chernobyl

Day prior to the fatal explosion, April 25th 1986, was scheduled for maintenance and to test ability of the reactor's turbine generator to generate sufficient amount of electricity to power the reactor's safety systems. Since the RBMK-100 reactor needs water to be continuously circulated throughout the core as long there's nuclear fuel in the core, testing purpose was to determine whether turbines in the rundown phase could produce enough energy to start the water pumps which are needed for reactor to work. In accordance to this testing during the daytime of April 25 all required conditions were met in order to start the testing and so reactor electricity output had been gradually reduced to 50% of reactor capabilities and then suddenly and totally unexpectedly regional power station that supplied this area with electricity went offline. Kiev grid controller then ordered postponement of further reduction of output because electricity was needed to satisfy the evening peak demand. Because of this unwanted circumstance testing was postponed and later put in the hands of the night shift of the plant that had very little experience in nuclear power plants because majority of them was brought from coal powered plants.


At 11:00 p.m., April 25, controller gave permission to continue the procedure and so the power output of reactor Number four needed to be reduced from its nominal 3,2 GW thermal to 0.7-1.0 GW in order to conduct the test at the prescribed lower power level. But the real problem was the fact that new crew wasn't aware of the prior postponement of the reactor slowdown made by the first crew and they followed the original test protocol which resulted in rapid power level decrease. Crew believed that the rapid fall of the power was because of malfunction in one of the automatic power regulators, which was as it turned out later completely wrong conclusion. When the power level decreases too rapidly reactor produces more nuclear poisonous product xenon-135, and crew managed to decrease the power to only 30 MW which is approximately only 5% of hat was really expected. After this mistake crew pulled the automatic control rods out of the reactor as the safety measure but that didn't help much because the reactor's power increased only to 200 MW which was still less than a third of the minimum required for the experiment. Even despite this fact crew continued with the experiment and so at 1:05 a.m. on April 26 water pumps that should have been driven by the turbine were turned on and this increased the water flow beyond what is specified in safety measures in 1:19 a.m. And in the precise time of 1:23:04 the fatal experiment began. The unstable state of the reactor wasn't shown on control panel so crew wasn't aware of the danger that was threatening them. The turbine was disconnected from the reactor and water pumps were cut off of the energy supply which resulted in the increased level of steam in the center of the reactor and the temperature as well and so pockets of steam formed voids in the coolant lines.

The working principle of the RBMK-100 reactor has large positive void coefficient. Void coefficient is the number that evaluates how much does thermal productivity of the reactor increase or decrease, in this case positive void coefficient had rapidly increased power of the reactor since water that usually absorbs neutrons decreased and in this state reactor became very unstable and unpredictable. At 01:23:40 operators pressed the AZ-5 button that ordered the shutdown of a reactor in the case of emergency, fully inserting all control rods, including the manual control rods that have been pulled out earlier. But the slow speed of the control rod insertion mechanism which lasts from 18-20 seconds to complete, and flawed rod design did the opposite by increasing the reaction rate. At this stage because of the increased energy production, deformation of the control rods followed as they became stuck after being inserted only one third of the way and they couldn't stop the reaction. Seven seconds later in 1.23:47 reactor power jumped to almost 30 GW which is ten times more than usual. This caused the melting of fuel rods and rapid increase of steam pressure, resulting in one large steam explosion which displaced and destroyed the reactor lid, ruptured the coolant tubes and blew off part of the roof. After part of the roof blew off, graphite fire happened as the result of reaction between the inrush of oxygen from the air with the extremely high temperature of the reactor and the graphite moderator that was placed on the control rods ends. Graphite fire was the main cause of radioactive cloud's spread to further areas.

Condition at the time of accident was very bad, mainly because of these two factors: crew was unprepared for possible disaster and there was also lack of adequate equipment and both of these factors led to more negative consequences and bunch of wrong decisions. Radiation levels in the worst-hit areas were about 20,000 rontgen per hour, and for comparison only, lethal dose of radiation is about 500 rontgen over 5 hours. This resulted with the crew exposure to the lethal radiation levels in just a couple of minutes. Unfortunate was also the fact that crew didn't knew just how big radiation really is because main dosimeter was inaccessible due to the explosion and all other dosimeters had limits of just 0.001 R/s, therefore just showing that radiation levels are above their upper limit. This is the main reason why crew wrongly assumed that radiation level is just about 3.6 R/h while real level was about 5600 times bigger. Because of this false data crew chief Aleksandar Akimov assumed that reactor was left intact despite the (obvious) evidences of pieces of graphite and reactor fuel lying around the building. Not only that, Akimov later ignored the readings of new dosimeter under the assumption that the new dosimeter must be incorrect as well. Akimov stayed with the crew until the morning putting efforts to pump the water into reactor and none of them wore any protective gear. Most of the crew, including Akimov himself, died from radiation exposure within three weeks. Victims weren't just the inexperienced crew members because firefighters soon arrived to try to extinguish the fire that broke after the explosion. Nobody told them that this wasn't a regular fire but the explosion of the nuclear reactor and they thought this was a regular electrical fire. In 5 a.m. fire was extinguished but many firefighters received lethal doses of radiation.



On 27th April, day after the fatal explosion, soviet government reacted after being convinced in the high levels of radiation and they evacuated the population of the nearby town Pripyat. Total number of 29 people (rescuers, firefighters and crew members) died soon after the accident and about 350,000 people were evacuated from contaminated areas near the reactor. According to UN agencies' estimations further number of 4,000 to 9,000 people died from consequences of this disaster, whose radiation level was estimated around the level of 400 atomic bombs like the one thrown on Hiroshima. The ecosystem near the reactor was badly hit as well because four square miles of the nearby forest changed their color to some color between the brown and purple and was later called by BBC as the "Red Forest". Huge number of animals also died and some of them lost the ability to reproduce.

There are two basic and opposite theories about the main cause of the Chernobyl accident. First theory puts all the blame on plant's crew while second theory attributes the accident to flaws in the RBMK-reactor design. There is also the conspiracy theory saying how reactor problems were well known fact that was deliberately hidden from the crew and that this is the main reason why majority of the crew didn't knew almost nothing about RBMK reactor.

Main grounds for the flawed design theory are dangerously large positive void coefficient which increases the power of nuclear reaction when in the reactor's cooling water steam bubbles start to appear, which very easily leads to the uncontrolled reaction, if there's no outside interference. Major design flaw was also the structure of control rods. Control rods are inserted into the reactor with purpose to slow down the reaction, and in the RBMK reactor design control rod end tips were made of graphite, hollow and filled with water while the rest, a truly functional part of the control rod which absorbs the neutrons and halts reaction was made of boron carbide. Because of this design, in moment when control rods were initially inserted into the reactor, the graphite which is a neutron moderator in fact increased the reactor power output instead of reducing it. Thus for the first few seconds of control rod activation, the reactor power greatly increased, instead of decreasing as they hoped it would. The unprepared and inexperienced crew wasn't aware that they in fact achieved the opposite effect. Also the water channels run throughout the core vertically which means that water temperature increases as water moves up and thus makes the temperature gradient in the core. This effect is particularly expressed if the top portion turns completely to the steam because this part is then (after transformation) no longer properly being cooled and therefore reactivity increases greatly. There was also just partial containment of the RBMK reactor in order to reduce costs which would have been big considering the size of the RBMK reactor and storing fission byproducts played they role as well in increasing the power of the reaction since reactor was fully functional for two years and therefore storing fission byproducts.

Main reasons for the flawed operators theory are the fact that plant crew didn't obey strict procedures and were ignorant of safety measures, primarily because they were lacking in experience, then there was the insufficient communication between the safety officers and the operators in charge of the experiment and mostly the fact that night crew wasn't introduced with the fact that first crew already started experiment as they started doing it once again from the beginning.

Before the explosion plans were to add two more reactors but after the explosion of nuclear reactor 4 all work on the yet unfinished reactors No.5 and No.6 was immediately halted and damaged reactor 4 was sealed off and 660 ft of concrete was placed between the accident site and the operational buildings. First three reactors weren't halted because of an energy shortage in Ukraine. But subsequently they were all taken offline, reactor Number two after the fire in 1991, reactor Number one in 1996, and the last remaining reactor Number three was turned off in the official ceremony by the former Ukrainian president Leonid Kuchma in 2000.

But still the potential danger lies in so called "sarcophagus" and its protective field of concrete that covers the damaged reactor and its ability to contain radiation. Sarcophagus was hasty built in order to prevent further radiation spreading and it isn't referred to as the optimal long-term solution, since many years have passed from the time it was built and it's aging poorly. Some estimation say that sarcophagus is in such a poor state that even small earthquake can cause the roof to collapse which would mean spreading of the new radioactive cloud. The big problem is also the water that continues to leak in sarcophagus, spreading the radioactive particles throughout the building and lately major problem is also the dust, since many radioactive particles similar to ashes are piling together. This problem was partially solved with the installation of air filtration system in 2001. In September 2007 Ukrainian government approved the building of the steel case around and over the reactor which will cost $1,4 billion. This steel case will measure 623 ft wide and 660 ft long and its building should be over until 2012. Once this building gets completed, dismantling of the damaged reactor 4 should begin.

Chernobyl nuclear disaster is the worst nuclear disaster in the mankind history which had positive effect on the public opinion about the potential danger that nuclear energy holds. This nuclear disaster showed jus how expensive mistakes when working with nuclear reactor can be, and forced governments using nuclear energy to search new, much safer types of nuclear reactors, and to increase nuclear power plant's safety measures to its possible maximum. Debates are still running whether the cause for this nuclear disaster lies in the inexperienced crew or is the design of the reactor solely to blame although is most likely both combined. 29 people died because of the exposure to radiation soon after the explosion, and UN estimates that the number of victims because of contamination of nearby areas will be couple of thousands more.



Population increase increases total energy needs and therefore demand for this type of energy grows, which unlike other traditional non-renewable energy sources almost doesn't emit dangerous greenhouse gases and new nuclear power plants had proven to be very safe and reliable and what's even more important ecologically acceptable. But as did Chernobyl example show, mistakes in dealing with nuclear reactor are very expensive and therefore maximum safety measures must be whole time on, the crew must be highly trained and the nuclear reactor design must be very safe. Also there always must be option what to do in case things get wrong. Only by satisfying these conditions we can prevent new nuclear disasters and ensure safe working principle of nuclear power plants which are in this moment satisfying about 16 % of the world energy needs. Chernobyl example was good example to learn lessons from, and they have been really learned and nuclear power plants have become places with maximum safety measures. With this of course the problem of nuclear waste and its storage wasn't solved, especially not in the poor countries and possible big problem could be terrorism as terrorists could turn their sight to nuclear energy and nuclear weapons. This is the thing international organizations are already working on as do the governments of the states that are using this energy source, but of course danger always exist.