A 01h in the morning, April 25, the reactor is operating at its rated output in the normal regime, the produced steam supplying the two turbines. At that time, permission to start the test was given. The power was slowly reduced and April 25, at 13:05, the power of the reactor was reduced to 50%. At this power, a single turbine was necessary to absorb the steam produced, and the second turbine has been stopped.
The test ought then continue with a reduction of power up to 30%. But electrical energy requirements are needed, officials of electric power in the USSR refused to give permission to continue the reduction of power and therefore the reactor continued to operate at 50% of its power for 10 new hours. At 23h10, April 25, staff received permission to the test and to resume the reduction of power. Unfortunately, at 00h28 on April 26, the operator omit to reset control devices and power fell to around 1% instead of the 30% needed. To this power by 1%, the reactor was almost arrested and insufficient energy to continue the experiment. The sharp reduction in power causes, in all reactors, a clearance of xenon in the fuel rods. Xenon is a radioactive gas, but in addition, it strongly absorbs neutrons, increasing again reducing power and thus can cause the engine to a complete stop. The power in the core is so weak that the water in the tubes did not boil as it should normally do, but remains in the liquid phase, and liquid water has a high absorbency towards the neutron . To combat these two effects, the operator then withdrew all of the control rods from the reactor core and the power raised to 7%, power still too low for the test requested, but he could not go higher because of the the presence of xenon and water in the liquid phase.
The situation was very unstable, it was to try to advance a car by speeding while all brakes are tight.
Running a RBMK reactor with all control rods outside is a serious mistake. In fact, these rods are also used to control the emergency shutdown of the reactor, and once fully outside, it takes too long time to reintegrate them into the upper part of the heart in case of need and the stopping becomes very slow . The Soviet procedures are very strict on this point and say that you should never run the engine with less than 30 bars in place.
Nevertheless, when the accident occurred, only 6 to 8 control rods were present in the reactor. On April 26, 01h, in violating the procedure on the control rods, the technician kept to 7% power reactor. The reactor had not been designed to operate at such low power and many automatic systems could not ensure normal operation. Also, the operator had to manually control the flow of water returning from the turbine because automatic control was not working at such low power. It is a very complex task to ensure manually and he did not reach to manage it effectively. The reactor was so unstable that it was about to be stopped by the action of the emergency rods. But that would have defeated the test, however, then many safety emergency stop were disabled.
Having spent a half-hour trying to stabilize the reactor, at 01h22, technicians considered that the state was also stable it could be and decided to start the test itself. But first, they decided to deactivate an alarm, the one of the the automatic shutoff. The reactor would have been stopped automatically if the turbine remaining was disconnected as this happens during the test, but the staff wanted to keep a chance to repeat the test, then this alarm was also disabled.
At this point, the reactor was in an abnormal condition. The majority of stop devices were disabled. The control/security rods had for the most part been withdrawn, and the power was abnormally low. The core was filled with water almost boiling point, but not quite. We mentioned that liquid water is a good absorber of neutrons, so if the water starts to boil, liquid water will be replaced by the steam and fewer neutrons are absorbed. In normal operation, this is not a problem because the reactor is designed to arrange to make do with, but with low power, with the core completely filled with water, a brutal boiling may cause an increase in power at a time when, with the control rods totally outsidet, the stopping system would be very slow to react.
In fact, the biggest weakness of the stopping device lies in the design of the control/stop rods. These rods slide in vertical tubes and are cooled by the circulation of water. Under normal circumstances, these control rods in moving in and out of the reactor to monitor the power - entering to reduce power (by adding more neutron absorber) and leaving for increase it. Also, entering the heart, the bars replaced the water, and were replaced by the water in emerging. The problem with this system is that water is a neutron absorber, so the effect of moving bars remains low. To increase their impact, at the lower end of the bar has been set another bar roughly one meter, called "displacer" in graphite. The graphite, as has been said, not quite absorbing neutrons and thus, when a control bar enters into the core, it does not replace water, but graphite, and the effect is more important. And similarly when it emerges.
But, in the case of the Chernobyl reactor, the control rods were raised so high that even the graphite was above the bottom of the reactor, and the tubes in the core contained only water. If, in this state, the bars were lowered into the core, the first effect would be to replace the water, which absorbs neutrons well, with graphite, which absorbs neutrons little. In other words, the introduction of control/stopping rods, which are supposed to stop the reactor, would have precisely the opposite effect.
The control rods in the Chernobyl RBMK reactor
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During normal operation of the reactor, rods are in the position III. The graphite "displacers" absorb only few neutrons and in the event of a fall of the bars, graphite is replaced by the alloy Bore-Iron, which absorbs very strongly the neutrons, lowers power significantly (position IV)
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In the Chernobyl reactor, prior to the accident, most of the bars were found in position I. In this position, the tubes in which the control rods slide are filled with water. Going down, the bars force water out and replace it with the graphite "displacers" which absorb much less neutrons than water, thereby causing an increase in reactor power.