Understanding the accident of Fukushima Daiichi

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Friday March the 11th at 2:46 p.m. an exceptionally powerful earthquake hit the pacific coast of Honshu the main island of Japan at 3:30 6:00 p.m. less than an hour after the earthquake a tsunami swept over the coast the waves went all the way up to ten kilometers inland result over 20,000 people dead or missing destroyed towns ports and land devastated nuclear power plants were also affected one in particular namely the Fukushima Daiichi Fukushima Daiichi is 250 kilometers northeast of Tokyo the nuclear power plant has six reactors each reactors successively commissioned during the 1970s units 1 2 & 3 were operating at full power the core in unit 4 was unloaded units 5 & 6 were in cold shutdown Fukushima reactors have a different technology than the pressurized water reactors built by the French operator EDF they are boiling water reactors called BW ours we say reactor because the heat in the core is produced by fission reactions boiling water because the water that removes the heat from the core turns into steam and the steam goes directly to the turbine the turbine drives the generator that produces electricity afterwards the steam is condensed with the help of a seawater cooling system and returns to the core a boiling water reactor has only one single system combining feed water and steam the core is composed of fuel assemblies containing uranium it is controlled by control rods introduced from the bottom that can stop the fission reactions in case of an emergency fission of uranium nuclei produces radioactive atoms that in turn produce heat and this continues to occur even after reactors shut down this is called residual heat keeping the fuel confined and cooled is a major safety issue the fuel is isolated from the environment by different containment barriers just like the famous Russian dolls a first barrier the fuel cladding of zirconium alloy a second barrier the steel reactor vessel in combination with steam and water cooling systems finally the third barrier the containment building in concrete with a leak tight steel liner the fuel is kept under water in the reactor as well as in the adjacent pool where the spent fuel is unloaded the pool is located at the top of the reactor vessel to facilitate the transfer of fuel under water when the earthquake hit the coast seismic sensors triggered the insertion of control rods although fission reactions stopped the residual heat had to be removed the off-site power supply was lost in the emergency diesel generators took over automatically they supply electricity to the backup systems needed for core cooling in reactors 2 & 3 it is a turbo pump the steam generated by the reactor operates the turbo pump which feeds water into the reactor vessel the steam is condensed in the wet well suppression pool within the containment in reactor 1 there was no turbo pump but a heat exchanger which condensed steam from the reactor vessel the condensed water was reintroduced into the reactor vessel by gravity this heat exchanger provided core cooling by natural convection for more than 10 hours until then everything seemed under control however reactor 1 due to excessive cooling forced the operators to temporarily isolate the heat exchanger in compliance with operating procedures the tsunami wave arrived less than an hour after the earthquake the waves went over the seawall flooding the lower parts of buildings and disabled the emergency diesel generators on reactor 1 the operator was unable to reactivate the heat exchanger the core was no longer cooled it would be the first to melt on units 2 & 3 the batteries were still operational they operated some of the valves the turbine driven pumps ran for nearly 24 hours and then stopped the cores were no longer cooled the meltdown scenario is almost the same in all three reactors only the dates change the water in the reactor vessel evaporated the fuel became uncovered heated up to a temperature of 2300 degrees Celsius the fuel melted and mixed with the materials from the structure to form a magma called corium the corium flowed down to the bottom of the reactor vessel according to Japanese officials it pierced the reactor vessel before falling on the concrete basement inside the containment what quantity of corium fell how deep do duty road the concrete did it pierce the steel liner even today it is not possible to learn more about the state of the corium in the three reactors at the same time still in the reactor vessel the steam was loaded with radioactive elements and Hydra to explain this phenomenon let's have a look at the early stages of fuel degradation heated at high temperature the fuel cladding is oxidized and cracks releasing volatile radioactive elements in addition to this the zirconium of the fuel clad created a reaction with the steam by absorbing the oxygen and by releasing hydrogen normally when mixed with air hydrogen catches fire and explodes however the containment building was filled with nitrogen an inert gas that avoids the presence of oxygen at this stage there was no risk as the steam pressure rose to a dangerous level in the reactor vessel the depressurizing valves opened gas was forced into the wet well suppression pool by a venting line the water acted as an efficient filter by trapping much of the radioactive elements but the water was no longer cooled because the emergency diesel generators were out of order and it soon began to boil thereby reducing its filtration capacity the wet well suppression pool in the communicating containment began to enter into an overpressure situation to avoid containment rupture the operator decided to release the gas into the atmosphere normally the venting line should have led all the gas outside the building by the chimney of the plant but hydrogen was escaping through uncontrolled leakage pathways and was released into the reactor building hydrogen reacts violently with oxygen in the air the explosion blew apart the frame at the top of the building apparently without damaging the containment building radioactive elements not yet trapped in the wet well suppression pool were released into the environment due to the absence of usable fresh water on the site the operators decided to inject seawater into the reactor vessel this solution far from ideal since salt is chemically active had at least the advantage of cooling and stabilizing the corium in the four days following the tsunami the four reactors were damaged by explosions and three of them with core melt although it has kept its structure intact reactor 2 is the current source of the most important radioactive releases into the soil as well as into the sea the explosion took place inside the building operators have probably encountered difficulties depressurizing the containment and the wet well suppression pool broke this loss of leak tightness led to the discharge into the atmosphere of unfiltered radioactive elements and to the spreading of highly contaminated water in the building's leading to highly polluting discharges into the sea the explosion of reactor 4 was due to hydrogen even though the core was completely unloaded the hydrogen came from reactor 3 via a joint pipe the reactors storage pools were also a great concern because they have lost their cooling systems and in addition to this they were not protected by any containment very little spent fuel was stored in pool 1 however there was much more in pools 2 3 & 4 especially pool 4 which contained the equivalent of the three cores in all three pools the water started to boil and without the help in extremists of cold water from helicopters and from a firehose the spent fuel have caused considerable radioactive release into the environment gradually the situation began to stabilize by the end of March 2011 fresh water had replaced seawater in July the reactor cooling system was again in operation in closed circuit thereby avoiding discharges of contaminated water into the environment in December 2011 Japanese authorities officially declared that the nuclear power plant reached the cold shutdown state an expression used when the cooling water does not evaporate anymore and remains liquid below 100 degrees Celsius this nuclear crisis was managed by men working under extremely difficult conditions cut off from the rest of the world with no news from their families after the tsunami without any power supply threatened by radiation they fought with all their force to cool the reactors trying to make in vain the backup systems work again or by using improvised means after this race against time to cool the plant followed a year where about 20,000 workers succeeded each other trying to regain control of the plant by first enhancing the dike against another tsunami mapping the site contamination clearing every access to the site immobilizing radioactive dust treating and disposing of contaminated water and avoiding further radioactive release in the years ahead the challenge will be to remove the spent fuel from the pools for final storage and radioactive waste repositories and then eventually on the long term under the critical eye of international experts the issue will give way to a challenge namely to remove the melted fuel from the three damaged reactors and to dismantle the site as we can see a huge tasks awaits the Japanese a task that started in March of 2011 and that will last for several decades you

Info

Channel: Institut de Radioprotection et de Sûreté Nucléaire - IRSN

Views: 2,317,413

Rating: 4.8398371 out of 5

Keywords: fukushima accident, nuclear accident, nuclear safety

Id: YBNFvZ6Vr2U

Channel Id: undefined

Length: 13min 1sec (781 seconds)

Published: Tue Jun 19 2012

Reddit Comments

I read what I believe was the official English report on the disaster (I think it was called "The 2011 Fukushima Nuclear Power Plant Accident" by Hatamura, et. al.). As is usually the case, there was a lot of design flaws and operator error that lead to the disaster.

Going from memory, the core elements of the disaster were 1) having the backup batteries, generators and fuel on the ground, so when the tsunami hit they were largely useless and 2) the local chain of command hesitated thinking they could salvage issues and weren't able to get direction from higher ups that wanted to scram the whole thing and 3) progressive power loss resulted in incorrect and inconsistent readings on various critical elements, so the operators didn't know what was going on inside the reactors and consequently made poor and incorrect decisons.

People like to compare this to Chernobyl, but there really isn't any, except at the most surface level. The cores were all contained (Chernobyl's cores were blown apart and exposed directly to the environment) and the radiation released was minimal.

The main lesson learned, as I recall, was pretty simple: don't put your backup generators and batteries on the ground where they're going to be flooded with seawater from the easily predicted tsunami that happens once in a while (I believe that was in the original site planning document, but was overlooked/overridden during construction). There were some other, relatively minor operational issues, e.g., some of the passive heat venting mechanisms failed due to poor maintenance or overheating, but the key seemed to be the operators either did not have the authority locally to scram the reactors or did not believe they did, so they waited too long to start the power down sequence and events got out of their control.

👍︎︎ 16 👤︎︎ u/mitakeet 📅︎︎ Feb 28 2019 🗫︎ replies

Interesting seems removing the corium will proof to be the biggest challenge. In Chernobyl it's still there.

👍︎︎ 8 👤︎︎ u/M3d4r 📅︎︎ Feb 27 2019 🗫︎ replies

Does anyone know if there have been developments toward any kind of passive cooling system that could operate without generators in cases like this?

👍︎︎ 4 👤︎︎ u/SgtWicket1 📅︎︎ Feb 27 2019 🗫︎ replies

Great video find OP. Thanks for this.

👍︎︎ 2 👤︎︎ u/BloodyVegan 📅︎︎ Mar 01 2019 🗫︎ replies

The BWR was designed by GE in the 60’s. 3 GE engineers in the 70’s discovered serious safety flaws in the BWR design. All 3 were eventually fired. Also, some safety TEPCO engineers brought serious compliance issues to TEPCO’s management and were ignore prior to the tsunami. It in accident investigation and prevention if you identify the chain and break the chain of events or poor decisions a mishap will not occur.

IMHO as an accident investigator, who in their right mind would build nuclear power plants on a coastline well known for serious earthquakes and Tsunamis, let alone placing major disaster averting equipment such as generators and battery backup below water level?

https://www.dailykos.com/stories/2011/03/15/956586/-Whistleblower-Expose-of-GE-Inspection-CoverupRARE-EU-Authored-US-BWR-Damage-Report

👍︎︎ 2 👤︎︎ u/Dom_Blonde 📅︎︎ Mar 06 2019 🗫︎ replies

So, I’m totally ignorant to the safety design phase of building nuclear power plants, but it seems to me they just really shouldn’t have built it where they built it, and they should protect them WAY better than they did-at least in this scenario.

Was this just a case of ‘everything that could have possibly gone wrong went wrong’? It could this have been avoided with better security measures?

👍︎︎ 1 👤︎︎ u/Camdingirth 📅︎︎ Mar 01 2019 🗫︎ replies