The Cause

The major cause of the incident is the mistakes by the the operational workers. The workers are inexperiences, and he did what a simple mistakes will done if not taken care properly which cause the reactor to explodes. What he did actually he set the regulator off, causes the reactor to behave unproperly.

The second cause of the incident is the operator had disabled one last safety system still functioning that will shut down the reactor if accident happens. This safety system will enables the reactor to automatically shut down if there is problem happening at the reactor.

The third cause of the incident is the operator also block own automatic shut down on low water level and this caused loss of turbines because the operator fears that the shut down would abort the test and they have to repeat the test again.

All these causes lead to the reactor being unstable and explodes thus creating history as the world worst nuclear disaster in the world.

Source : http://history1900s.about.com/od/horribledisasters/p/Chernobyl.htm