Chernobyl Legacy Reportage By Paul Fusco

All these actions led to an extremely unstable reactor configuration. Nearly all the control rods were removed, which would limit the value the safety rods when initially inserted in a scram condition. Further, the reactor coolant had reduced boiling, but had limited margin to boiling, so any power excursion would produce boiling, reducing neutron absorption by the water. The reactor was in an unstable configuration that was clearly outside the safe operating envelope established by the designers.
Experiment and explosion
At 1:23:04 a.m. the experiment began. The steam to the turbines was shut f, and a run down the turbine generator began, together with four ( eight total) Main Circulating Pumps (MCP). The diesel generator started and sequentially picked up loads, which was complete by 01:23:43; during this period the power for these four MCPs was supplied by the coasting down turbine generator. As the momentum the turbine generator that powered the water pumps decreased, the water flow rate decreased, leading to increased formation steam voids (bubbles) in the core. Because the positive void coefficient the RBMK reactor at low reactor power levels, it was now primed to embark on a positive feedback loop, in which the formation steam voids reduced the ability the liquid water coolant to absorb neutrons, which in turn increased the reactor's power output. This caused yet more water to flash into steam, giving yet a further power increase. However, during almost the entire period the experiment the automatic control system successfully counteracted this positive feedback, continuously inserting control rods into the reactor core to limit the power rise.