Engineers were carrying out safety tests on one of the plants reactors. They wanted to know if the steam turbines would have enough mechanical enertia to keep the cooling water running if the reactor shut down, or the power from the national grid was cut.
The intention was to reduce the power in the reactor to 25% but, because of some peculiar nuclear properties of the reactor, the power was cut to almost nothing. this meant that the water cooling rate was severely reduced. the temperature of the water rose but did not boil.
The engineers overrode safety systems and withdrew the control rods. The output of the reactor increased, this caused the water in the cooling tubes to vaporize, thus reducing their neutron absorbing capacity. The reactor output increased from 25% to 10,000% in 4 seconds. The cooling tubes melted before the engineers were able to put back the control rods. the resulting contact between steam and graphite caused an explosion (evolution of hydrogen and carbon monoxide) that blew the top off the reactor.