 This video is sponsored by NordPass. More about that later on. In 1975 and 1982, a Soviet-designed RBMK reactor would experience a fuel-melting event and would signal the dangers of a flawed design, but these signals would fall on deaf ears. This one is a double bill and an intro to the RBMK reactor. It will form part one of a new series of videos on Chernobyl. Think of this as how the Hobbit is to the Lord of the Rings, which will be the Chernobyl disaster of 1986. Now, both of these events in this video have very little in forms of official reports, and this was due to the period of the time in which they existed, where a Soviet government would cover up any hint that Eastern block technologies were lacking in comparison to their Western counterparts. Today's subject I will place here on the Plainly Difficult Disaster Scale. Leningrad NPP is a nuclear power station in Solznovibor, Leningrad, a blast, 43 miles west of modern-day St. Petersburg. However, when the plant was first constructed, the city was actually called Leningrad. Construction began on the site in 1967 and was the first to use the ill-fated RBMK reactor design. Unit 1 first began operation in 1973, with units 2 to 4 opening in 1975, 1979 and 1981 respectively. Each installed reactor had the same net power output of 925 MW of electricity, however they produced much more in thermal energy. Now let's look at the RBMK reactor and its design. Reactor's history can be traced back to the mid-1950s with the light-water-called graphite-moderated 30 MW of thermal energy, AM1, reactor in Obnisk. The design was further expanded upon in the 1960s with the AMB100 and the AMB200 designs. Between 1964 and 1966 Soviet designers needed to field a new reactor that was cheap, easy to build and maintain, and be capable of electricity generation and also be able to produce plutonium for nuclear weapons. To facilitate this, the designers opted to use light water for coolant and graphite for moderation, a style which is unique to the RBMK. Now, moderator is used to reduce the speed of fast neutrons released from fission so they can better facilitate a chain reaction. Ideally, moderators work without capturing any neutrons, leaving them as thermal neutrons. So because of this, most reactors need a moderator to work efficiently, and different designs use different methods for moderation, for example graphite, light and heavy water. The combination of graphite moderator and light-water coolant offers up a strange result. As when compared to the PWR reactor, which is one of the world's most popular designs, the coolant and moderator in the form of light water are the same. The PWR is a very safe design, as once the coolant heats up and turns into steam, it loses its effectiveness as the moderator, slowing down the chain reaction, improving stability and eventually cooling down. However, the RBMK didn't have this due to having graphite as its moderator, and as it does not evaporate easily, still does its job even if the coolant boils away. When the coolant heats up turning into steam, it loses its neutron absorption capabilities, creating free neutrons, leading to an increase in reactivity. This is known as a positive void coefficient, which is not instantly a dangerous design if the working parameters of the reactor do not rely on the neutron absorption of the water to stay safe. How the RBMK reactor design did rely heavily on the steam content of its core for its reactivity. This has to be countered by the control rods, which regulate the overall power of the reactor, leading to a very fine balancing act. If not properly managed, an instant of a runaway can happen as the coolant heats up, increasing the reactivity, heating up the core more, creating more steam, leading to greater reactivity. This is called a feedback loop. Create a similar result to a diesel engine runaway as shown in this very well shot video. The use of the graphite core allowed low enrichment uranium-235 fuel to be employed for power generation at only a quarter of the expense of heavy water reactors, which had higher startup costs and much more complex maintenance. Also low enrichment unsurprisingly cost less to make, which was good for the Soviet accountants. The fuel was enriched to 2% and formed into pellets which were packed into a 3.65 meter long Zercaloi tube forming a fuel rod. 18 fuel rods were arranged cylindrically into a carriage to form a fuel assembly, which was then placed inside one of the reactor cores 1693 fuel channels. Within the reactor, each fuel assembly is positioned in its own vertical pressure tube 7 meters long. Each channel is individually cooled by pressurized water, which is allowed to boil exiting at the top at about 290 degrees centigrade. The steam is passed through a separator where it is sent to a high then low pressure turbine, which turns the generators producing electricity, after which the coolant is run through a condenser cooled by a separate coolant circuit from a nearby water source. From there it is sent back to the core to complete the cycle. The reactor had two separate coolant loops which mirrored each other passing through its own half of the core. Each loop had four pumps, three for normal operation and one for backup. The graphite moderator consisted of multiple blocks placed next to each other with a gap filled with a mixture of helium and nitrogen gas that formed the core region of the reactor and was around the size of a small house. The graphite didn't receive any type of cooling meaning its operational temperature was around 700 degrees centigrade, however the gas in between the blocks helped with heat conduction. The reactor was equipped with bore and carbide control rods with graphite tips and these were used to shut down and regulate the power of the reactor. Most of them were inserted from the top, however a number of shorter rods were inserted from the bottom for axial power control. The reactor had a safety shutdown in the form of the AZ5 button, which once activated would initiate a reactor scram. The main top inserted rods provided automatic manual or emergency control. The automatic rods were regulated by feedback from in-core detectors. In addition to this some shorter rods were inserted into the bottom of the core to help combat hot spots of uneven criticality. In total the RBMK at Leningrad had 170 control rods. The final part of the RBMK was its containment or lack thereof. You see the doctrine at the time of the RBMK was that it was always going to be operated within design specs hence no risk of disaster. But it didn't take into account human nature for buggering things up. What little protection to the outside world was provided was in the form of a reinforced concrete lined cavity that acts as a radiation shield. The reactor itself sat on a steel plate and was topped by a thousand ton steel cover. The steam separators were also housed in their own separate concrete containment. A contributory factor to the lower containment than other reactors was that it can be refuelled whilst in power operation. To achieve this a large crane is situated above the reactor. This meant that building a massive concrete dome containment structure would be time consuming and more important costly. Now that is a basic overview of the RBMK. The version installed at Leningrad was a first generation design. However there was a second generation design which was the type installed in Chernobyl unit 4. The second gen was virtually the same however one of the main differences was increased control rods to 211 and a reduction of fuel channels to 1661 in the graphite core. 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NordPass has both desktop and mobile applications for macOS, Windows, Linux, Android and iOS featuring a zero knowledge architecture which means your data is encrypted before it even leaves your device to be stored on Nord servers. You can also share passwords securely for websites like streaming services with people you trust. If you want to give NordPass a go then you can for free at www.nordpass.com forward slash Plainly Difficult. You can even upgrade to a premium package which allows up to six active devices and a data breach scanner as well as many other bits and pieces. This leads us back to Leningrad and relatively new RBMK Unit 1 on 30th November 1975. It is very difficult to reconstruct the accident as at the time the reactor was shrouded by secrecy, however some parts of the first person account does exist from a trainee who later worked at Chernobyl. On 29th November the reactor was being started up after some regular maintenance. Whilst powering up and reaching around 800 megawatts one of the reactor's turbo generators had control issues leading to the need to lower the power to 500 megawatts to take it offline. This was near 12 o'clock midnight and as with most disasters was the time for a shift changeover. The night shift was tasked with the continuation of raising the power of the reactor. By accident one of the operators shut down the remaining turbo generator leading to a reactor trip causing a scram of the control rods. This in combination with the reduction of the power due to the TG being taken offline increased the level of zen and poisoning within the reactor. Not wanting to cause too much of a delay the chief operator issued the order to put the functional TG back online and to restart the reactor. The unit was restarted to the minimum controllable level however the poisoning of the reactor was at an unacceptable value reducing the operational margin to just eight control rods. This meant that nearly all of the control rods had to be fully removed from the reactor in order to achieve an output level for the TG to resume power generation. Again a scram was triggered shutting down the reactor due to uneven neutron power. This is due to the size of the RBMK core meaning that different areas could gain criticality whilst other areas remain poisoned which would lead to hotspots. Another restart was attempted and was more successful in reaching the minimum controllable level. At 6.15 am the reactor capacity was raised to 1000 megawatts of thermal energy. By 6.33 am the power was raised to 1720 megawatts. The operators had conducted too much of a fast rise of the reactor power, deadly for the highly poisoned and small margin state of the reactor. Power levels to load the TG were achieved when operators received warnings of low water flow levels. The core had developed hotspots and the operators attempted to fight these by lowering some of the manual control rods whilst leaving the automatic ones to manage the overall power output. Scarily a new alarm sounded in the control room indicating moisture in the graphite channel 1333. The presence of moisture in the graphite moderator hinted at a rupture in the high pressure cooling tube around the fuel meaning a potential meltdown. At this point the AC5 button was pushed activating a scram shutting down the reactor. It was found that severe damage to the reactor core, one technological channel had collapsed and 32 fuel assemblies had burned due to operating the reactor in an unsafe manner. The damaged channel proved difficult to remove using the refuelling machine causing issues for workers. Due to improper confinement around 1.5 MCI was released into the atmosphere and five kilometers from the affected power unit radiation levels were recorded at 600mph immediately after the fuel damage. The accident was caused by the operators trying to make good the error of accidentally shutting down the operating TG. By doing so they worked outside the regulations for low power operation but the management culture at the time didn't consider this a major issue. Well how wrong they were. The Soviet Ministry responsible for nuclear industry was in charge at Leningrad making any information about incidents state secrets. This meant that no operators at other RBMKs would have even known about the fuel damage let alone learn from the mistakes. Right let's go on to the other lesser known Chernobyl incident and that is the fuel damage at unit 1 in 1982. Chernobyl NPP is a now shut down plant in Ukraine. Unit 1 was the same type that was used at Leningrad and was installed in 1972 achieving operation in 1977. There isn't a great deal to write about again as it was covered up at the time with the help of Viktor Brookanov who will most likely show up again on this channel in the future. However what is known from a KGB internal memorandum was like at Leningrad the RBMK reactor had been shut down for maintenance and have received fuel damage. The maintenance was scheduled to be complete on the 13th of September 1982 and before this the reactor had to be tested. Upon restarting and reaching 700 megawatts during the test run a fuel channel number 6244 was starved of water overheated and melted severely damaging the core and again leading to moisture in the graphite moderator. The issue wasn't noticed for between 20 and 30 minutes before an AZ-5 button activation allowing significant contamination of the coolant. Now it's unknown for certain what caused the coolant starvation as initially the operators were blamed however later on a failed valve was also seen as a potential cause but it is difficult to know for certain due to the cover up. The channel was severely damaged and the repair turned into a dirty radiological cleanup. It took almost a year to fix the damage from the accident and the core area adjacent to the destroyed channel was out of use indefinitely. The biggest issues from both these incidents was operators not working within the parameters set out from, either from negligence or pressure from management. The worryingly dangerous culture set out by management and their wider political sphere of nuclear power in the USSR would culminate in a much bigger involving an RBMK in 1986. Thank you to NordPass for sponsoring this video, sponsorships like this really help keep the lights on at Plain and Difficult HQ. 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