEPCO) reactor operators tried to reduce the containment pressure and the temperature of the suppression pool by venting. Eventually the pump flow rate was insufficient to cool the reactor and the water in the reactors eventually dropped to a point where the reactor fuel was no longer surrounded by water. The result was the fuel became overheated and started to melt. Once the cladding that surrounds the fuel begins to melt, a chemical reaction occurs that generates hydrogen. By the time the operators had opened the containment vent valves significant quantities of hydrogen had been produced. Shortly after the venting operation began, a hydrogen explosion in the reactor building, outside of the primary containment, occurred at reactor 1 on March 12 and several days later at reactor 3. This significantly complicated emergency response efforts.
In an effort to cool the fuel and prevent further fuel damage, TEPCO began injecting sea water into the reactors on March 13. Meanwhile, a fire in the vicinity of the Fukushima Daiichi reactor 4 fuel pool area followed by an explosion in the same area prompted concerns that fuel within the used fuel storage pool was severely damaged. This assumption was later proven incorrect. The hydrogen generated in reactor 3 may have migrated to the reactor 4 building, resulting in the explosion. Because of conflicting reports on the amount of water in the used fuel storage pools, Japanese officials used helicopters and fires hoses to douse the pools with water. This is important because water is used to cool the fuel and acts as a barrier to releasing radiation to the environment.
By December 2011, TEPCO announced that the company had reached an important milestone in its recovery efforts when the reactors had successfully been brought to a state of cold shutdown. The company continues to monitor radiation levels to protect local citizens and plant workers and works to cleanup and decontaminate the areas affected by the reactor accident. Visit TEPCO’s website to learn more about the company’s roadmap toward restoration.