 Attention! Attention! Attention! Attention! Attention! Attention! Attention! 1986 is the year that would change everything in the nuclear industry. A disaster would burn the concept of atomic fallout into the zeitgeist of the 1980s. And a legacy would act as a constant reminder of the risks of improper management of a nuclear reactor. The disaster became a testament on how poor design and mismanagement when placed on a Venn diagram can equal irradiating most of Eastern Europe. Of course today we are looking at the Chernobyl disaster of 1986 and this one has earned the magical 10 on my Painted Plain Difficult Disaster Scale. Believe it or not, this video is actually part of a series of videos on the Chernobyl disaster and in a wider context the RBMK reactor type. And in light of that it might be worth checking out my video on the Lenin grad and other Chernobyl meltdown. Don't worry I'll still be here when you get back. Ah you're back, welcome. Yep I know it was a bit of an overdone trope so let's get back to the main reason why you're here. Our story starts back in 1970 and in Ukraine about 100 kilometers 62 miles from Kiev. When ground was broken for a new city that would become known as Pripyat, an atom grad that was designed to house the workers for a new nuclear plant. It should be said that the original plans were to build the city and the plant just 25 kilometers or 16 miles from Kiev but it was thought to be too close to a major city which turned out to be a good decision. The concept of an atom grad is similar to say Tomsk, which were closed cities although Pripyat was opened later on in its short lifetime as a city. These municipalities were designed to be self-contained communities that had a sole purpose and that was keeping the power plant working and by extension the workers that were employed there happy. The community at Pripyat was developed to be the perfect home for a young family with its amusement park and modern at the time architecture. In the same year the ground where the new plant would exist was prepared for construction. The Chernobyl nuclear power plant began construction proper in 1972 after the central government decided on what type of reactors the site would house. Originally the idea for a pressurized water reactor was thought to suit the new plant but it was overruled by Ala Tolly Alexandrov, the chief scientist who was at the time supervisor of the project of RBMK reactor plants. The reactor type was preferred due to the design being cheaper to produce and as we found out with Leningrad that cost was a pretty high priority some might say even over safety. The nuclear power plant had the first of its four reactors ready to operate in 1977 followed by number two in 1978. As a quick side note, units one and two were the first generation RBMK 1000 reactors and because of this they had slightly different core loadings and containment structures but the remaining two reactors at Chernobyl were of a slightly more modern second generation and they were named units three and four which went online in 1981 and 1983 respectively. There were plans for another two units but these were abandoned after 1988 when risks of contamination left the project untenable. The nameplate capacity of Chernobyl nuclear power plant was unit one at 800 megawatts of electricity unit two at a thousand megawatts of electricity unit three also had a thousand watts of electricity and the same with unit four. In this video I'll be focusing on the second generation RBMK its design and more specifically unit four but I did look into the first generation in the Leningrad video. The RBMK was a bit of a unique design in the reactor world but it was a product of evolution rather than revolution in that it used graphite to moderate the chain reaction with light water as a coolant. This combination allowed it to use lower enrichment uranium-235 for fuel which is not surprisingly significantly cheaper than other reactor designs. The RBMK had a very large core region consisting of a height of seven meters and a diameter of 11.8 meters which was mounted in a steel cylinder that was placed on top of a biological shield. The steel cylinder was 16 millimeters thick and had a diameter of 14.52 meters by a height of 9.75 meters dug into the floor of the reactor building. Inside a reinforced concrete line pit between the steel wall and the concrete lining a water tank was placed followed by a sand packing. Below the bottom biological shield there were several basements used for pipe work for the reactor core cooling loops. The core consisted of graphite blocks size 25 by 25 centimeters stacked up with holes drilled through the center for fuel and control channels. The size of the core could cause issues in reactivity control during operation but we will talk about that a bit later on. On top of the core another biological shield was placed with a steel cover on top of it which formed the floor of the reactor hall. The cover plate was capable of withstanding a pressure build up resulting from steam released by two simultaneous channel ruptures. All of this allowed the design to be refuelled whilst in operation which again helped with costs by reducing downtime. As a side effect of this the confinement structure of RBMK reactors is less than ideal as the tall refuelling machine had to be housed inside the reactor hall. Because of the large floor space and subsequent roof requirements meant that the vast quantities of heavy concrete wouldn't have been architecturally or more importantly financially viable. The RBMK didn't have a prototype and was instead put straight into production with the first reactor being installed in Leningrad in 1970. The original designs omitted confinement instead claiming that the fuel being in its own channel with flowing cooling water was an acceptable alternative. The main takeaway from this is confinement was poor resulting in disastrous results in the case of efficient product release. The coolant was pumped through the core from the bottom common header through high pressure tube that jacketed the fuel elements where the water would boil. Coolant water also circulated in the control rod channels. The primary cooling of the reactor was via two separate cooling circuits one of each for half of the core meaning everything was doubled up leading to four main circulating pumps which were three operating and one standby per loop. As the coolant boils it exits the top of the core and is passed through a water steam separator which takes the steam to a high then low pressure turbines which generate electricity. The steam is then passed through a condenser that turns it back into water to be pumped back into the core completing the primary cooling circuit. The condenser is called by using a separate secondary water cooling circuit meaning the primary cooling circuit is self-contained. As mentioned before the reactor employed graphite blocks for its moderator. Now moderator is used to reduce the speed of fast neutrons released from fission so they can better facilitate a chain reaction getting more use out of the fuel. Ideally moderators work without capturing any neutrons leaving them as thermal neutrons. But where the design has its potential flaw is having a separate coolant that can also absorb neutrons which can make the reactor unstable in certain situations. This brings up the big elephant in the room and that is void coefficient. You see it wasn't just a graphite that had moderation characteristics but the coolant water as well because H2O naturally slows down and absorbs neutrons. This brings a problem for the reactor as with the increase in temperature from fission the coolant boils off into steam known as voids i.e a bubble that does not absorb neutrons. With the steam not absorbing neutrons this leads to an increase in the power of the reactor. The rbmk did not rely solely on its coolant for moderation because that's what the graphite was for but the water did absorb neutrons meaning that as it boiled off the operators had to compensate for this with the control system. If not properly managed an incident of a runaway can happen as the coolant heats up increasing the reactivity heating up the coolant more creating more steam leading to greater reactivity and this is called a feedback loop. Due to its reactor state more and more neutrons are produced and their density grows exponentially fast creating a positive void coefficient. The risks of a feedback loop were greater at certain power levels but this will be covered later on. The design relied heavily on the void coefficient for its reactivity. This is unlike a pwr setup in which the coolant is also the sole moderator meaning the chain reaction can't be sustained if all the coolant has boiled off creating a more stable design but this is also much more expensive. Now this leads us quite neatly on to reactor control. As I said before the core region was rather big and this meant that the rbmk was kind of like multiple smaller reactors in that hotspots could occur which needed to be individually managed and this leads on to the strange design choice for both top loaded and bottom loaded control rods. There were 167 top loaded rods and 32 shortened rods loaded via the bottom and another 12 automatic controls inserted from the top. The shortened rods were used to manage axial power distribution which was necessary due to the core size to manage hotspots. The main control rods had a 4.5 meter long graphite rod termed a displacer attached to the end of the length of the control rods except for the 12 rods that were used in automatic control. The displacer connected to its rod via a telescope with a water filled space of 1.25 meters separating the displacer and absorbing rod. This was to stop water filling the space left behind by the control rod after it has left the core region which would parasitically absorb neutrons because removing one neutron absorber in the form of a boron control rod to be replaced by another neutron absorber albeit weaker in the form of H2O kind of defeats the point of the control rod. Thus if there was no displacer then more control rods would be needed as each one would have less of an effect on the reactivity of the core. The dimensions of the rod and displacer were that when the control rod was fully extracted the displacer sat centrally within the fueled region of the core with 1.25 meters of water at either end. But this setup in certain situations could cause a positive scram effect. On receipt of a scram signal causing a fully withdrawn rod to fall water was displaced from the lower part of the channel as the rod moved downwards. This caused a localized insertion of positive reactivity in the lower part of the core. This was because the RBMK had a high moderated to fuel ratio meaning the lower part of the core would experience a spike in reactivity as the graphite displacer entered it as it replaced the neutron absorbing water with a neutron moderating graphite. But this situation was only dangerous at certain power levels of the reactor which was meant to be mitigated by the knowledge and training of the operators and also importantly by their management. Part of this mitigation was by having an operating reactivity margin. The ORM is the number of equivalent control rods of nominal worth remaining in the core. The total time required for a scram or insertion of the emergency control rods into the core when starting from the upper limit stop switches was around 18 seconds which was relatively slow. But this was due to the size of the control rod channels and the fact that the coolant had to be pushed out of the way. The scram could be initiated by the control system or manually via the AZ5 button. The control system employed on the RBMK-1000 utilized ionizing chambers and various sensors both inside the core and outside which were effective from power levels above 10% of total power. The external detectors were all mounted mid-level of the core meaning no low actual power monitoring could be undertaken. This meant that low power operation below 10% had to be done by the operators relying on experience and feel than monitoring the equipment. But manual low power control was only really intended for a cold startup so to speak. When the reactor was free of the neutron absorbing poison Xenon-135. Right I need to mention about neutron poison as it will play a vital part later on and was also a contributory factor in another RBMK incident that meltdown in Leningrad in 1975. During fission some products are given off one of which is Xenon-135 which has a high neutron absorption rate which is okay when the reactor is in a steady state as it burns off this undesired byproduct as poison burning is proportional to reactor power. But when the power of the reactor is decreased the Xenon-135 is not burned off and can shut down the chain reaction by absorbing too many neutrons. This is known as an iodine pit. All is not lost as Xenon-135 has a relatively short half-life of around 9.2 hours so reactor core is considered poison after several half-lives of past. But this is not good from an economic standpoint as the unit would be shut down for a few days which is where good management is meant to come into effect where safety is prioritized over money but in the form of USSR taking a reactor out of action for a few days could have a detrimental effect on one's career longevity. Our story starts with an issue inherent to all reactors of any type a thing called decay heat. You see reactors produce heat from fission which is used to create steam to drive the turbines for electrical power. Some of this power is taken to drive the electronically operated coolant pumps which keep the coolant flowing through the core. The reactor needs around 28,000 litres per hour during full power operation. Some heat is produced not directly from fission but instead from the products produced from the chain reaction and this is known as decay heat. And when the reaction stops this continues at least for a while after shutdown. During power operation this is fine as electricity is being produced but after shutdown the pumps need to keep circulating to remove the decay heat. Usually electricity needed for the pumps can be drawn from the national grid but what about if there is a full power outage then backup is needed. This presents a problem if for any reason being scheduled or not a reactor is shut down and there is a power cut how will the pumps keep working? Well the solution is pretty simple and that is a diesel backup generator. Each of Chernobyl's reactors had three backup diesel generators which took in excess of 70 seconds to reach full power too much time in the event of a full power cut. In theory some of this time could be covered by the spool down of the turbo generators which was theorised to cover around 45 seconds. Not the full time needed but possibly enough to prevent fuel melting. To prove that this could work it had to be tested out and in 1982 1984 and 1985 this was attempted but all tests failed to produce the electricity needed to run the pumps. The test was to be conducted again in April 1986. Strangely the test procedure that was written hadn't taken into account the eccentricities of the RBMK reactor especially at low power outputs. The test was to be conducted during a routine shutdown but the test to go ahead some vital systems had to be switched off most notably the active passive emergency cooling system used in a loss of cooling accident. This required the chief engineer to sign it off. In preparation for the test the following had to be set up. The reactor power was to be reduced to between 700 and 1000 megawatts of thermal energy and the steam turbine generator was to be at normal operating speed. After all this was sorted the preparations for the electrical test could go ahead. The steam supply to the turbine generator would be closed off. The turbine generator performance would then be monitored to see if it could bridge the power for coolant pumps until the emergency diesel generators took over. When the emergency generator supplied electrical power the turbine generator would be allowed to continue freewheeling down. The normal plan shutdown procedure of the reactor was then to be completed. The date of the test was to be the 25th of April 1986 15 minutes past 2pm completed by the day shift. As such the operators were briefed on the procedure and a team of electrical engineers was present. At around 1am on the 25th of April the reactor was gradually powered down to around 50% in time for the day shift. At 1.05pm the turbo generator TG7 was switched off. The four main circulating pumps, two electrical feed water pumps and other electrical equipment that was connected with this turbo generator were switched over to bus bars for the turbo generator TG8. The ECCS was disabled with the correct authorization from the chief engineer. But just after 2pm the Kiev electrical controller asked for the test to be postponed to cover the evening electrical peak demand. The request was due to another power station in the region unexpectedly going offline. Chernobyl's director agreed and the test was pushed back but the ECCS was not reinstated. The status of the ECCS showed the lack of safety culture at the plant. Although the system wouldn't really be a factor in the disaster it highlights the management culture at the time. The day shift soon gave way to the evening shift which in turn was soon to be replaced by the night shift. To oversee the test Anatoly Dyatlov deputy chief engineer took the helm in the control room. Due to his seniority he outranked all operational staff on shift at the time. Alexander Akimov was the chief of the night shift and Lenin Toptunov was the operator responsible for the reactor's operational regimen and the movement of the control rods. At around 2300 hours the Kiev electrical controller allowed the continuation of the reactor shutdown and by 10 past 2300 hours the reactor power was brought down to 700 megawatts of thermal energy. The night shift operators had to prepare for the test in a short period of time. In theory after the test was completed the team would have a straightforward night just monitoring the decay heat until the next crew change over. As the power dropped off the high amounts of iodine 135 decayed into the poison xenon 135 as it was not being burned off quick enough. This produced the power of the reactor without any operator intervention to 500 megawatts of thermal energy at which point manual intervention was started in order to maintain the level. At 28 minutes past 12 on the 26th of April the reactor power had dropped to just 30 megawatts of thermal energy with a near shutdown like state. It is not 100% known why but both equipment failure and operator error have been attributed to the reactor state. As set out in the operating instructions the reactor should have been powered down to wait for the xenon 135 to decay but shutting down the reactor without doing the test wouldn't have made many friends in management. To increase the power the control rods began to be removed to try and overcome the poisoning of the core reduced coolant void and graphite cooldown. In the attempts to raise the power the operational reactivity margin was violated by removing more of the absorbing control rods and by 1 a.m. around 200 megawatts of thermal energy was reached. The reactor was now in a dangerous state not wanting to leave empty handed Dyatlov insisted that the test be conducted at the current power level. In reality the core was heavily poisoned and was in a dangerous state with very few absorbing rods for emergency control. During the preparations of the test the operators powered up the two backup coolant pumps leading to all eight working. The increase in water flow reduced the voids within the core which would have set off an automatic scram but this was bypassed by the operators. As the coolant absorbed the neutrons in the core only eight control rods were left as a safety margin. The minimum that the ORM stipulated was that around 28 rods should have been in the core. The reactor started to experience a feedback loop. With not many rods in the core to manage the power the situation was that the water in the reactor found itself being the main neutron absorber and disaster was just waiting to happen. At 1.23 in the morning the actual test had now begun. The command given to stop the valves for turbine number eight closed and the rundown operation started. As the turbines spooled down the power to the feed water pump slowed increasing the temperature of the coolant creating more voids. With less water absorbing neutrons the power of the reactor began to rise. A feedback loop was beginning to start in the lower part of the reactor core. The operators pressed the AZ5 button in response to the worrying increase in power and the control rods started their travel into the reactor core. As the control rod attached graphite displaces moved down to the lower part of the core in their control channels the neutron absorbing water was pushed out of the way creating a momentary spike in power replacing it with neutron moderating graphite. During the spike some of the fuel rods ruptured blocking the control columns stopping the control rods at 1.3 insertion. This left the graphite water displaces stuck in the lower part of the core increasing the runaway power excursion. Within three seconds the reactor power rose to 530 megawatts of thermal energy. In the control room a few shocks were felt and the control rod indicators showed that the insertion had stopped. As the channel pipes began to rupture mass steam generation occurred as a result of depressurization of the reactor cooling circuit. Fish and materials leaked into the coolant and steam pressure grew inside the core as the coolant boiled off creating a greater neutron population. The reactor power hit 30 000 megawatts of thermal energy although it is thought that the power could have gone much higher than that during the excursion. The steam blew off the top steel cover and top biological shield of the reactor as more than the two design fuel channels failed releasing debris into the reactor building. This was what would later be described as the first explosion but it would not be the last. A second more powerful explosion occurred between two and three seconds later and this destroyed the roof and ejected super heated pieces of graphite out of the reactor hall peppering the area around the building with shrapnel. Some red hot material fell onto the roof of the machine hall and started a fire. It is thought that around 25% of the core material was ejected but now uncovered reactor core mixed with the outside air and ignited a graphite fire. A blue glow could be seen outside the reactor building from ionizing radiation emitting from unit 4. There was nothing left of the reactor confinement structure which meant little stopped the harmful contamination from escaping. Some of the debris that was ejected started a fire on the roof of reactor 3's building but insanely the unit wasn't shut down straight away. Instead the operators were told to take iodine tablets and don't respirators. At 1.45 in the morning the first responders from the Chernobyl fire team arrived under the command of Lieutenant Vladimir Prevok and began trying to extinguish the flames on unit 3's roof. All around them laid pieces of unit 4's graphite core. The firefighters were only aware of the fire and because of this they didn't have any radiological protective clothing on. As they worked they breathed in some of the fish and material in the form of dust and were exposed to the deadly radiation. Many would come down with acute radiation sickness shortly after. Some of the first responders felt a metallic taste in their mouths similar to what others had reported post exposure for example from the victim of the Wood River Junction criticality. The fires on the roofs of units 3 and 4 were under control between 10 past 2 and 20 past 2 in the morning and the fire was fully put out at 5 o'clock in the morning. However the graphite fire in what was once unit 4 continued to burn expelling its radioactive plume of dust steam and smoke into the atmosphere. The operators started to pump water into the core but this did little to bring the situation under control. The estimated dose of 5 to 6 Roentgens per second was experienced around some parts of the reactor building which is equivalent to more than 20,000 Roentgens per hour which is enough to give you a lethal dose in minutes. The effective dose to meter monitoring of the operators was unavailable when the two 1,000 Roentgen per second capable meters were inoperable. The only remaining meters maxed out at 0.001 Roentgens per second which immediately read off the scale when used meaning that no one in the control room actually knew what they had received. The poor awareness of the dangers they were in left the operators believing that the core was still intact. Crew chief Alexandra Akimov stayed in the building until the morning ordering staff to try and pump water into the core. Many of these workers would be dead within a few weeks. You see the RBMK core was generally fought by staff to be incapable of becoming uncovered and as such the blatant evidence to the opposite was ignored. At 2pm on the 26th of April an announcement that you would never want to hear was broadcast in the city of Pripyat. For the attention of the residents of Pripyat the city council informs you that due to the accident at the Chernobyl power station in the city of Pripyat the radioactive conditions in the vicinity are deteriorating. The Communist Party is officials and the armed forces are taking necessary steps to combat this. Nevertheless with the view to keep people safe and as healthy as possible the children being top priority we need to temporarily evacuate the citizens in the nearest towns of Kiev region. For those reasons starting from the 27th of April 1986 at 2pm each apartment block will be able to have a bus at its disposal supervised by police and city officials. The first to leave the town was women and children and within 3 hours the city was empty a ghost town and left a decay. But this will be a subject of another video. Emergency accident management operation began on the 28th of April. The first priority was to try and tackle the radioactive material release. The first attempts were to try and put out the graphite fire. To try and put out the fire multiple items were to be dropped into the core. Each item that was to be dropped over the core was intended to tackle each different part of the fire and the radioactive release. First the radiological side was tackled by dumping a total of 5000 tons including about 40 tons of boron carbide, 2400 tons of lead and 1800 tons of sand, clay and 800 tons of dolomite. The method of deployment was to be via helicopter. Initially the first pilots received high doses as they hovered above the uncovered core. The pilots were then ordered to fly over the core dumping during the flight, which reduced pilot exposure but reduced the effectiveness of each sortie and had the side effect of damaging the remaining structures and actually spread the contamination even more. In total 1800 sorties were flown where they didn't do much and even worked as a thermal insulator increasing the fuel damage. Boris Shrebena became the Soviet crisis management supervisor and Valery Legazov from the Kurchatov Institute of Atomic Energy became a key advisor of the government commission formed to investigate the cause of the disaster and to plan the mitigation of its consequences. Initially the Soviet government wanted to not make the disaster public but that couldn't stay a secret for long. Radiation alarms were set off all over Europe and unsurprisingly questions were beginning to be asked. At around the same time as the helicopters were dumping material on the reactor the USSR officially admitted to an incident at Chernobyl. At around 9pm a 22nd broadcast the first of its type was announced. The firefighting efforts and pumping of coolant water left the basement flooded which caused another potential disastrous outcome another steam explosion. You see the fuel from the core was melting towards the basement which had flooded but below that was more water from the bubbler pools which was a large water reservoir for the emergency cooling pumps and as a pressure suppression system. Needless to say that this was the next point of focus for the accident management team. Engineers Alexei Ananikov and Valery Bezpolov and supervisor Boris Baranov volunteered to empty the bubbler pools. This was done by valves controlling sluice gates which were housed in a room that had now become flooded. All three men donned protective suits and breathing equipment and made their way down the corridor. The men were successful and the bubbler pools began to empty. Surprisingly all three survived not only the day but continued to live long after the disaster. Averting the immediate steam explosion but that wasn't into the worry of a deadly explosion. As described in the film China Syndrome the core could continue to melt through the floor of the containment and into the earth eventually hitting the water table meaning another steam explosion. The next focus of the accident management team was stopping the molten core from escaping the bottom of the reactor basement. A design that was fought up by physicist Lenid Bolshov that involved digging out a cavity below the reactor building basement. The concept used coiled formation of pipes cooled with water and covered on top with a thermally conductive graphite layer. The graphite cooling plate was to be encapsulated between two sacrificial concrete layers each one a metre thick. In theory as the molten uranium oxide hit the graphite it was to rapidly cool down catching the core meltdown. Thankfully the theory didn't have to be put into practice as the core had finally come to rest in one of the lower basements called the reactor building. Part of which formed the famous elephants foot which was discovered in December 1986 which had a reactivity of 8,000 runcans or 80 greys per hour. For reference a whole body dose of around five greys can be lethal. The molten material is known as corium and is a mixture of the material found inside the core and is mainly made up of silicon dioxide with traces of uranium graphite zirconium and titanium. The excavated cavity and cooling system now unneeded was filled with concrete to help the structural integrity of the heavily damaged reactor building. A plan was conceived to contain the contamination release of unit 4 in the form of a concrete lid which would come to be known as the sarcophagus. Construction began in May 1986, 24 days after the disaster to encapsulate some 200 tons of radioactive corium, 30 tons of highly contaminated dust and 16 tons of uranium and plutonium. The construction would last for 206 days between June to late November of the same year. But before the core could be enclosed under the sarcophagus the next phase of remediation was to remove all the core material that scattered the area and one of the most dangerous places was the roofs of the building surrounding unit 4. Initially robots were thought to be able to undertake the clearing works but due to radiation interference only around 10% of the debris could be recovered. This necessitated the use of good old fashioned people. The most basic of equipment was needed for the task which included protective clothing and a shovel. A toast of 5000 people known as liquidators were called upon to undertake the cleanup on the roof each only being allowed between 45 to 90 seconds of total working time to reduce the risk of exposure. The calculated risk was 25 rem or 250 millisieverts of radiation per person. There were many more liquidators also used for other clearing up works in the wider area and finally after this the sarcophagus could be closed over the turbine hall and reactor building. The construction was undertaken over eight stages all whilst unit 4 released contamination and next to unit 3 was still needed to be operable. Concrete containment walls were built around unit 4 and the dividing walls erected between it and unit 3. The sarcophagus was placed on top of the concrete containment walls. More than 400,000 cubic meters of concrete and 7,300 tonnes of metal were used during the construction of the sarcophagus. The building encapsulated an estimated 740,000 cubic meters of heavily contaminated debris together with contaminated soil. Holes were installed in the structure to allow pressure release albeit through a filtration system. The shelf life of the structure was only thought to be around 20 to 30 years necessitating a new containment structure but this will probably be covered in the next video in this series. So with an official death toll of 31, a nuclear power plant essentially unusable and many decades ahead of remediation works, how was unit 4 allowed to be operated in such a dangerous state? Unsurprisingly, it comes down to management. The warning signs of the potential for fuel damage in an rbmk-1000 reactor reared their heads several times the reactor types history. A partial fuel melting in Leningrad over 10 years before in 1975 had been covered up and information was not passed on to other rbmk operational staff at different power plants. Again, the fuel melting incident that happened in 1982 at Chernobyl unit 1 no less was also covered up, this time by Viktor Brookanov, meaning lessons again were not passed on. The risks of a positive effect of scram causing a power spike were known in December 1983 at Ignalio unit 1. Unlike other incidents involving rbmk's, the information from the event was passed on to other plants, but were thought to have not been important enough to act on, as the control rods didn't get stuck and did continue to stop the reaction. The IAEA was involved in the investigation into the disaster using the international nuclear safety advisory group, which had been created by the IAEA in 1985. All of the reports have never been able to categorically say for a hundred percent what the cause of the core uncovering was, and there is much speculation to whether the power excursion happened pre or post scram, but operating the reactor in the state it was in meant that something was going to fail. Regardless of the inherent floor designs of the rbmk, it was operated way out of the agree parameters, even set by the lax Soviet nuclear industry. It can be attributed to the culture of working where failure was not accepted and vital operating incidents were not learned from. As well as this, the design floors were outright covered up and held back from the operators. In 1987, a criminal trial was brought against five plant employees between the 7th and 30th of July, in a temporary courtroom set up in the House of Culture in the city of Chernobyl, Ukraine. They were deputy chief engineer Anatoliy Dyatlov, Chernobyl plant director Viktor Brookanov, the former chief engineer Nikolai M. Fomin, the shift director of reactor 4 Boris Rogozin, and chief of reactor 4 Alexander Kovlenko. Three other men, Alexandra Akymov, Lenid Topchenov and Valery Pebervazenko would have been tried as well, but the cases were dropped after their deaths due to radiation exposure. They were all found guilty of varying charges from criminal mismanagement of potentially explosive enterprises to negligence, receiving between 10 and 2 years amongst them. Dyatlov, who was in charge during the test, was given 10 years, would only serve a fraction of this being released due to bad health. He would die in 1995 from complications from his exposure. Brookanov, who had helped cover up the partial meltdown at unit number 1 in 1982, was also given 10 years but still lives to this day and also served a shorter term in his sentence. Even though I suppose the perpetrators were punished, the culture of the USSR nuclear industry meant that rules were regularly and blatantly broken and overlooked, especially when it came to important things like the ORM, which ultimately put the reactor in such a deadly state. This was all highlighted in the 1992 INSAG7 report, which also said in its summary, the plant fell well short of the safety standards in effect when it was designed, and even incorporated unsafe features, compounded by inadequate and effective exchange of information between both operators and between operators and designers. There was an inadequate understanding by operators of the safety aspects of their plant, also affected by insufficient respect on the part of the operators for the formal requirements of operational and test procedures. The industry in the USSR had an insufficiently effective regulatory regime in place to counter pressures for production. The pressures from central government and lack of awareness of the design risks of the reactor meant that management acted dangerously in the false pretense that the RBMK was indestructible. Pretty much the entire management structure was throwing around a grenade that they didn't know had the pin pulled out of, meaning disaster was only a matter of time. Now I'll be following up this video, with another video at some point, about the wider impact of the disaster on local and wider population, as well as digging deeper into liquidators and their heroic work. Thanks for watching, I hope you enjoyed the video. 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