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What happened?
A 1 am, on April 25, the reactor is operating at its rated output in the normal regime, the produced steam supplies the two turbines. At that moment, permission to start the test was given. The power was slowly reduced and on April 25, at 1:05 pm, 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 disconnected.
The test must then be continued with a reduction of power down to 30%. But electrical energy requirements are needed, officials of electric power, in Kiev, refused to give the permission to continue the reduction of power and therefore the reactor continued to operate at 50% of its power for more than 9 new hours. At 11:10 pm, on April 25, staff received permission to get back to the test and to resume the reduction of power. Unfortunately, at 0:28 am, April 26, the operator omitted to reset control devices and the power fell to around 1% instead of the 30% needed. At this power of 1%, the reactor was almost stopped and the energy insufficient to continue the experiment. The brutal power reduction causes, in all reactor, a clearance of xenon in the fuel rods. Xenon is a radioactive gas, but in addition, it strongly absorbs neutrons, aggravating again the power reduction and then can cause a complete shutdown of the reactor. The power in the core is so weak that the water in the tubes cannot boil as it should normally do, but remains in liquid phase, and the liquid water has a high absorbency in regard to the neutrons. To combat these two effects, the operator then withdrew all of the control rods from the reactor core and the power increased toward 7%, a power still too low for the requested test, but he could not go higher because of the the presence of xenon and water in liquid phase.
The situation was very unstable, it was like of to try to advance a car by speeding while all brakes are tight.
Operating 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 pulled up, it takes a long time to reinsert them into the upper part of the core if needed and the shutdown becomes very slow . The Soviet procedures are very strict on this point and specify that the reactor must not operate with less than 30 bars in place.
Nevertheless, when the accident occurred, only 6 to 8 control rods were present in the reactor. At 1 am, on April 26, in violation of the procedure on the control rods, the technician kept to 7% the power of the reactor. The reactor had not been designed to operate at such a low power and many automatic systems cannot ensure normal operation. So, the operator had to manually control the flow of water returning from the turbine because automatic control was not working at such a low power. It is a very complex task that to assure manually these operations and he did not succeed to manage them effectively. The reactor was so unstable that it was on the point to be stopped by the action of the emergency rods. But however, that would have made fail the test, then many safety emergency stop were disabled.
Having spent a half-hour trying to stabilize the reactor, at 1:22 am, technicians considered that the state was as stable than 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 remaining turbine should be disconnected as it 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 have, for the most part, been withdrawn, and the power was abnormally low. The core was filled with water almost boiling, but not quite. We mentioned that the liquid water is a good absorber of neutrons, and so if the water starts to boil, liquid water will be replaced by the steam and fewer neutrons will be absorbed. In a normal operation, this is not a problem because the reactor is designed to deal with, but at a so low power, with the core completely filled with water, a brutal boiling may cause an increase in power at a time where, with the control rods totally withdrawn, the stopping systems 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 move in and out of the reactor to monitor the power - by entering to reduce power (by adding more neutron absorber) and by leaving for increase it. So, by entering the core, the rods replaced the water, and were replaced by the water by being pulled up. The problem is that water is also a neutron absorber, so the effect of moving the rods remains low. To increase their impact, at the lower end of the rod has been set another rod in graphite, of roughly one meter, called "displacer". The graphite, as has been said, absorbs less the neutrons and thus, when a control bar enters into the core, the boron does not replace the water, but the graphite, and the effect is more important. And similarly when it emerges.
But, in the case of the Chernobyl reactor, the control rods were withdrawn so high that even the graphite was above the lower part of the reactor, and the tubes, in the core, contained only water. If, in this state, the bars are lowered into the core, the first effect would be to replace the water, which absorbs well the neutrons, by the graphite, which absorbs less the neutrons. In other words, the introduction of the control/stopping rods, which are supposed to stop the reactor, would have precisely the opposite effect.
The control rods of the Chernobyl RBMK reactor
During normal operation of the reactor, the rods are in the position III. The graphite "displacers" absorb only few neutrons and in the event of a fall of the bars, the graphite is replaced by the alloy Bore-Iron, which absorbs very strongly the neutrons, lowering significantly the power (position IV).
In the Chernobyl reactor, prior to the accident, most of the rods were in position I or II. In these positions, the tubes in which the control rods slide down are filled with water. Going down, the bars push off the water and replace it with the graphite "displacers" which absorb less the neutrons than water, thereby causing an increase in reactor power.