Understanding the accident of Fukushima Daiichi

Summary

On March 11, 2011, a massive earthquake and subsequent tsunami struck Japan, causing a nuclear disaster at the Fukushima Daiichi power plant. The event led to the meltdown of three reactors, hydrogen explosions, and the release of radioactive materials into the environment. The crisis was managed under extreme conditions, with workers battling to cool the reactors and prevent further contamination. The cleanup and decommissioning process is expected to last several decades, highlighting the long-term challenges of nuclear accidents.

Takeaways

• 🌏 A massive earthquake struck the Pacific coast of Honshu, Japan, on March 11th, causing widespread destruction and triggering a tsunami.
• 💧 The tsunami waves reached up to ten kilometers inland, leading to the death or disappearance of over 20,000 people and extensive damage to towns, ports, and land.
• 🔋 The Fukushima Daiichi nuclear power plant, located 250 km northeast of Tokyo, was severely affected by the disaster.
• ⚛️ Fukushima Daiichi has six boiling water reactors (BWRs), which differ from the pressurized water reactors by having a single system for feedwater and steam.
• 🚫 The earthquake caused the seismic sensors to trigger the insertion of control rods, stopping the fission reactions but leaving residual heat to be managed.
• 🛑 The tsunami disabled the emergency diesel generators, leading to a loss of core cooling in reactors 1, 2, and 3, which eventually led to meltdowns.
• 🔥 Hydrogen buildup due to the reaction of zirconium with steam led to explosions in the reactor buildings, further complicating the situation.
• 🌡️ The loss of cooling systems in the spent fuel storage pools posed a significant risk of radioactive release if not managed properly.
• 🌊 Seawater was injected into the reactor vessels as a last resort to cool and stabilize the corium, despite the long-term chemical issues it would cause.
• 🛠️ The crisis was managed by workers under extremely difficult conditions, who fought to cool the reactors and regain control of the plant.
• 🏗️ The long-term challenge involves spent fuel removal, radioactive waste management, and eventually decommissioning the site, a task expected to last several decades.

Q & A

What was the date and time of the powerful earthquake that struck the Pacific coast of Honshu, Japan?

The earthquake occurred on Friday, March the 11th at 2:46 p.m.

How did the tsunami impact the coastal areas of Japan after the earthquake?

The tsunami swept over the coast less than an hour after the earthquake, with waves going up to ten kilometers inland, resulting in over 20,000 people dead or missing and widespread destruction.

What is the Fukushima Daiichi nuclear power plant's location in relation to Tokyo?

The Fukushima Daiichi nuclear power plant is located 250 kilometers northeast of Tokyo.

How many reactors does the Fukushima Daiichi nuclear power plant have, and what type of reactors are they?

Fukushima Daiichi has six reactors, each commissioned during the 1970s, and they are boiling water reactors (BWRs).

What is the primary function of a boiling water reactor (BWR)?

In a BWR, the heat produced by fission reactions boils water, which then turns into steam to drive a turbine connected to a generator that produces electricity.

What is the process of dealing with residual heat in a nuclear reactor after shutdown?

Residual heat is the heat that continues to be produced even after the reactor is shut down. It is managed by keeping the fuel confined and cooled, which is a major safety issue.

What barriers are used to contain the fuel in a nuclear reactor?

The fuel is contained by multiple barriers: the fuel cladding made of zirconium alloy, the steel reactor vessel with cooling systems, and the containment building made of concrete with a leak-tight steel liner.

What happened to the Fukushima Daiichi reactors during the earthquake and tsunami?

The earthquake triggered the insertion of control rods, stopping fission reactions. The tsunami disabled the emergency diesel generators, leading to a loss of cooling for the reactors, which eventually resulted in core meltdowns in units 1, 2, and 3.

What is corium, and how did it form during the Fukushima Daiichi accident?

Corium is a molten mixture of nuclear fuel and reactor materials that formed when the fuel melted and mixed together at temperatures of around 2300 degrees Celsius during the meltdown.

What measures were taken to cool and stabilize the corium after the meltdown?

Seawater was injected into the reactor vessel to cool and stabilize the corium, despite the chemically active nature of salt.

What were the challenges faced by the workers during the initial crisis at Fukushima Daiichi?

The workers faced extremely difficult conditions, including being cut off from the rest of the world, without news from their families, no power supply, and the threat of radiation.

What is the current state of the Fukushima Daiichi nuclear power plant as of December 2011?

As of December 2011, the Japanese authorities officially declared that the nuclear power plant reached a cold shutdown state, where the cooling water remains liquid below 100 degrees Celsius and does not evaporate.

What are the long-term challenges for the decommissioning of the Fukushima Daiichi site?

The long-term challenges include removing the spent fuel from the pools for final storage, managing radioactive waste repositories, and eventually dismantling the site under the supervision of international experts, a task that is expected to last for several decades.

Outlines

00:00

🌪️ The Great East Japan Earthquake and Tsunami

On March 11, 2011, a powerful earthquake struck the Pacific coast of Honshu, Japan, followed by a devastating tsunami. The Fukushima Daiichi nuclear power plant, located 250 km northeast of Tokyo, was severely impacted. The plant's boiling water reactors (BWRs) were designed to produce electricity through steam generated by fission reactions. Despite the earthquake triggering control rods to stop fission, residual heat posed a significant safety challenge. The tsunami's arrival disabled emergency diesel generators, leading to a loss of core cooling and the eventual meltdown of three reactors. The disaster highlighted the critical need for effective containment and cooling systems to manage residual heat and prevent catastrophic failure.

05:01

🔥 Meltdown and Venting at Fukushima Daiichi

The Fukushima Daiichi disaster escalated as the loss of cooling led to the meltdown of three reactors. The fuel rods overheated, causing a chemical reaction with steam that released hydrogen, which in turn led to explosions. Efforts to vent steam to reduce pressure inadvertently released radioactive elements into the environment. The situation was further complicated by the uncontrolled leakage of hydrogen and the subsequent explosions that damaged the reactor buildings. The spent fuel storage pools, which lost their cooling systems, posed an additional risk of radioactive release. The crisis was managed by workers under extreme conditions, who fought to cool the reactors and stabilize the situation, ultimately leading to the official declaration of a cold shutdown state in December 2011.

10:01

🛠️ Post-Disaster Recovery and Long-Term Challenges

Following the disaster, the recovery efforts at Fukushima Daiichi involved a massive workforce of approximately 20,000 workers. They focused on reinforcing the site against future tsunamis, mapping contamination, securing access points, immobilizing radioactive dust, and treating contaminated water to prevent further environmental impact. By the end of March 2011, the situation began to stabilize with fresh water replacing seawater for cooling. The reactor cooling system was restored to a closed circuit in July, reducing the risk of contaminated water discharge. The long-term challenges include the removal of spent fuel from the pools, the management of radioactive waste, and the eventual decommissioning of the site, all under the scrutiny of international experts. This monumental task, which began in March 2011, is expected to span several decades.

Keywords

💡Earthquake

An earthquake is a sudden shaking of the Earth's surface caused by the rapid release of energy in the Earth's lithosphere. In the video, the earthquake that hit the Pacific coast of Honshu, Japan, is the triggering event that led to the subsequent tsunami and the nuclear disaster at the Fukushima Daiichi plant.

💡Tsunami

A tsunami is a series of ocean waves caused by an underwater disturbance such as an earthquake, volcanic eruption, or underwater landslide. The script describes how the tsunami following the earthquake swept over the coast, causing widespread destruction and contributing to the severity of the nuclear incident.

💡Fukushima Daiichi

Fukushima Daiichi is the name of the nuclear power plant that was severely affected by the earthquake and tsunami. The term is central to the video's theme as it was the site of one of the most significant nuclear accidents in history, with far-reaching environmental and social impacts.

💡Boiling Water Reactor (BWR)

A Boiling Water Reactor is a type of nuclear reactor that uses the heat generated by nuclear fission to boil water, producing steam that drives a turbine to generate electricity. The Fukushima Daiichi plant used this technology, which is crucial to understanding the mechanisms and challenges faced during the disaster.

💡Residual Heat

Residual heat refers to the heat that continues to be produced in a nuclear reactor even after the fission process has stopped. This concept is key to understanding the ongoing cooling needs of the reactor core, which, if unaddressed, can lead to a meltdown as described in the script.

💡Containment Barriers

Containment barriers are multiple layers of protection designed to prevent the release of radioactive materials from a nuclear reactor. The script mentions several barriers, such as the fuel cladding, reactor vessel, and the containment building, which are essential in maintaining safety.

💡Core Meltdown

A core meltdown is a catastrophic failure of a nuclear reactor's core, where the fuel rods melt due to insufficient cooling. The video describes how the loss of cooling in reactors 1, 2, and 3 at Fukushima Daiichi led to meltdowns, a central event in the disaster.

💡Hydrogen Explosion

A hydrogen explosion, as mentioned in the script, occurred when hydrogen gas, released from the reactor's zirconium cladding reacting with the steam, mixed with air and ignited. This led to explosions that damaged the reactor buildings and contributed to the release of radioactive materials.

💡Venting

Venting is the controlled release of gases from a nuclear reactor to prevent overpressure. In the video, the operators had to vent the reactors to avoid containment rupture, but this also led to the release of radioactive elements into the environment.

💡Seawater Cooling

Seawater cooling refers to the use of seawater to cool the reactor core when other cooling methods are unavailable. The script describes how seawater was injected into the reactors at Fukushima Daiichi as a last resort, despite the long-term issues it could cause due to salt contamination.

💡Cold Shutdown

A cold shutdown is a state in which a nuclear reactor's cooling water remains liquid and does not evaporate, indicating that the reactor is stable and the fuel is no longer producing significant heat. The script mentions that the Fukushima Daiichi plant reached this state in December 2011, marking a significant step in managing the crisis.

💡Radioactive Release

Radioactive release refers to the emission of radioactive materials into the environment. The video describes how the disaster at Fukushima Daiichi led to significant releases of radioactive materials into the soil and sea, highlighting the long-term environmental impact of the incident.

Highlights

On March 11, 2011, a powerful earthquake struck the Pacific coast of Honshu, Japan, followed by a devastating tsunami.

The Fukushima Daiichi nuclear power plant, located 250 km northeast of Tokyo, was affected by the disaster.

Fukushima Daiichi has six boiling water reactors (BWRs), each with unique technology compared to French pressurized water reactors.

Residual heat from nuclear fission reactions posed a significant safety challenge even after reactor shutdown.

Seismic sensors triggered control rod insertion, halting fission but necessitating residual heat removal.

Emergency diesel generators automatically took over power supply for core cooling after the off-site power was lost.

The tsunami disabled the emergency diesel generators, leading to the failure of core cooling in reactor 1.

Batteries in units 2 & 3 powered some valves and turbine-driven pumps for nearly 24 hours before failing.

Meltdown in reactors 1, 2, and 3 occurred as the cores were left uncooled, with fuel reaching temperatures over 2300 degrees Celsius.

Corium, a molten mixture of fuel and structural materials, formed and flowed to the bottom of the reactor vessels.

Hydrogen buildup due to zirconium-steam reactions led to explosions in the reactor buildings.

Radioactive elements were released into the environment due to containment and suppression pool failures.

Seawater was injected into the reactor vessels as a cooling measure, despite the risk of salt contamination.

Spent fuel storage pools lost cooling, risking further radioactive release, and required emergency water replenishment.

By the end of March 2011, the situation began to stabilize with fresh water replacing seawater for cooling.

The reactor cooling system was restored to a closed circuit in July, preventing further contamination of the environment.

Japanese authorities declared the nuclear power plant reached a cold shutdown state in December 2011.

The crisis was managed by workers under extreme conditions, demonstrating remarkable resilience and dedication.

Long-term challenges include spent fuel removal, radioactive waste management, and site decommissioning.

Transcripts

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

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