Occurrence of the Chernobyl accident on April 26, 1986 was mainly due to a combination of severe design deficiencies of the reactor and its shutdown system, coupled with violation of procedures when conducting an experiment on the reactor unit. The Chernobyl-type accident could not occur at the nuclear power stations in Daya Bay which have reactors of entirely different design.
The cause of the Three Mile Island (TMI-2) accident on March 28, 1979 was apparently attributed to a combination of component failure, deficient instrumentation, and human errors. The operators were unable to diagnose or respond properly to a "loss of coolant accident" due to a defective pressure relief valve. Although there was a partial melting of the reactor core at TMI-2, the radiological release was mainly confined within the containment building and the release to the environment is minimal and insignificant. There were also no injuries or adverse health effect as a result of the accident.
Occurrence of the Fukushima accident in March 2011 was mainly caused by a major earthquake happened on March 11, 2011 and the large tsunami it created which disabled the off-site power supply and almost all on-site power supply and damaged some cooling facilities of Fukushima Daiichi Nuclear Power Station, leading to loss of proper cooling to the reactors and the spent fuel pools. Consequently, three reactor cores were largely melted and containment buildings of four reactors suffered different degrees of damage. The reactors in Fukushima are boiling water reactors which are different from those in the nuclear power stations in Daya Bay.