Chernobyl Legacy Reportage By Paul Fusco

The reactor power dropped to 30 MW thermal (or less)—an almost completely shutdown power level that was approximately 5 percent the minimum initial power level established as safe for the test. Control-room personnel therefore made the decision to restore the power and extracted the reactor control rods, though several minutes elapsed between their extraction and the point that the power output began to increase and subsequently stabilize at 160–200 MW (thermal). In this case the majority control rods were withdrawn to their upper limits, but the low value the operational reactivity margin restricted any further rise reactor power. The rapid reduction in the power during the initial shutdown, and the subsequent operation at a level less than 200 MW led to increased poisoning the reactor core by the accumulation xenon-135. This made it necessary to extract additional control rods from the reactor core in order to counteract the poisoning.
The operation the reactor at the low power level with a small reactivity margin was accompanied by unstable core temperature and coolant flow, and possibly by instability neutron flux. The control room received repeated emergency signals the levels in the steam/water separator drums, relief valves opened to relieve excess steam into a turbine condenser, large excursions or variations in the flow rate feed water, and from the neutron power controller. In the period between 00:35 and 00:45, it seems emergency alarm signals concerning thermal-hydraulic parameters were ignored, apparently to preserve the reactor power level. Emergency signals from the Reactor Emergency Protection System (EPS-5) triggered a trip which turned f both turbine-generators.
After a period, a more or less stable state at a power level 200 MW was achieved, and preparation for the experiment continued. As part the test plan, at 1:05 a.m. on 26 April extra water pumps were activated, increasing the water flow. The increased coolant flow rate through the reactor produced an increase in the inlet coolant temperature the reactor core, which now more closely approached the nucleate boiling temperature water, and reducing the safety margin. The flow exceeded the allowed limit at 1:19 a.m. At the same time the extra water flow lowered the overall core temperature and reduced the existing steam voids in the core. Since water also absorbs neutrons (and the higher density liquid water makes it a better absorber than steam), turning on additional pumps decreased the reactor power still further. This prompted the operators to remove the manual control rods further to maintain power.
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.