 Natural disasters have been a fear of mankind throughout history, from a volcano near Pompeii to an earthquake in the Pacific Ocean. A natural disaster can devastate a whole region, causing mayhem and destruction in its wake. A deadly earthquake and resulting tsunami off the coast of Japan would result in nearly 20,000 missing and dead, a country economically wounded and a chain of events that would cause a nuclear disaster on a scale which the world had not seen since Chernobyl. If you like what we do here at Plainly Difficult, consider helping the channel grow by liking, commenting and subscribing. Let's get started. We have covered many reactor incidents on this channel, however something that makes today's subject unique is that the damage to equipment and the environment is worse than anything else I've covered on this channel. Needless to say, today we are going to dive into the Fukushima disaster. I'm going to rate this event here on the Plainly Difficult Disaster Scale, which is higher than the INES which rated it at a 7, the same level as Chernobyl, so that obviously means that my rating scale is better. The Fukushima Daiya Ichi NPP site lies approximately 220km north of Tokyo, at almost the midpoint of the Pacific coast. It straddles Okuma and Futabara townships in Fukushima Prefecture, which is around here on the map. The site is approximately 3.5km large. It is operated and managed by Tokyo Electric Power Company or TEPCO. The site houses six boiling water reactors, three built by General Electric, two by Toshiba and one by Hitachi, although all were designed by GE. Planning on the site began in 1967 on Unit 1, with its commercial operation beginning in 1972. For the next seven years, the remaining five units came online, with Unit 6 commissioned in October 1979. During the 12 years between 1967 and 1979, BWR design improvements led to variations between the six units on the site. After Unit 1, which was an earlier BWR-3 design, Units 2 to 5 were BWR-4 designs, and Unit 6 was a BWR-5 design. Because of this, the power outputs of the three designs were different with Unit 1 having 460MW of electricity, Units 2 to 5 having 784MW of electricity, and Unit 6 having 1100MW of electricity. Fukushima Daiichi plant is connected to the power grid by four lines, the 500km line, the 275km or Kuma lines, and the 66km Yonomori line to the Shin-Fukushima substation. The site was built on a bluff 25m above sea level. Originally it was intended to be at 35m, however the designers lowered the height to reduce a strain on seawater pumps, as well as making the foundations closer to stable bedrock, which helped keep the plant more earthquake resistant. This lowered height was thought to be tsunami safe in conjunction with an adequate seawall. The reactors on site were in two groups. The first, when looking at the station from the sea, contains Units 4, 3, 2 and 1 going from left to right. The rightmost group contains Units 5 and 6. Like all commercial nuclear reactors, the end goal is the generation of heat for the purpose of making steam. This is used to drive a turbine, which then generates electricity. Right before we dive into the disaster, let's have a look at how a boiling water reactor works. As this type of reactor is not often covered on this channel, for the purpose of simplicity, this is a general overview of how the GEBWRs work, however there were many variations between the three types used at the site. A BWR design uses demineralised, light water for both cooling and moderation, much like a pressure water reactor. This type of reactor is the second most common type after the PWR. However, unlike a PWR, which uses the heat of the coolant to create steam in a secondary coolant loop, in a BWR the boiling of the primary coolant is used for the steam. The steam is collected in the top of the reactor before passing towards the turbines. The steam generated from the fission passes through the turbine, after which it goes to a condenser to be returned back to water. Inside the condenser, a secondary loop of water is used and is kept separate from the primary to stop cross contamination. At Fukushima, seawater was used for condenser cooling water and auxiliary equipment cooling water. A seawall 10m high from the seabed was built in front of the power station, with open channels behind it that led from the power plant to the ocean. Water was drawn in through sluice gates and pump rooms installed for each unit. From there it is transferred to the condenser by pumps installed in the pump rooms. The fuel used at Fukushima consisted of fuel pins bundled in square arrays. The fuel pins consist of low enrichment uranium oxide or mixed uranium and plutonium oxide. Fuel pellets enclosed and sealed in zirconium alloy cladding tubes. Each of the reactors had control rods for regulating the power and had 97, 137 and 185 across the three different BWR types. The rods were inserted into the reactor from underneath. The control rods had the ability to scram the reactors shutting down fission in an event of an emergency. In the event of a power cut, backup power is generated from diesel power generators. This backup system was important for powering the coolant pumps in the event of an emergency. The reactors have two types of containment. The first being the reactor primary containment vessel, the second being the building in which it is housed. The core is kept within a containment vessel. Around the vessel there is an outer containment which is enclosed by a concrete plug. The plug can be moved by a crane over the spent fuel pool. The spent fuel pools is where used fuel rods can be stored. The PCV has two major compartments. The reactor vessel is located in the dry well. The DW is connected to a second compartment, the suppression chamber, which holds a large amount of water and enough space to suppress pressure increases. The water within the suppression chamber can be used to scrub radioisotopes from any gases released within the containment vessel. The primary containment vessels were filled with nitrogen to provide prompt control of any hydrogen generated during an incident. The secondary confinement is provided by the reactor building itself and is designed to try and hold any contaminants in a last line of defence of environmental protection. The site had three control rooms as each was used to control two reactors. Although each reactor had its own panel, this setup allowed close working between reactor teams. Each reactor's emergency diesel generators and associated equipment were stored underneath the turbine buildings between 7 to 8 metres below grade. The backup power systems also used DC batteries. These were located in the basements of the control buildings for units 1, 2 and 4 and in the mezzanine levels of the turbine buildings for units 3, 5 and 6. Units 1 to 5 had two diesel generators each and unit 6 had three. The batteries gave the power plant up to 8 hours of emergency power in the case of electrical isolation from the grid. Issues were raised by some engineers during construction as placement of safety critical equipment below grade had a higher risk of flooding, especially when combined with the lower bluff of the site. However, these issues were swept away by TEPCO's decision to follow GE's design plans to the letter and, oh boy, that would come back to bite them. This leaves us to March 2011. Three of the six units were shut down for refuelling, leaving one, two and three in operation. There were some 6,400 workers on site, approximately 2,400 consisting of 750 TEPCO personnel and around 1,650 contractors were working in the controlled area with approximately 2,000 carrying out work in the support of the planned refuelling. Unit 4 had its fuel rods removed and units 5 and 6 still had the fuel elements inside the reactors, however the control rods were inserted into the core to stop fission. On the 11th of March, at 1446 local time, an earthquake of a magnitude of nine lasted for two minutes. It was caused by a sudden release of energy at the interface where the Pacific tectonic plate forces its way under the North American plate. The earthquake was the largest ever recorded in Japan and the world's fourth largest since records began in 1900. At the time of the earthquake, sensors at the plant detected ground movement and initiated a scram in the three operating reactors. This was built in to the design of the plant and the action controlled the reactor's reactivity. The offsite power grid connections were lost during the earthquake. Because of this, the emergency systems needed power to be supplied by the 13 on-site generators. Even after shut down, the reactors needed power to monitor and pump coolant through the cores, as after fission, the fuel elements still provide decay heat strong enough to cause fuel element melting. To help with the cooling, in normal shutdown, the turbines are bypassed and the coolant in steam form goes straight to the condensers, after which it is pumped back to the core, complete in the cycle. However, during this event, the reactor was completely isolated from the turbine building due to the loss of power caused by the earthquake. With the usual way of cooling now isolated, a backup system using the suppression pool was used. Unit 1 used a different system to the remaining reactors. It used two closed cooling loops by sending the primary coolant through a heat exchanger using a secondary tank of cooling water. Once the heat was exchanged from the reactor coolant to the secondary water, the primary coolant then returned to the reactor via gravity. However, this system was cooling down the reactor too efficiently, fearing damage to the reactor vessel from extreme heat changes, one of the systems was turned off. This was set out in the operational procedures. The units 2 and 3, an open system was used which necessitated additional water. This was powered by steam from the primary coolant turning a small turbine, which in turn injected water back into the reactor. The steam that ran the turbine went to and accumulated in the suppression pool inside the primary containment vessel which served as a heat sink for absorbing the waste heat. The water needed to continue cooling the reactor was supplied from a condensate tank. Once the tank was empty or the suppression pool was full, water was then supplied from the pool for cooling. However, not only the operating reactors on the 11th of March had decay heat. During refuelling, the spent fuel rods are placed in pools near each reactor. These are also still hot so effective cooling is normally provided by electrical power. If it was just an earthquake, the event would have been effectively managed using the onsite generators and batteries. However, 40 minutes post seismic activity, the first tidal wave just under five meters tall crashed up against the sea wall which effectively protected the plant. However, just 10 minutes later, a second 14 meter high tsunami wave was heading for the power station. The wave effortlessly crashed over the sea wall which only provided protection of around five meters above the normal sea level. The water flooded the turbine rooms, shorting out their electrical systems. The wave damaged the unhoused sea water pumps. This meant that essential plant systems that use sea water including the liquid cooled emergency diesel generators could not be cooled to ensure their effective operation. The flooding water made its way into all buildings including the vital basements that housed the generators and their associated electrical equipment. This resulted in loss of emergency AC power. One air-cooled generator survived and continued to supply emergency power to unit six. This meant, however, that units one to five had total power failure. The power station was designed to be able to work off DC batteries for up to eight hours in the event of a loss of AC power generation. However, some of these systems were also affected by the flooding inundating the functional DC systems. Power started to dwindle in units one, two and four, 15 minutes post-flood due to loss of all AC and DC power. The operators of units one and two could no longer monitor the reactor pressure and the reactor water levels or key systems and components used for core cooling. This situation had no procedure set out for the operators. Because of this, the emergency control room supervisors were flying blind and had to find some way of resetting power to the stricken plant. Units three, five and six maintained power, allowing the operators to observe the power status as the main control room indications and controls were still functioning. Units three and five still had DC battery power and six still had full AC power. To extend the time between full plant blackout on units three and five, all non-essential systems were shut down. In unit three, the operators manually restarted the reactor core cooling system, controlling and monitoring the reactor water injection with the available DC power. Unit one's attempt to restart the shut down cooling system failed due to loss of power and the reactor vessel created more pressure as the core began to increase in heat. The high pressure disallowed the use of an external cooling source, for example, from an external pump. After the approval of the prime minister, a nuclear emergency was declared at three minutes past seven local time. With no indications due to power loss, the operators at unit one and two assumed a worst case scenario of no way of cooling the reactor cores, which would mean potential core uncovering of unit one and unit two. In other words, no coolant in the core, which could lead to a meltdown. This information was passed onto the government bodies at around one minute past nine PM. An order at 2123 was given by the government to evacuate an area of around three kilometers near the plant. High levels of radiation were detected in the unit one reactor building, indicating core damage at 2151. At 10 to 12 at night, the reactor vessel at unit one showed signs of being over pressured beyond design spec. The site superintendent ordered preparations for venting of the unit one containment vessel. At 2 10 AM on the 12th of March, a team was able to enter the room where unit twos reactor core isolation cooling system equipment was located and read the parameters to determine the system status. The system was confirmed working in unit twos core as a steam being created inside the core was successfully powering the small turbine in the backup cooling system. Upon hearing that unit twos cooling was still functional, operators focused on dealing with the unfolding drama at unit one. At 4 AM, an alternate cooling system was put into operation using fire trucks pumping sea water into unit one as the vessel pressure had reduced. Unit one's containment measurement at 4 19 AM on the 12th of March showed that pressure had decreased since the last measurement without any operator action and without an official venting route, indicating that some unintentional containment pressure relief had occurred through an unknown path. This coupled with an increase in radiation levels hinted at reactor vessel damage. Because of this, the government extended the evacuation zone to 10 kilometers. Plans for venting of unit one's containment was set to start at 9 AM on the 12th of March. As soon as the Fukushima prefecture authorities confirmed at 9 02 AM of the completion of the evacuation of Akuma town, the team started the manipulation of valves in order to arrange the path for the venting of unit one's containment to atmosphere. At 2 PM, the final valve was manipulated. Success of the venting operation was confirmed by a decrease in containment pressure at 2 30 PM. Initially, no increase in radioactivity was seen, however, this would change one hour post-venting and was measured at one millisievert an hour near unit one. At unit three, after 20 and a half hours of operation, the reactor core isolation cooling system ceased to operate at 11.36 AM. The system was unsuccessfully restarted resulting in the heat within the reactor core creating steam lowering the coolant level. An automatic high pressure coolant jet system started after the level dropped past the set level. At half past three PM, work to connect the mobile voltage power supplies to units one and two using an undamaged transformer in unit two was completed and a low voltage grid for supply of AC power to unit one was re-energized. However, before the benefits of the return of power could be reaped, unit one exploded. The upper part of the reactor building of unit one was severely damaged in the explosion. Although no damage was caused to the primary containment, the secondary in the former building was now compromised. Three hours later, the evacuation zone was extended to 20 kilometers. The cause of the explosion was thought to be from hydrogen that had escaped from the reactor via an unknown path. Teams returned to the site to repair the damaged electrical and water feedlines. After these were repaired, the reactor had been without cooling for around four hours. Although things were kind of back under control for unit one, unit three started to become the next drama during the disaster. Fearing that the turbine-driven high-pressure cooling system would fail with the lowering steam within the reactor, unit three was planned to be hooked up to an external water pumping source. To facilitate this, the high-pressure system needed to be switched off for valves to be opened to be hooked up to the new cooling system. The valves could not be opened. Whilst the operators struggled for 45 minutes, the pressure within the reactor rose disallowing the external water pumping option. Pating themselves into a corner, the operators attempted and failed to restart the high-pressure system, essentially leaving unit three with no cooling system at all. An emergency at unit three was declared at 5.10 a.m. on the 13th of March. At 5.15 a.m., fire engines were called up to pump water into the reactor core like had been done for unit one. Again, like with unit one, a venting path was needed to lower pressure. To reduce the reactor pressure below the fire engine pump pressure required the activation of pressure relief valve. This was achieved by the use of 12-volt batteries taken from cars at the site, which were collected in the common main control room of units three and four. The reactor pressure vessel fell below what was needed to use the fire engine pumps. An injection of boriated fresh water into the unit three reactor started at 9.25 a.m. Unit three had also gone for more than four hours without cooling. At 2.15 p.m., high radiation dose rate of around one millisieverts an hour was measured near the site boundary. 15 minutes later, the radiation dose rate exceeded 100 to 300 millisieverts an hour at the entry doors of the unit three reactor building. At 6.30 a.m. on the 14th, the water level in unit three dropped as cooling water supplies that were being pumped began to dwindle. At 11.01 a.m., an explosion occurred in the upper part of the unit three reactor building, similar to what happened at unit one, destroying the structure above the service floor. In addition to the destruction of the alternate water injection arrangement, the capability to vent the containment in unit two was lost. At around 1.00 p.m. on the 14th, unit two began to experience cooling issues as well. In what seems to be a repeating theme, reactor pressure increased as the cooling water level decreased. To enable low pressure fire truck water pumping, safety valves were operated to help drop the reactor pressure. The fire trucks began to pump water at 8 p.m. At around 9.55 p.m., confinement radiation levels had increased substantially to 5,360 millisieverts an hour in the dry well and 383 millisieverts an hour in the suppression chamber. At 10.30 p.m., the unit operators tasked with establishing the venting line to relieve the containment pressure when able to open the vent valves. Venting from the containment was unsuccessful meaning that there was little that the operators could do to stop pressure buildup within the reactor containment. And in the early hours of the 15th, the inevitable happened. In the early hours of the 15th of March, explosions were heard at units two and four with the top half of unit four's building being damaged. A drop in pressure readings on the suppression chamber of unit two was seen. This hinted in the containment of the reactor had been compromised, meaning an uncontrolled release of radioactive isotopes. Following the events in unit two, all personnel except essential workers required for monitoring and emergency response were instructed by the site superintendent to go to a radiologically safe location. 650 people evacuated to Fukushima, Diani nuclear power plant site, approximately 12 kilometers away. White smoke or steam was seen to be coming from the top of unit two's reactor building. A radiation measurement was taken of 11.93 millisieverts an hour at the main gate at 9 a.m. An order was issued by the government authorities at 11 a.m. requiring all residents within a 30-kilometer radius of the power plant to take shelter indoors. A team needed to enter the reactor building of unit four to investigate the status of the spent fuel pool. However, upon entry, they recorded radiation levels of 1,000 millisieverts an hour. After an aerial survey on the 16th, water levels were confirmed in unit four's pool. However, it was unknown about the condition of unit three, making its pool a priority to ensure enough water was in place to stop the spent fuel from becoming uncovered. Spraying started on the 17th with helicopters dropping around 30 tons of seawater. Later, fire hoses and water cannons were used to fill the pool. Additionally, on the 20th, unit four received water spray as well. Later on into March, power was gradually restored to units one to four with units five and six receiving power from the only working air-cooled generator. Units three and four were the last to receive power after being completely cut off for more than two weeks. At this time, seawater was swapped for a boreated fresh water in the cooling efforts of the reactors, meaning the situation was gradually coming back under control. Unit five was the first to be put into cold shutdown mode at 14.30 on the 20th of March, 2011. This was followed by unit six at 19.27 on the same day. However, the remaining units had a long journey ahead as a more stable situation was achieved in April, 2012. TEPCO published in May, 2012, an estimate of the amount of radioactivity released at Fukushima, which was about 1,020 peta-becuels over the 12th to the 31st of March. 500 peta-becuels of iodine-131, 10 peta-becuels of cesium-137, and 10 peta-becuels of cesium-134. The remaining was noble gases mainly made up of xenon-133. Releases to the ocean over the 26th of March to the 30th of September were thought to be about 11 peta-becuels of iodine-131, 3.5 peta-becuels of cesium-134, 3.6 peta-becuels of cesium-137, a total of 18.1 peta-becuels. The release into the ocean came from the water pumped into reactors during the forced cooling, which had subsequently been contaminated by the damage reactor cause. However, the publications were only estimates, and there is much uncertainty as to how much was released into the sea. This is mainly due to the area being flooded at the time of the contamination of the water. It was estimated that Fukushima released around one-tenth of the contamination of Chernobyl. Tests on caught fish around the area were shown to have the same levels of contamination in 2012 as they had post-accident in 2011. Hinting had a more prolonged release of contaminants into the ocean. Post-accident and exclusion zone of 20 kilometers was set up around the plant, meaning residents within the zone had to leave. However, other towns and villages outside the 20-kilometre zone were also evacuated for decontamination. In March 2012, three area definitions were set up to describe each evacuation status of the towns and villages around the stricken power plant. Area one is known as the evacuation cancellation prepared zone. These are areas where it is confirmed that the annual dose of radiation will definitely be 20 millisieverts a year or less, and the evacuation order will soon be lifted. Residents are allowed to visit their property, but not stay the night. And people in the area are not required to take protection from radiation. Area two is where residents are not permitted to live, known as the restrictive residence zone. Areas where the annual integral dose of radiation is expected to be 20 millisieverts a year or more, and where residents are ordered to remain evacuated in order to reduce the risk of radiation exposure. Residents can also return to this area, but not stay the night as well. Area three, the difficult to return zone, is where it is expected that residents will not be returning home for a long time. These are the most restrictive areas and entry is prohibited unless specifically allowed to, and whilst in protective clothing. The annual dose of radiation in these areas is expected to be 20 millisieverts a year or more within five years, and the current integral dose of radiation per year is 50 millisieverts a year or more. Gradually, as decontamination efforts were undertaken, many places affected in 2011 became habitable. However, such areas as Futaba and parts of Nami and Okuma are still inaccessible to residents. The roughly 195,000 residents who lived in the vicinity of the plant were screened by the end of May 2011. No elevated health risks were predicted. All of the 1,080 children tested for thyroid gland exposure show results within safe limits, according to the June IEA report. In December, around 1,700 residents were checked with the majority showing exposure levels below one millisievert a year, with the remaining all but 10 receiving below five millisieverts a year, with the final 10 in excess of 10 millisieverts a year. A 2012 Hirosaki University study reported 46 out of 62 people tested had activated thyroids from iodine 131. The average dose was 4.2 millisieverts and 3.5 millisieverts in adults and children respectively. This was much lower compared to the tests on Chernobyl evacuees, who received an average of around 490 millisieverts. It was estimated that there were around 700 deaths from disaster-related incidents, for example people uprooted from homes and hospitals because of the evacuation. Another big contributor to health risks from such an incident is the psychological trauma linked to being near a nuclear disaster. Some of the worst affected people were the key workers at the plant who were faced with the fast-moving, ever-increasing disaster. Many who worked on the recovery efforts had to complete tasks in less than favorable conditions, all whilst not knowing the safety of their friends and family due to the tsunami in the region. As of 2018, the Japanese government has acknowledged four workers had radiation-caused illnesses, with one dying of lung cancer at the age of 50. A frozen wall was installed around the site to try and limit the seepage into groundwater. Since the meltdowns, TEPCO was fighting a losing battle with water that flowed downhill towards the sea, which made its way via Fukushima's fractured reactor buildings. However, the frozen soil barrier had failed to completely stop water seepage. For the best part of the next decade, works at the site have been painfully slow, with not one but three melted-down reactors to deal with. In April 2011, two robots sent into reactor building one, recorded radiation levels as high as 1120 millisieverts an hour. In May, another robot was sent into unit one and discovered levels in excess of 2,000 millisieverts an hour. In 2012, radiation levels were found between 31.1 and 72.9 severts an hour inside the containment of unit two. The status of the fuel in units one to three was unknown as it had melted out of the reactor's cores for several years post-event. In 2015, another robot was sent inside the reactor confinement for unit one. However, intense radioactivity immobilized it. Finally, in 2017, a remote-controlled robot took the first pictures of the melted core of reactor three. In 2019, a robot with two fingers made first contact with the fuel debris in the primary containment vessel of reactor two. A roadmap to start removing fuel from the three units was set out with unit three storage pool beginning in 2018. Plans to begin to remove fuel from the reactors is set to begin in 2021. The Japanese government so far has put $27 billion into cleaning up the mess with around 75,000 workers scrubbing the roads, walls, roofs, gutters, and drains. Around 600 million cubic feet of grass, trees, and topsoil have been removed and stuffed into millions of black bags. But the road to completion of decommissioning and decontamination left by the 2011 disaster will take many more decades until it's finished. Now, the IAEA report, as always, is well worth a read for a full dive into the disaster. If you'd like me to cover the aftermath in better detail, let me know in the comments and I may make it into a future video. I hope you enjoyed the video. If you'd like to support the channel financially, you can on Patreon from $1 per creation. That gets you access to votes and early access to future videos. I have YouTube membership as well from 99 pence per month and that gets you early access to videos too. And all I have to say is thank you for watching.