The Fukushima Nuclear Disaster
As opposed to the other two accidents mentioned below which are attributed to technical or human errors, the Fukushima disaster was triggered by earthquake and aggravated by the tsunami.
The plant comprises six separate boiling water reactors (BMRs). Experts consider this accident to be the second largest nuclear accident after the Chernobyldisaster, but more complex as multiple reactors were involved. At the time of the quake, Reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. The remaining reactors shut down automatically after the earthquake, the electricity grid was knocked out and generators started operating the cooling water supply. The plant was protected by a seawall designed to withstand a 5.7 m tsunami but not the 14 m maximum wave which arrived 40–60 minutes after the earthquake.
The entire plant was flooded and the diesel generators got damaged. All power for cooling was lost and reactors started to overheat, due to natural decay of the fission products. Three reactors experienced complete melt down and hydrogen explosions led to release of radioactivity into the environment.
The Chernobyl Disaster
The design of the Chernobyl was totally different from the Three Mile Island reactor in US. It was moderated by graphite and cooled by water. The accident occurred during a design test when safety interlocks were bypassed and the reactor was never actually shut down, thus the fission process continued uncontrollably, and the sudden surge in output shattered the reactor, opening up a gaping hole in the upper part of the building. The combustible graphite moderator then caught fire, sending out vast clouds of radioactive material over a huge area.
Despite several irregularities during tests on the reactor’s power supply, including an unexpected drop in reactor output, operators continued the test, repeatedly violating operating procedures. Structural flaws in the reactor also exacerbated the situation.
In 2006, the World Health Organization estimated that as many as 9,000 people may have died as a result of the Chernobyl disaster.
The Three Mile Island Accident
This accident pertains to pressurized water reactor (PWR). A malfunction in the secondary water supply caused the pumps to stop working. Although the Emergency Core Cooling System started as it was designed to do, human error led to a mistaken decision to shut it down. Large amounts of coolant escaped leading to heat build up in the reactor. It resulted in severe core meltdown.
Some of the radioactive material from the fuel rods leaked into the primary coolant system, and was released into the atmosphere. However, since the reactor building and the structures remained intact, most of the radioactivity remained safely contained within the reactor building. The impact on the outside world was minimal.
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