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Bypassing safety systems a dangerous strategy

Manostaxx The text that follows is owned by the site above referred. Here is only a small part of the article, for more please follow the link SOURCE: http://www.controleng.com/search/search-single-display/bypassing-safety-systems-a-dangerous-strategy/ea9b734e5857057085fb07b60351a1da.html Safety and maintenance are two concepts that ought to go together. However, even two things in perfect harmony can get out of tune when there is […]

Acid/Alkaline Theory of Disease Is Nonsense

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SOURCE: https://www.quackwatch.org/01QuackeryRelatedTopics/DSH/coral2.html

Gabe Mirkin, M.D.

Have you seen advertisements for products such as coral calcium or alkaline water that are supposed to neutralize acid in your bloodstream? Taking calcium or drinking alkaline water does not affect blood acidity. Anyone who tells you that certain foods or supplements make your stomach or blood acidic does not understand nutrition.

You should not believe that it matters whether foods are acidic or alkaline, because no foods change the acidity of anything in your body except your urine. Your stomach is so acidic that no food can change its acidity. Citrus fruits, vinegar, and vitamins such as ascorbic acid or folic acid do not change the acidity of your stomach or your bloodstream. An entire bottle of calcium pills or antacids would not change the acidity of your stomach for more than a few minutes.

All foods that leave your stomach are acidic. Then they enter your intestines where secretions from your pancreas neutralize the stomach acids. So no matter what you eat, the food in stomach is acidic and the food in the intestines is alkaline.

Dietary modification cannot change the acidity of any part of your body except your urine. Your bloodstream and organs control acidity in a very narrow range. Anything that changed acidity in your body would make you very sick and could even kill you. Promoters of these products claim that cancer cells cannot live in an alkaline environment and that is true, but neither can any of the other cells in your body.

All chemical reactions in your body are started by chemicals called enzymes. For example, if you convert chemical A to chemical B and release energy, enzymes must start these reactions. All enzymes function in a very narrow range of acidity. (The degree of acidity or alkalinity is expressed as “pH.”). If your blood changes its acidity or alkalinity for any reason, it is quickly changed back to the normal pH or these enzymes would not function and the necessary chemical reactions would not proceed in your body.

For example, when you hold your breath, carbon dioxide accumulates in your bloodstream very rapidly and your blood turns acidic, and you will become uncomfortable or even pass out. This forces you to start breathing again immediately, and the pH returns to normal. If your kidneys are damaged and cannot regulate the acidity of your bloodstream, chemical reactions stop, poisons accumulate in your bloodstream, and you can die.

Certain foods can leave end-products called ash that can make your urine acid or alkaline, but urine is the only body fluid that can have its acidity changed by food or supplements. ALKALINE-ASH FOODS include fresh fruit and raw vegetables. ACID-ASH FOODS include ALL ANIMAL PRODUCTS, whole grains, beans and other seeds. These foods can change the acidity of your urine, but that’s irrelevant since your urine is contained in your bladder and does not affect the pH of any other part of your body.

When you take in more protein than your body needs, your body cannot store it, so the excess amino acids are converted to organic acids that would acidify your blood. But your blood never becomes acidic because as soon as the proteins are converted to organic acids, calcium leaves your bones to neutralize the acid and prevent any change in pH. Because of this, many scientists think that taking in too much protein may weaken bones to cause osteoporosis.

Cranberries have been shown to help prevent recurrent urinary tract infections, but not because of their acidity. They contain chemicals that prevent bacteria from sticking to urinary tract cells.

Taking calcium supplements or drinking alkaline water will not change the pH of your blood. If you hear someone say that your body is too acidic and you should use their product to make it more alkaline, you would be wise not to believe anything else the person tells you.


Dr. Mirkin, who practices medicine in Kensington, Maryland, is board-certified in four specialties: allergy and immunology; sports medicine; pediatrics; and pediatric immunology. He has served as a teaching fellow at Johns Hopkins Medical School, Assistant Professor at the University of Maryland, and Associate Clinical Professor in Pediatrics at the Georgetown University School of Medicine. He has written 16 books on sportsmedicine, weight control, and low-fat eating. His Web site offers broadcasts and reports on thousands of topics. He also offers a free weekly e-mail newsletter.

The Worst Nuclear Accidents: What Really Happened at Chernobyl and Fukushima

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SOURCE: https://www.decodedscience.org/the-worst-nuclear-accidents-what-really-happened-at-chernobyl-and-fukushima/5591

Fukushima

Ranking nuclear accidents, Chernobyl and Fukushima were, on a scale of 1 to 7, a 7 in severity. But these accidents were not the same. With different site designs, the level of radiation released and the degree of harm to workers and the general public differed significantly.

Chernobyl Nuclear Plant Design

Chernobyl site and plant: The Chernobyl complex consists of four nuclear reactors, with two additional reactors under construction at the time of the accident in 1986. The Soviet-designed Chernobyl reactor, RBMK-1000, is a graphite-moderated boiling water reactor.

How the RBMK-1000 Works

Put simply, the fuel rods are zirconium alloy clad and contain uranium dioxide fuel. Inserted into a graphite moderator, water is pumped from the bottom through the fuel channels as it heats, boils, and turns to steam. The steam then drives the turbines.

Control rods, in case of emergency or energy control, can be lowered into the core to control the power output, or shut the reactor down. The RBMK design does not have a primary containment, which is typically a structure of steel and concrete around the reactor vessel built to withstand a pressure increase, and contain radioactive release in the event of an accident. After the accident at Unit-4, Units 1-3 continued operation. The last one was shut down in 2000. There are 11 other RBMK’s still operating in Russia.

Fukushima by Lincun

Fukushima Nuclear Power Plant Design

The complex consists of Fukushima Dai-ichi 1, 2, 3, 4, 5, 6, and Fukushima Daini 1, 2, 3, 4, one of the worlds largest reactor complexes. Fukushima Dai-ichi are GE boiling water reactors (BWR) designed in the 1960s, and built in the 1970s.

How the BWR Works

Each fuel assembly is zirconium clad with uranium dioxide fuel. As water is pumped up through the core, it is heated and turned to steam, thus driving the turbines. The design protecting against radiation release is control rod insertion, in case of emergency, to control the fission of the core. The second defense is a primary containment vessel. Built of reinforced steel in a concrete shell, it surrounds the reactor pressure vessel containing the fuel rods. A suppression pool, filled with water, located below the reactor vessel and part of the primary containment vessel, acts as a heat sink in case of an accident. Dai-ichi units 1, 2, 3, and 4 were involved in the accident described below.

The Chernobyl Accident

On April 26, 1986, Unit 4 of the nuclear power station at Chernobyl had a severe and life-threatening accident that released massive amounts of radiation into the environment. During a test of the turbines, an operator made errors in disabling some automatic shutdown options, causing the rapidly unstable fuel to be impossible to control. The operator tried to control the core reaction by inserting control rods, but due to the hot-fuel-and-water reaction, the control rods jammed the channels halfway down. Steam generation then spread throughout the core, causing an explosion and releasing fission products into the air. A few seconds later, a second explosion threw fuel fragments and hot graphite skyward. A number of fires started, and burned for days, releasing large amounts of radioactive elements into the environment. With no primary containment the release of radioactivity was staggering.

Fukishima Accident

On March 11, 2011, Japan and subsequently, Fukushima, was hit by a 9.0 earthquake and its resulting tsunami. Fukushima Dai-ichi 1, 2, 3, and 4 were involved in  a severe accident as a result.

The plants shut down effectively after the earthquake, but were damaged by the tsunamis that followed. The first tsunami wave hit approximately 41 minutes later, with a second wave which caused the submergence of the emergency diesel generators, electrical switchgear, and batteries. This plunged the station into an electrical blackout, isolating the reactors from their ultimate heat sink. In shutdown mode, with no way to achieve heat removal from the core, steam production increased, reactor vessel pressure rose significantly, and the water level covering the core lowered.

  • The core started melt down, resulting in the generation and release of large amounts of hydrogen, which escaped from the primary containment.
  • Units 1, 2, and 3 had hydrogen explosions which blew off the roof and walls of the top part of the reactor buildings surrounding the primary containment.
  • Unit 2 also experienced a rupture in the suppression pool.
  • The fuel for Unit 4 was stored in a fuel pool where a fire started. Volatile and easily airborne radionuclides , fission products, were carried out with the steam and hydrogen.

Radioactivity at Chernobyl and Fukushima

Both plants released significant amounts of radionuclides. For Fukushima, these were limited to outgased products with the most significant being I-131 (half-life, 8 days), Cs-137 (half life, 30 years) and Cs- 134 (half life, 2 years). The radiation at the point of the accident became 1000 times the normal on-site dose rates but has decreased significantly over time. Cs- 137, the problem, travels easily in the environment and very difficult to clean up. No radiation illness or death caused by the Fukushima accident has occurred.

Chernobyl vs Fukushima

Chernobyl’s accident spewed core material into the air. With no primary containment to block the release, many more fission products and core material became airborne; approximately 100+ times that of the Fukushima release. Among the 600 workers present, 2 died within hours and 28 died within the first few months, from radiation sickness. Exposure of  106 more to high levels have yet not resulted in death. The spread of radionuclides affected millions of people across Europe. Today, an increase of thyroid cancers have been detected.

Differences Between Fukushima and Chernobyl

Comparison of the two level 7 accidents shows that they are quite different. The power plants and radionuclide differences caused different dispersion patterns. The Fukushima reactors, protected by the primary containment vessel, released far less radioactivity than Chernobyl, which had no primary containment. Couple the Chernobyl design flaws with poorly-trained operators, and it becomes obvious why this was truly a nuclear disaster, the long-term effects of which are still being studied.

Sources

NRC. Backgrounder on Chernobyl Nuclear Power Plant Accident. Accessed Nov. 12, 2011.

BBC News. How does Fukushima differ from Chernobyl? (2011). Accessed Nov. 12, 2011.

World Nuclear Organization. Fukushima Accident 2011. (2011). Accessed Nov. 12, 2011.

© Copyright 2011 Judy Haar, All rights Reserved. Written For: Decoded Science

What is the worst kind of power plant disaster? Hint: It’s not nuclear.

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SOURCE: http://io9.gizmodo.com/5783526/what-is-the-worst-kind-of-power-plant-disaster-hint-its-not-nuclear

Media coverage of the Japan nuclear reactor leak makes it seem like the worst kind of power plant disaster that you would ever face. But when you look at the actual statistics and history of similar disasters, nuclear power plants are not the most dangerous energy sources – even when terrible accidents happen.

The disaster in Japan is horrific, and we aren’t trying to say it isn’t a terrible situation. The question we’re trying to answer rationally here is whether nuclear power plant accidents cause more damage than other kinds of power plants. We’ve put together a list of five of the worst power plant disasters in recent history, measured by death toll, monetary damage, and regions affected. The lesson? The issue isn’t so much the kind of energy you use, but how you design the power plants that contain it.

As you can see, when accidents happen, the deadliest and costliest source of energy is water – especially when it’s held back by poorly-designed dams. The Chernobyl disaster doesn’t come close to the damage done when a dam at a hydroelectric plant bursts.

Oil and natural gas are among the most expensive energy sources in terms of damage done.

In addition, we have only measured the cost to human life here. The Kingston Fossil Plant coal fly ash slurry spill and the Deepwater Horizon oil spill – both enormously expensive oil industry disasters – destroyed enormous amounts of wildlife on land and in the water, even if the human toll was low.

1975: Shimantan/Banqiao Dam Failure
Type of power: Hydroelectric
Human lives lost: 171,000
Cost: $8,700,000,000
What happened: Shimantan Dam in China’s Henan province fails and releases 15.738 billion tons of water, causing widespread flooding that destroys 18 villages and 1500 homes and induces disease epidemics and famine

1979: Morvi Dam Failure
Type of power: Hydroelectric
Human lives lost: 1500 (estimated)
Cost: $1,024,000,000
What happened: Torrential rain and unprecidented flooding caused the Machchu-2 dam, situated on the Machhu river, to burst. This sent a wall of water through the town of Morvi in the Indian State of Gujarat.

1998: Nigerian National Petroleum Corporation Jess Oil Pipeline Explosion
Type of power: Oil
Human lives lost: 1,078
Cost: $54,000,000
What happened:Petroleum pipeline ruptures and explodes, destroying two villages and hundreds of villagers scavenging gasoline.

1944: East Ohio Gas Company
Type of power: Liquified natural gas (LNG)
Human lives lost: 130
Cost: $890,000,000
What happened: Explosion at LNG facility destroys one square mile of Cleveland, OH.

1907: Monongah Coal Mine
Type of power: Coal
Human lives lost: 362
Cost: $162,000,000
What happened: Underground explosion traps workers and destroys railroad bridges leading into the mine.

Compare these to:

1986: Chernobyl Nuclear Power Plant
Type of power: Nuclear
Human lives lost: 4,056 (Source for this number: United Nations Scientific Subcommittee on the Effects of Atomic Radiation)
Cost: $6,700,000,000
What happened: Mishandled reactor safety test at Chernobyl nuclear reactor causes steam explosion and meltdown, necessitating the evacuation of 300,000 people from Kiev, Ukraine and dispersing radioactive materials across Europe.

NOTE: Monetary damage is measured in 1996 US dollars, except in accidents since that time measured in the dollar values of that year.

A lot of this research was based on public policy professor Andrew Sovacool’s extremely informative monograph “The Accidental Century,” which looks at power plant disasters in the twentieth century in great detail. Another good resource is this 1998 report from PSI, a Swiss engineering institute, which (as Robert Gonzalez points out in comments) explores the risks of various power systems as well as the long-term effects of accidents in the energy industry.

Reporting by Robert T. Gonzalez

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Pressurised heavy water reactor (PHWR)

The PHWR reactor design has been developed since the 1950s in Canada as the CANDU, and from 1980s also in India. PHWRs generally use natural uranium (0.7% U-235) oxide as fuel, hence needs a more efficient moderator, in this case heavy water (D2O).** The PHWR produces more energy per kilogram of mined uranium than other designs, but also produces a much larger amount of used fuel per unit output.

** with the CANDU system, the moderator is enriched (i.e. water) rather than the fuel – a cost trade-off.

The moderator is in a large tank called a calandria, penetrated by several hundred horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy water under high pressure in the primary cooling circuit, reaching 290°C. As in the PWR, the primary coolant generates steam in a secondary circuit to drive the turbines. The pressure tube design means that the reactor can be refuelled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit. It is also less costly to build than designs with a large pressure vessel, but the tubes have not proved as durable.

 

 A Pressurized Heavy Water Reactor (PHWR/Candu) diagram

 

A CANDU fuel assembly consists of a bundle of 37 half metre long fuel rods (ceramic fuel pellets in zircaloy tubes) plus a support structure, with 12 bundles lying end to end in a fuel channel. Control rods penetrate the calandria vertically, and a secondary shutdown system involves adding gadolinium to the moderator. The heavy water moderator circulating through the body of the calandria vessel also yields some heat (though this circuit is not shown on the diagram above).

Newer PHWR designs such as the Advanced Candu Reactor (ACR) have light water cooling and slightly-enriched fuel.

CANDU reactors can accept a variety of fuels. They may be run on recycled uranium from reprocessing LWR used fuel, or a blend of this and depleted uranium left over from enrichment plants. About 4000 MWe of PWR might then fuel 1000 MWe of CANDU capacity, with addition of depleted uranium. Thorium may also be used in fuel.

Nuclear Power Reactor – Boiling water reactor (BWR)

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Boiling water reactor (BWR)

This design has many similarities to the PWR, except that there is only a single circuit in which the water is at lower pressure (about 75 times atmospheric pressure) so that it boils in the core at about 285°C. The reactor is designed to operate with 12-15% of the water in the top part of the core as steam, and hence with less moderating effect and thus efficiency there.  BWR units can operate in load-following mode more readily then PWRs.

The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are thus part of the reactor circuit. Since the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and radiological protection provided during maintenance. The cost of this tends to balance the savings due to the simpler design. Most of the radioactivity in the water is very short-lived*, so the turbine hall can be entered soon after the reactor is shut down.

* mostly N-16, with a 7 second half-life

A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750 assemblies in a reactor core, holding up to 140 tonnes of uranium. The secondary control system involves restricting water flow through the core so that more steam in the top part reduces moderation.

 

 A Boiling Water Reactor (BWR) diagram

Nuclear Power Reactor – Pressurised water reactor (PWR)

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Pressurised water reactor (PWR)

This is the most common type, with over 230 in use for power generation and several hundred more employed for naval propulsion. The design of PWRs originated as a submarine power plant. PWRs use ordinary water as both coolant and moderator. The design is distinguished by having a primary cooling circuit which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive the turbine. In Russia these are known as VVER types – water-moderated and -cooled.

 

A Pressurized Water Reactor (PWR) diagram

 

A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a large reactor would have about 150-250 fuel assemblies with 80-100 tonnes of uranium.

Water in the reactor core reaches about 325°C, hence it must be kept under about 150 times atmospheric pressure to prevent it boiling. Pressure is maintained by steam in a pressuriser (see diagram). In the primary cooling circuit the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type. The secondary shutdown system involves adding boron to the primary circuit.

The secondary circuit is under less pressure and the water here boils in the heat exchangers which are thus steam generators. The steam drives the turbine to produce electricity, and is then condensed and returned to the heat exchangers in contact with the primary circuit.

Components of a nuclear reactor

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There are several components common to most types of reactors:

Fuel. Uranium is the basic fuel. Usually pellets of uranium oxide (UO2) are arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.*
* In a new reactor with new fuel a neutron source is needed to get the reaction going. Usually this is beryllium mixed with polonium, radium or other alpha-emitter. Alpha particles from the decay cause a release of neutrons from the beryllium as it turns to carbon-12. Restarting a reactor with some used fuel may not require this, as there may be enough neutrons to achieve criticality when control rods are removed.

Moderator. Material in the core which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.

Control rods. These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it.*  In some PWR reactors, special control rods are used to enable the core to sustain a low level of power efficiently. (Secondary control systems involve other neutron absorbers, usually boron in the coolant – its concentration can be adjusted over time as the fuel burns up.)
* In fission, most of the neutrons are released promptly, but some are delayed. These are crucial in enabling a chain reacting system (or reactor) to be controllable and to be able to be held precisely critical.

Coolant. A fluid circulating through the core so as to transfer the heat from it.  In light water reactors the water moderator functions also as primary coolant. Except in BWRs, there is secondary coolant circuit where the water becomes steam. (See also later section on primary coolant characteristics)

Pressure vessel or pressure tubes. Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the surrounding moderator.

Steam generator. Part of the cooling system of pressurised water reactors (PWR & PHWR) where the high-pressure primary coolant bringing heat from the reactor is used to make steam for the turbine, in a secondary circuit. Essentially a heat exchanger like a motor car radiator*. Reactors have up to six ‘loops’, each with a steam generator. Since 1980 over 110 PWR reactors have had their steam generators replaced after 20-30 years service, 57 of these in USA.

* These are large heat exchangers for transferring heat from one fluid to another – here from high-pressure primary circuit in PWR to secondary circuit where water turns to steam. Each structure weighs up to 800 tonnes and contains from 300 to 16,000 tubes about 2 cm diameter for the primary coolant, which is radioactive due to nitrogen-16 (N-16, formed by neutron bombardment of oxygen, with half-life of 7 seconds). The secondary water must flow through the support structures for the tubes. The whole thing needs to be designed so that the tubes don’t vibrate and fret, operated so that deposits do not build up to impede the flow, and maintained chemically to avoid corrosion. Tubes which fail and leak are plugged, and surplus capacity is designed to allow for this. Leaks can be detected by monitoring N-16 levels in the steam as it leaves the steam generator.

Containment. The structure around the reactor and associated steam generators which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any serious malfunction inside. It is typically a metre-thick concrete and steel structure.

Newer Russian and some other reactors install core melt localisation devices or ‘core catchers’ under the pressure vessel to catch any melted core material in the event of a major accident.

There are several different types of reactors as indicated in the following table.

Nuclear power plants in commercial operation

Reactor type Main Countries Number GWe Fuel Coolant Moderator
Pressurised water reactor (PWR)
US, France, Japan, Russia, China
277
257
enriched UO2
water
water
Boiling water reactor (BWR)
US, Japan, Sweden
80
75
enriched UO2
water
water
Pressurised heavy water reactor (PHWR)
Canada, India
49
25
natural UO2
heavy water
heavy water
Gas-cooled reactor (AGR & Magnox)
UK
15
8
natural U (metal),
enriched UO2
CO2
graphite
Light water graphite reactor (RBMK & EGP)
Russia
11 + 4
10.2
enriched UO2
water
graphite
Fast neutron reactor (FBR)
Russia
2
0.6
PuO2 and UO2
liquid sodium
none
TOTAL 438 376
IAEA data, end of 2014.  GWe = capacity in thousands of megawatts (gross)
Source: Nuclear Engineering International Handbook 2011, updated to 1/1/12
For reactors under construction: see paper Plans for New Reactors Worldwide.

 

Nuclear Power Reactors -RBMK Reactors

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  • The RBMK is an unusual reactor design, one of two to emerge in the Soviet Union in the 1970s.
  • The design had several shortcomings, and was the design involved in the 1986 Chernobyl disaster.
  • Major modifications have been made to RBMK reactors still operating.

The Soviet-designed RBMK (reaktor bolshoy moshchnosty kanalny, high-power channel reactor) is a pressurised water-cooled reactor with individual fuel channels and using graphite as its moderator. It is also known as the light water graphite reactor (LWGR). It is very different from most other power reactor designs as it derived from a design principally for plutonium production and was intended and used in Russia for both plutonium and power production.

The combination of graphite moderator and water coolant is found in no other power reactors in the world. As the Chernobyl accident showed, several of the RBMK’s design characteristics – in particular, the control rod design and a positive void coefficient – were unsafe. A number of significant design changes were made after the Chernobyl accident to address these problems.

A Light Water Graphite-moderated Reactor (LWGR/RBMK)

A Brief History of Nuclear Accidents Worldwide

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Serious nuclear accidents have been few and far between—but their stories will help prevent future catastrophes.

Fukushima Daiichi

Fukushima, Japan, March 2011

The earthquake and tsunami that struck eastern Japan on March 11, 2011, caused a serious accident at the Fukushima Dai-ichi nuclear power plant on the northeastern coast of Japan.

How did it happen?
The earthquake cut off external power to the reactors. tsunami, which reached levels more than twice as high as the plant was designed to withstand, disabled backup diesel generators, crippling the reactor cooling systems. Battery power was quickly exhausted, and overheating fuel in the plant’s operating reactor cores led to hydrogen explosions that severely damaged three of the reactor buildings. Fuel in three of the reactor cores melted, and radiation releases from the damaged reactors contaminated a wide area surrounding the plant and forced the evacuation of nearly half a million residents.


Chernobyl

Chernobyl, Ukraine (former Soviet Union), April 26, 1986

Chernobyl is considered the world’s worst nuclear disaster to date. It occurred on April 26, 1986, when a sudden surge in power during a reactor systems test resulted in an explosion and fire that destroyed Unit 4. Massive amounts of radiation escaped and spread across the western Soviet Union and Europe. As a result of the disaster, approximately 220,000 people had to be relocated from their homes.

How did it happen?
Unit 4 was to be shut down for routine maintenance. A test was conducted to determine the plant equipment’s ability to provide sufficient electrical power to operate the reactor core cooling system and emergency equipment during the transition period between a loss of main station electrical power supply and the start-up of the emergency power supply. Workers did not implement adequate safety precautions or alert operators to the electrical test’s risks. This lack of awareness led the operators to engage in actions that diverged from safety procedures. Consequently, a sudden power surge resulted in explosions and nearly complete destruction of the reactor. The fires that broke out in the building contributed to the extensive radioactive releases.


Three Mile Island

Middletown, Pennsylvania, USA, March 28, 1978

The partial meltdown at Three Mile Island Unit 2 is considered the most serious nuclear accident in U.S. history, although it resulted in only small radioactive releases.

How did it happen?
The accident began with failures in the non-nuclear secondary system, followed by a human-operated relief valve in the primary system that stuck open, which allowed large amounts of nuclear reactor coolant to escape. Plant operators’ initial failure to correctly identify the problem compounded it. In particular, a hidden indicator light led to an operator manually overriding the automatic emergency cooling system because he mistakenly believed that too much coolant water in the reactor had caused the steam pressure release. Eventually the reactor was brought under control, although the full extent of the accident was not understood until later.


Enrico Fermi Unit 1

Frenchtown Charter Township, Michigan, USA, October 5, 1966

Coolant flow blockage in two fuel channels led to the partial meltdown of two fuel assemblies at Fermi Unit 1.

How did it happen?
Fermi Unit 1 was the nation’s first and only commercially operating liquid metal fast breeder reactor. Vibrations caused a component within the reactor vessel to loosen, which blocked coolant flow when hydrodynamic forces carried it up the fuel subassemblies’ inlet nozzle. Workers did not notice what had occurred until core temperature alarms sounded. Several fuel rod subassemblies reached temperatures of up to 700 degrees Fahrenheit, causing them to melt. After the reactor was shut down for repairs, it was returned to partial operation periodically until 1972, but it was never again fully operational. It was officially decommissioned in 1975.


SL-1

Idaho Falls, Idaho, USA, January 3, 1961

The withdrawal of a single control rod caused a catastrophic power surge and steam explosion at the SL-1 boiling water reactor that killed all the workers on duty at the time.

How did it happen?
On January 3, 1961, workers were in the process of reattaching to their drive mechanisms control rods they had disconnected earlier that day to enable test equipment to be inserted in the reactor core. They lifted the central control rod 20 inches, instead of the four inches that was required.  This error caused the reactor to go critical and its power to surge 6,000 times higher than its normal level in less than a second. As a result, nuclear fuel vaporized and a steam bubble was created. The steam bubble expanded so quickly that it pushed water above it against the reactor vessel, which caused it to jump out of its support structure. It hit an overhead crane and then returned to the reactor vessel. In the process, all of the water and some of the fuel was released from the reactor vessel. All three workers on duty received lethal doses of radiation, in addition to trauma from the explosion.


Sodium Reactor Experiment

Los Angeles, California, USA, July 1959

A partial meltdown occurred at the Sodium Reactor Experiment (SRE) due to cooling flow blockage that caused the reactor core to overheat.

How did it happen?
The Sodium Reactor Experiment experienced extensive fuel damage during a power run. Thirteen of forty-three fuel elements overheated when the cooling flow provided by the liquid sodium was blocked by tetralin, an oil-like fluid which had leaked into the primary sodium loop during prior power runs. This overheating caused the reactor core to fail. Fission products were released from the damaged fuel into the primary sodium loop. Some of the fission products leaked from the primary sodium loop into the high bay area, a region inside the building housing the reactor. Other fission products flowed with the helium cover gas over the liquid sodium in the reactor pool to gaseous storage tanks. Fission products from the high bay area and from the gaseous storage tanks were processed through the filters of a ventilation system and discharged to the atmosphere.


Windscale

Cumberland (now Cumbria), UK, October 10, 1957

Windscale Unit 1’s core caught fire and melted, which led large amounts of radioactivity to be released to the surrounding area.

How did it happen?
Before the accident, Unit 1 was activated to release built-up energy in the graphite of the core. The fuel was cooler than the normal operating temperature and was warming more slowly than expected. A second release led to a higher temperature than workers expected. Eventually the temperature was more than 750 degrees Fahrenheit, so air was vented to cool it. The reactor caught fire, igniting an estimated 11 tons of uranium. Workers first used carbon dioxide to try to put out the fire, but that strategy failed. Next they used water, which eventually succeeded. It took workers a total of three days to put out the fire. In the meantime, radiation escaped through the chimney and contaminated much of the surrounding area and reached as far as mainland Europe. More than 200 cancer deaths are attributed to the disaster, which is considered to have been the worst to occur in the West.

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