What happened on ChNPP in April, 26th, 1986 year?
Chernobyl catastrophe is gradually forgotten, though seemed, that the most grandiose technogenic accident in history of mankind on the scales and consequences - accident on the Chernobyl nuclear power plant, forever will be engraved in humanís memory, will serve as terrible caution to the people living today and for their descendants, that always it is necessary to address atomic nucleus as YOU, that the thoughtless, pragmatic attitude to an atomic energy, whether it will be the nuclear weapon or "peace atom" is much more dangerous, than to release genie from a bottle.
The author decided to remind about Chernobyl catastrophe, he had to take part in liquidation of catastropheís consequences, to leave his health there on the coast of Pripyat. Though this accident has no direct attitude to a military science and history, but in particular the "stupid and illiterate, rough and silly" army had to correct mistakes of the "intelligent geniuses of science, focus of all the best, that is in our society" by lives and health of soldiers and officers.
The technical aspect of this terrific tragedy is considered in the article. I speak to experts in advance, that much here is given in extremely simplified kind, here and there even to the prejudice of scientific accuracy. It is made so that it became clear to a person even very far from physics, atomic power engineering - what and why nevertheless happened at night from 25th to 26th, April,1986 year.
Preconditions to disaster
Reactor Ė is not only a source of electric power, but also is its consumer. While nuclear fuel will not be unloaded from the active zone of reactor, it is necessary to pump water through it continuously in order to prevent overheat of FUEL ELEMENTS. Usually a part of electric capacity of turbines is selected for own needs of reactor. If the reactor is stopped (fuel replacement, preventive works, an emergency stop), power supply of reactor goes from the next blocks of the external electric system.
Feeding from reserve diesel engine-generators is stipulated on extreme emergency case. However, in the best case, they can start to produce the electric power not earlier, than in one-three minutes. There is a question: by what to feed pumps while the diesel engine-generators will give on a mode? It was necessary to find out - what is the time from the moment of switching-off of steam supply on turbines, they will produce a current, sufficient for emergency supply of the basic systems of reactor, rotating by inertia. The first tests have shown, that turbines cannot provide with the electric power the basic systems in a mode of rotation by inertia (running-out mode).
Experts of "Dontechenergo" offered the own control system of magnetic field of turbine that promised to solve a reactorís energy supply problem at emergency switching-off of steam supply on the turbine. On April, 25th it was supposed to test this system in work as all the same this day it was planned to stop for repair work the 4-th power unit.
However it was required first of all to use something as ballast loading in order to have possibility to make gagings on the running out turbine. Secondly, it was known, that at falling of thermal capacity of reactor up to 700-1000 megawatt, the system of an emergency stop of reactor (SESR) will function, the reactor will be stopped and it wonít be possible to repeat experiment several times as its xenon poisoning will occur.
It was decided to block SESR system, and to use reserve MCP (main circulation pump) as ballast loading.
It was the FIRST and the SECOND terrible mistakes which caused all the rest. First of all, absolutely there was no need to block SESR. Secondly, everything was possible to use as ballast loading, except from circulating pumps. They in particular connected among themselves electrotechnical processes absolutely far from each other and the processes occurring in reactor.
Chronicle of disaster
On April, 25th 1986 year. 1.00. Gradual decrease in power of reactor began.
13.05. Power of reactor was decreased from 3200 megawatt up to 1600. The turbine Ļ7 was stopped. The feeding of electrosystems of reactor was transferred on the turbine Ļ8.
14.00. The system of an emergency stop of reactor SESR was blocked. At this time the dispatcher of "Kievenergo" has ordered to detain a stop of the block (the weekend, the other half of day, consumption of energy grows). The reactor operates on half-capacities, and SESR is not connected again. It is a gross error of the personnel, but it not affected on development of events.
23.10. The dispatcher removes an inhibition. The personnel start to reduce power of reactor.
On April, 26th 1986 year. 0.28. Power of reactor decreased to a level when the control system of movement of operating cores should be transferred from local to the general (in usual mode of group of cores it is possible to move independently from each other - so more conveniently, and at low capacity all cores should be controlled from one place and move simultaneously). It wasnít made. It was the THIRD terrible mistake. Simultaneously the operator commits the FOURTH terrible mistake. He does not give out to the machine a command "to hold capacity". As a result capacity of reactor promptly decreases up to 30 megawatt. Boiling in channels decreased sharply, xenon poisoning of reactor begun. The personnel of turn commit the FIFTH terrible mistake (at this time I would give other estimation to actions of turn. Itís not a mistake any more, but itís a crime. All instructions prescribe in such situation to stop reactor). The operator brings out from an active zone all operating cores.
1.00. Power of reactor succeeded to be lifted up to 200 megawatt against the prescribed by the tests program 700-1000. It was the second criminal act of turn. It wasnít not possible to raise up higher the power, because of growing xenon poisoning of reactor.
1.03. The experiment began. The seventh pump was connected to six operating main circulating pumps as ballast loading.
1.07. The eighth pump was connected as ballast loading. The system wasnít designed on operation of such quantity of pumps. Cavitational failure of MCP began (they simply didnít have enough water). They suck out water from drums of separators and its level in them dangerously went down. The huge stream of enough cold water through reactor lowered steam formation to a critical level. The machine completely deduced cores of automatic control from an active zone.
1.19. Owing to dangerously low water level in drums of separators, the operator increases supplying of feed water (condensate) in it. Simultaneously the personnel commits the SIXTH terrible error (I would tell - the second criminal action). It blocks stop systems of reactor on signals of an insufficient water level and pressure of steam.
1.19.30 Water level in drum separators started to grow, but because of decrease in temperature of water, arriving in an active zone of reactor and its considerable quantity, boiling there stopped. Last cores of automatic control left an active zone. The operator commits the SEVENTH terrible error. He completely deduces from an active zone even last cores of hand control, losing thereby possibilities to operate the processes occurring in reactor. The matter was that the height of reactor was 7 meters and it responded well on moving of operating cores when they moved in average part of an active zone, and in process of their removal from the centre, controllability worsens. Speed of moving of cores was 40 cm in second.
1.21.50 Water level in drums of separators a little exceeded norm and the operator deactivated a part of pumps.
1.22.10 Water level in drums of separators stabilized. Much less waters now arrives in an active zone, than till this moment. Boiling again begins in an active zone.
1.22.30 Because of discrepancy of the control systems which wasnít signed on similar operating mode it appeared, that water delivery in reactor makes nearby 2/3 from required. During this moment the station computer shows listing of parameters of reactor with indications that the reactivity margin is dangerously small. However the personnel simply ignored these data (it was the third criminal action these days). The instruction prescribes in such situation immediately in emergency order to stop reactor.
1.22.45 Water level in separators stabilized, the quantity of water arriving in reactor succeeded to be brought into compliance. Thermal capacity of reactor started to grow slowly. The personnel supposed that reactor work succeeded to be stabilized and it was decided to continue experiment. It was the EIGHTH terrible error. In fact practically all control cores were in the lifted position, the reactivity margin was inadmissible small, SESR was deactivated, and systems of an automatic stop of reactor on abnormal steam pressure and water level were blocked.
1.23.04 Personnel blocked system of an emergency stop of reactor, operating in case of stopping of steam supply on the second turbine if the first had already been switched off before. I will remind, that the turbine Ļ 7 had been already switched off at 13.05 25.04 and now the turbine Ļ8 operated only. It was the NINTH terrible error (And the fourth criminal action these days). The instruction forbids deactivating this system of an emergency stop of reactor in all cases. Simultaneously the personnel blocks steam supply on the turbine Ļ8. It is an experiment on gauging of electric characteristics of the turbineís operation in running-out mode. The turbine starts to lose turns, pressure in network decreases and MCP, feeding from this turbine begins to reduce turns.
The investigation established that if the system of an emergency stop of reactor on signal of stopping of steam supply on last turbine had not been deactivated the accident wouldnít have occurred. Automatics would have blanked-off the reactor. But the personnel supposed to repeat experiment several times on various parameters of control of generatorís magnetic field. The reactorís stop excluded such possibility.
1.23.30 MCP considerably lowered turns and water stream through an active zone of reactor considerably decreased. Steam formation began to grow quickly. Three groups of cores of automatic control went downwards, but could not stop increase of thermal capacity of reactor, as it wasnít sufficiently. Since steam supply on the turbine was deactivated, its turns continued to decrease, pumps supplied waters to a reactor ever less.
1.23.40 Chief of turn, having understood an event, orders to press button AZ-5. Control cores with the maximum speed fall downwards according to this command. Such massed introduction in an active zone of reactor of absorbers of neutrons urged to stop completely during short time processes of nuclear division.
It was the Last TENTH terrible error of the personnel and last immediate cause of accident. Though it is necessary to tell, that if this last error wouldnít have been made, all the same accident had already been inevitable.
And here is what happened - on distance of 1.5 meters under each core so-called "displacer" was suspended. It was the aluminum cylinder of 4.5 m length, filled with graphite. Its task was the following: at lowering of an operating core, the absorption of neutronsí increase occurred not sharply, but more smoothly. Graphite absorbed neutrons too, but a little bit weaker than boracium or cadmium. When control cores were lifted to a limit upwards, the bottom ends of displacer were above the bottom border of an active zone on 1.25 meters. There was water, which hadnít not boiled yet in this space. When all cores sharply gone downwards on signal ņ«-5, cores with a boracium and cadmium still actually hadnít entered into an active zone, and cylinders of displacers, operating like pistons, superseded from an active zone this water. FUEL ELEMENTS were cropped up.
There was a sharp jump of steam formation. Pressure of steam sharply increased in reactor and this pressure didnít allowed cores to fall downwards. They hanged, having gone only 2 meters. The operator switched off power supply of coresí muff. By pressing this button electromagnets which held operating cores attached to armature were deactivated. Absolutely all cores (either manual or automatic control) were disconnected from the armature and freely fell downwards under gravity after such signaling. But they had already hanged, propped up by steam, and did not move.
1.23.43 Reactor self- acceleration began. Thermal power reached 530 megawatt and continued to grow promptly. Two last systems of emergency protection operated - on level of capacity and on growth rate of capacity. But both these systems operated signaling AZ-5, and it still had been supplied 3 seconds ago manually.
1.23.44 Thermal power of reactor increased in 100 times and continued to grow in shares of second. FUEL ELEMENTS were heated, inflating particles of fuel broken off covers of FUEL ELEMENTS. Pressure repeatedly increased in an active zone. This pressure, overcoming pressure of pumps superseded water back in supplying pipelines.
Further pressure of steam destroyed a part of channels and steam lines over them.
It was the moment of the first explosion.
The reactor ceased to exist as operated system.
After destruction of channels and steam lines, pressure in reactor began to fall and water gone again to an active zone of reactor. Chemical reactions of water with nuclear fuel, heated by graphite, zirconium began. During these reactions rough formation of hydrogen and carbonic oxide began. Pressure of gases in reactor was growing promptly. The cover of reactor weight about 1000 tons grew, breaking all pipelines.
1.23.46 Gases which were in reactor, joined to aerial oxygen, having formed detonating gas which because of high temperature presence instantly blew up.
It was the second explosion.
The reactor cover flew up upwards, turned on 90 degrees and fell downwards again. Walls and overlapping of reactor hall collapsed. The quarter of graphite being there, fragments of heated FUEL ELEMENTS took off from reactor. These fragments fell to the roof of machine hall and other places, having formed about 30 seats of fire.
Fission chain reaction stopped.
The personnel of plant started to leave the workplaces approximately from 1.23.40. But only 6 seconds had passed from the moment of signaling AZ-5 till the moment of the second explosion. It was impossible to consider, what had happened for this time and the more so to have time to make something for the own rescuing. The employees who escaped at explosion had already left a hall after explosion.
At 1.30 the first fire-fighting team of lieutenant Pravik got under way to a fire place...
(Material was taken from a site www.obozrevatel.com)