Chernobyl Legacy Reportage By Paul Fusco

At 11:04 p.m., the Kiev grid controller allowed the reactor shut-down to resume. This delay had some serious consequences: the day shift had long since departed, the evening shift was also preparing to leave, and the night shift would not take over until midnight, well into the job. According to plan, the test should have been finalized during the day shift, and the night shift would only have had to maintain decay heat cooling systems in an otherwise shut-down plant; the night shift had very limited time to prepare for and carry out the experiment. Further rapid reduction in the power level from 50% was actually executed during the shift change-over. Alexander Akimov was chief the night shift, and Leonid Toptunov was the operator responsible for the reactor's operational regime, including the movement the control rods. Toptunov was a young engineer who had worked independently as a senior engineer for approximately three months.
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 In this case the majority control rods were withdrawn to their upper limits, but the low value the operational reactivity margin restricted any further rise reactor power.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.