At Chernobyl Unit 4, the design characteristics of the reactor coupled with operation errors resulted in an inability to control the nuclear fission reaction inside the reactor thereby resulting in output that greatly exceeded rated output and reactor damage.
Due in large part to the absence of a containment vessel for containing radioactive substances discharged from the reactor, a large amount of radioactive substances, including pieces of fuel, were scattered throughout the surrounding area.
At Three Mile Island Unit 2, troubles caused the leak of reactor cooling water and more cooling water was not injected due to the misreading of water level gauges by an operator thereby preventing the cooling of decay heat* and allowing fuel to be dry-fired. As a result, the fuel reached high temperatures and melted.
At the Fukushima Daiichi Nuclear Power Station, the tsunami caused a loss of all AC power thereby rendering reactor cooling equipment inoperable. As a result, decay heat could not be cooled, the fuel dry-fired, reached high temperatures and melted. Since cooling water injection could not be recommenced quickly, ultimately, the reactor was damaged. Furthermore, temperatures inside the containment vessel rose leading to high pressures, and a large amount of radioactive substances was discharged into the environment when the containment vessels were vented to alleviate this pressure.
* “Decay heat” refers to the heat given off by fuel after the nuclear fission reaction has been halted. After a reactor has been shut down, rated output falls to approximately 6% and decreases with time thereafter
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