Chernobyl Legacy Reportage By Paul Fusco

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.
At 1:23:40, as recorded by the SKALA centralized control system, an emergency shutdown or scram the reactor was initiated. The scram was started when the EPS-5 button (also known as the AZ-5 button) the reactor emergency protection system was pressed thus fully inserting all control rods, including the manual control rods that had been incautiously withdrawn earlier. The reason the EPS-5 button was pressed is not known, whether it was done as an emergency measure or simply as a routine method shutting down the reactor upon completion the experiment. There is a view that the scram may have been ordered as a response to the unexpected rapid power increase, although there is no recorded data convincingly testifying to this. Some have suggested that the button was not pressed but rather that the signal was automatically produced by the emergency protection system; however, the SKALA clearly registered a manual scram signal. In spite this, the question as to when or even whether the EPS-5 button was pressed was the subject debate. There are assertions that the pressure was caused by the rapid power acceleration at the start, and allegations that the button was not pressed until the reactor began to self-destruct but others assert that it happened earlier and in calm conditions. For whatever reason the EPS-5 button was pressed, insertion control rods into the reactor core began. The control rod insertion mechanism operated at a relatively slow speed (0.4 m/s) taking 18–20 seconds for the rods to travel the full approximately 7-meter core length (height). A bigger problem was a flawed graphite-tip control rod design, which initially displaced coolant before neutron-absorbing material was inserted and the reaction slowed. As a result, the scram actually increased the reaction rate in the lower half the core.
A few seconds after the start the scram, a massive power spike occurred, the core overheated, and seconds later resulted in the initial explosion. Some the fuel rods fractured, blocking the control rod columns and causing the control rods to become stuck after being inserted only one-third the way. Within three seconds the reactor output rose above 530 MW. The subsequent course events was not registered by instruments: it is known only as a result mathematical simulation. First a great rise in power caused an increase in fuel temperature and massive steam buildup with rapid increase in steam pressure. This destroyed fuel elements and ruptured the channels in which these elements were located. Then according to some estimations, the reactor jumped to around 30 GW thermal, ten times the normal operational output. It was not possible to reconstruct the precise sequence the processes that led to the destruction the reactor and the power unit building. There is a general understanding that it was steam from the wrecked channels entering the reactor inner structure that caused the destruction the reactor casing, tearing f and lifting by force the 2,000 ton upper plate (to which the entire reactor assembly is fastened). Apparently this was the first explosion that many heard. This was a steam explosion like the explosion a steam boiler from the excess pressure vapor. This ruptured further fuel channels—as a result the remaining coolant flashed to steam and escaped the reactor core. The total water loss combined with a high positive void coefficient to increase the reactor power.