Chernobyl Legacy Reportage By Paul Fusco

The test plan called for the power output reactor 4 to be gradually reduced to 700–1000 MW thermal. The power level established in the test program (700 MW) was achieved at 00:05 on April 26; however, because the natural production in the core a neutron absorber, xenon-135, reactor power continued to decrease, even without further operator action. And as the power reached approximately 500 MW, Toptunov committed an error and inserted the control rods too far, bringing the reactor to a near-shutdown state. The exact circumstances will probably never be known, as both Akimov and Toptunov died from radiation sickness.
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 In this case the majority control rods were withdrawn to their upper limits, but the low value the operational reactivity margin restricted any further rise reactor power.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.