Delays in the test (caused by the grid requiring power from Unit 4 longer than anticipated before it was tripped off for the test) caused the operators to get the core into an unstable condition (with most of the power being generated at the top, and with relatively hot water entering at the bottom) while determined to complete the test after the need for power was over.
It must be said here that accounts of specific control room actions differ—and that not all workers involved survived.
The water passing through the core was boiled by the nuclear heat and passed to steam drums where any entrained water was removed.
The steam then went to drive two identical turbine generators (to provide power to the grid).
Operators heard “banging sounds” from the area of the reactor, which was probably the initial rupture of tubes in the reactor.
Approach to the reactor was impossible because of the high radiation, although two control room operators and two trainees approached or (in the case of the trainees) entered the reactor space and received fatal doses of radiation.The Chernobyl complex, in what is now the independent Ukraine, was built over a long enough period that its four nuclear units (each “unit” is a combination of one reactor and two turbine generators) were of different designs, even though built side by side.The first two units were the earliest design, while Units 3 and 4 were of the second generation of RBMK-1000, which were intended to have an improved confinement for the reactor in case of accident.Even so, the structure was intended to contain only the rupture of two of the tubes in the reactor—which actually had about 1600 tubes.The reactor core used in this type of plant was physically so large (a cylinder 21 feet tall and 36 feet wide) that power could be increasing rapidly in one part of the core (either “radially” or on one side, or else “axially” meaning at the top or bottom) while decreasing in another.
Large amounts of radioactive materials were being released because the core itself was essentially exposed to the atmosphere.