The unit 4 reactor was to be shut down for routine maintenance on 25 April 1986.
It was decided to take advantage of this shutdown to determine whether, in the event of a loss of station power, the slowing turbine could provide enough electrical power to operate the main core cooling water circulating pumps, until the diesel emergency power supply became operative.
As the shutdown proceeded, the reactor was operating at about half power when the electrical load dispatcher refused to allow further shutdown, as the power was needed for the grid.
Some media had reported a seismic origin of the accident, however the scientific credibility of the paper at the origin of this rumour has been discarded.
The planned programme called for shutting off the reactor's emergency core cooling system (ECCS), which provides water for cooling the core in an emergency.
Although subsequent events were not greatly affected by this, the exclusion of this system for the whole duration of the test reflected a lax attitude towards the implementation of safety procedures.
It was not until about on 25 April that the grid controller agreed to a further reduction in power.
For this test, the reactor should have been stabilised at about 700-1000 MWt prior to shutdown, but possibly due to operational error the power fell to about 30 MWt section in the information page on RBMK Reactors) by - although the operators may not have known this.