How does a nuclear meltdown work? (w/ Video)

nuclear meltdown
This illustration of a nuclear reactor shows water entering the core and surrounding the fuel rods (vertical red bars). When the water level decreases, the fuel rods begin to heat up and face the risk of melting. Image from video below.

( -- When working properly, nuclear reactors produce large amounts of heat via nuclear fission reactions. The heat converts the surrounding water into steam, which turns turbines and generates electricity. But if you remove the water, you also remove the most important cooling element in a nuclear reactor and open up the possibility for nuclear meltdown.

A handful of nuclear meltdowns of varying degrees of severity have occurred since the 1950s, when researchers began building and testing nuclear reactors. The most serious instance happened in 1986 in Chernobyl, Ukraine. Plagued by design flaws and operator errors, the plant experienced fires, explosions, and radiation leakage. As a result, 30 people died of acute radiation syndrome, and thousands of cases of fatal cancers and birth defects have been reported in the following years. Today, limited access is allowed inside a 30-km (19-mile) exclusion zone surrounding the area.

By comparison, the Three Mile Island accident in Harrisburg, Pennsylvania, was much less serious. In 1979, a minor cooling system malfunction led to a series of events that caused a partial meltdown that damaged one of the reactors. However, very little radiation was released into the environment due to the surrounding primary containment vessel. Although the accident caused public concern, no deaths or adverse health effects have been officially attributed to the meltdown.

In Japan, the current nuclear crisis at the Daiichi power plant lies somewhere in between Three Mile Island and , according to recent news reports. Last Friday’s 9.0-magnitude earthquake and 10-meter (33-foot) tsunami waves that traveled up to 10 km (6 miles) inland overpowered several of the plant’s safety measures. Although employees at the plant have been risking their lives to try to keep the reactors cool, the chance of a serious meltdown seems to be increasing.

Inside a reactor

Inside the core of a are thousands of long, thin made of zirconium alloy that contain uranium. When a reactor is turned on, the uranium nuclei undergo , splitting into lighter nuclei and producing heat and neutrons. The neutrons can create a self-sustaining chain reaction by causing nearby uranium nuclei to split, too. Fresh water flows around the fuel rods, keeping the fuel rods from overheating and also producing steam for a turbine.

But if not enough water flows into the reactor’s core, the fuel rods will boil the water away faster than it can be replaced, and the water level will decrease. Even when the reactor is turned off so nuclear reactions no longer occur, the fuel rods remain extremely radioactive and hot and need to be cooled by water for an extended period of time. Without enough water, the fuel rods get so hot that they melt. If they begin to melt the core and the steel containment vessel, and release radiation into the environment, nuclear meltdown occurs.

What's happening at the Fukushima nuclear power plant. Video credit: Reuters.

Japan’s cooling problems

When the earthquake struck Japan, three of the six reactors (Reactors 4, 5, and 6) at the Fukushima power plant were already off for routine inspections. Earthquake tremors triggered the automatic shutdown of the other three reactors, Reactors 1, 2, and 3 (along with eight other nuclear reactors at other power plants). To stop the chain reaction, control rods that absorb neutrons were inserted in between the fuel rods.

But the fuel rods are still hot, since radioactive byproducts of past fission reactions continue to produce heat. When the earthquake tore down the power lines, the plant’s main cooling system stopped working. As a backup measure, diesel generators turned on to spray the fuel rods with coolant. But the tsunami that occurred shortly after the earthquake was larger than the plant’s designers had anticipated, and water flowed over the retaining wall and into the area with the generators, causing them to fail. The next backup measure for cooling the fuel rods was a battery system, but the batteries lasted only a few hours. Later, technicians brought in mobile generators and also attempted to inject seawater into the nuclear reactors, which makes them permanently unusable but could help prevent a complete meltdown.

While the nuclear technicians searched for better cooling options, the water levels continued to decrease, exposing the tops of the fuel rods. Pressure also began building in some of the reactors. So far, at least three explosions have occurred in Reactors 1, 2, and 3. The explosions happened when the fuel rods began to melt and release gases that reacted with the surrounding steam, producing hydrogen. To release some pressure and prevent explosions, technicians vented some of the reactors, which also released some radioactive material into the environment. Officials have said that the pressure in Reactor 2 dropped significantly after the explosion there, suggesting that the explosion breached the steel containment structure - the reactor’s “last resort” for containing leaked radiation.

Also, a fire ignited at Reactor 4, thought to be caused by a large pile of spent fuel rods in a pond. Spent fuel rods need to be kept fully submerged in water for cooling, but the lack of water has left some of the rods partially exposed. Smoke from the fire temporarily increased radiation levels around the reactor, so preventing future fires is very important. The Fukushima plant has seven ponds of spent fuel rods from the past few decades. By some estimates, there may be as many as half a million spent fuel rods that are still radioactive and could catch fire if not kept cool.

Japanese officials have stated that radiation around the nuclear reactors has risen to the level where it would adversely affect a person’s health. Officials have implemented a 20-km (12-mile)-radius evacuation zone, and have advised people to stay indoors. The US has told its citizens living in the area to stay at least 50 miles away from the power plant. Some people have been taking prophylactic iodine as a safety measure; consuming this non-radioactive iodine before exposure to radioactive iodine can fill a person’s thyroid and hopefully prevent absorption of the radioactive variety. Fortunately, westerly winds have so far blown much of the radioactive material out to sea.

Overall, because the extreme events that caused the cooling problems are so rare and unexpected, it’s difficult to predict exactly what will happen next for Japan’s nuclear plants.

Explore further

Japan worst-case scenario unlikely to cause catastrophic radiation release: expert

More information: via: IEEE Spectrum, The Wall Street Journal, Scientific American, and National Geographic

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Citation: How does a nuclear meltdown work? (w/ Video) (2011, March 17) retrieved 22 August 2019 from
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Mar 17, 2011
How does nuclear theft work?

It's time to get the nuclear industry off taxpayer-funded welfare. No more loan guarantees and liability limitations for that vicious, corrupt and polluting industry. If they can't afford to pay their own insurance in the much-loved free market, then they can bloody well fail.

And since no insurer will write a policy for a nuclear plant in the USA without both loan guarantees and liability limitations - the funding for said having been filched from the pockets of taxpayers by your ever-loving government - there will be no more nuclear plants in the USA.

End of story.

Mar 17, 2011
This article appears to treat all reactors the same, as if all reactors were of the "boiling water reactor" design commonly found in Generation 1 and 2 reacters used in the US and Japan and other places. There are many different reactor designs (see Wikipedia article on Nuclear reactor technology). Each design has different safety systems, both active and passive, and each has different failure modes. Please be less parochial and do your homework.

Mar 17, 2011
Oh, Thank God you aren't using the Glen Beck "M&M" teaching style.

Still amazes me that as advanced as we are, the actual electricity is generated from steam. Heated liquid.

Mar 17, 2011
These "experts" are criminally negligent and should be brought to justice. In a culture where criticism and whistle-blowing are anathemas, this is what you get. They all knew of the potential failures but no one came forward to prevent all hell breaking loose. Same goes for the poisoning of our oceans, our soil, and our aquifers. Prosecution, then nuclear chain gang.

Mar 17, 2011
"Once again, nature has challenged man's best efforts"? As the Reuters video ends. NATURE???? Nuclear reactors don't exist in nature. Man has challenged nature and is losing. As usual.

Mar 18, 2011
Nuclear reactors don't exist in nature.

Nuclear reactors existed in Nature ~ 2 Gyr ago when the natural U-235/U-238 ratio was high enough to sustain a spontaneous chain reaction.

They were predicted by the late Professor Paul K. Kuroda ["On the nuclear physical stability of the uranium minerals, J. Chem. Phys. 25 (1956) 781- 782; "On the infinite multiplication constant and the age of the uranium minerals, J. Chem. Phys. 25 (1956) 1295- 1296].

They were first discovered in 1972 in the Oklo Mine by scientists working for the French Atomic Energy Commission.

Neutron repulsion is the recently recognized energy source that powers fission of heavy nuclei and compact cores of stars and galaxies:

With kind regards,
Oliver K. Manuel

Mar 18, 2011
Natural reactor Wikipedia: http://en.wikiped..._reactor
Natural reactor: Sun. I think there are others out there too.

Mar 18, 2011
Smoke from the fire temporarily increased radiation levels around the reactor, so preventing future fires is very important
A fire implies heat generated from oxidation of a fuel. I suspect this was heat- generated from uncooled, white hot fuel rods. The "smoke" was likely hot oxides boiling off the fuel rods. Might have been some flammables ignited by hot fuel rods as well. But if radiation levels increased significantly during the fire, it was most likely white hot fuel rods.

Mar 18, 2011
NATURE???? Nuclear reactors don't exist in nature.

Yes they do, google it you halfbreed.

Mar 18, 2011
Stop moaning about a tragic accident, complaining about nuclear energy when your part of the problem. To be able to even post a comment you would have used a computer which intern is filled with its fair share of environmental issues. Technology has evolved but so has the humans race for greed, alternative energy may be out there but while there is money to be made you will never see it have a significant impact on our society. Humans best achievements will always be overshadowed by its worst. Human nature will always have the final say..

Mar 18, 2011
Although it's not over yet, if they manage to keep a lid on it going totally wrong, this shows how safe nuclear power is - Even a mag 9 earthquake + 10m tsunami couldn't cause it to be lethal.
I'm also always amazed how primitive it is though using the steam to turn a turbine, that technique is about 200+ years old isn't it?

Mar 18, 2011
Natural reactor: Sun. I think there are others out there too.

Yes the Sun is one type of self-sustaining natural reactor. Other stars are probably self-sustaining natural reactors, too.

Measurements [1] revealed that our Sun is energized by:

_a.) Neutron emission from the core - 60%
_b.) Neutron decay to Hydrogen - 5%
_c.) H-fusion into He - 35%

The waste products are a 10:1 mix of H:He. That is the composition of the brightly glowing sphere of waste products that emits heat, light and the solar wind.

The H:He ratio in the solar wind is typically 10:1.

The H:He ratio in the interstellar medium is 10:1.

Thus, other stars may also be powered by the same nuclear reactions that power the Sun [1].

[1] "Neutron Repulsion," The APEIRON Journal, in press (2011) 19 pages

Mar 18, 2011
It's time to get the nuclear industry off taxpayer-funded welfare. No more loan guarantees and liability limitations for that vicious, corrupt and polluting industry.
They're all vicious corrupt and polluting. Fossil fuel plants only take longer to strip the landscape and emit tons of filthy and radioactive waste.
After an unbelievable 9.0 earthquake and a 10m tsunami the workers at the plant are still managing to keep a 40 year old damaged nuclear facility together.
Today. Tomorrow they're soup. Curious that there was no redundant power feeds, and the generators exposed to tsunami, no?

Mar 18, 2011
the shocking part is that we can't recover the heat from spent fuel rods and make the 'waste' highly profitable.
There are designs which can use spent fuel:

Mar 18, 2011
In the USA, about 40K people die annually from the emissions of fossil-fueled (mostly coal) power plants. There have been ZERO deaths (in the USA) from nuclear plants -- EVER! The Canadians are, as we speak, building Pebble Bed Reactors to extract oil from tar sands. These reactors do NOT require the elaborate cooling systems and backups that boiling water reactors need to be safe. We can reprocess spent fuel and burn 99% of it in reactors designed to burn it, but we are "worried about plutonium" from spent fuel, even though there are tons of it laying around, mostly in the old USSR, where the hardly guard it at all. Get real people -- you should be really worried about running out of fossil fuel, because you will lose a whole array of petrochemicals (like fertilizer, plastics, etc.) and, yes, MAYBE, CO2 will cause global warming.

Mar 19, 2011
the shocking part is that we can't recover the heat from spent fuel rods and make the 'waste' highly profitable.

I agree, but it has to be cool enough to be manageable and safe, but hot enought to gain cost effective energy. Delicate design balance.

Mar 20, 2011
I love how a Fox News anchor was saying that with Libya and Japan, the WORLD IS ON THE BRINK!!!!

Uh, on the brink of what? Libya is not going to change the world one way or the other, and while Japan has been hit with everything but Godzilla coming ashore, they along with the help of all the other nations sending aid will eventually recover from this.

But according the Fox News, we're ON THE BRINK!!!

Mar 20, 2011
TO CORRECT A FEW MISCONCEPTIONS. First the energy from decay heat isn't "lost" in an operating reactor. Second, For many years, there has been an Expended Core Facility in Idaho, but no one wants to have "expended fuel" shipped thru their state. As soon as billions were spent on Yucca Mountain, the State no longer wanted it (Thanks Harry). They have our money now. As wehrush said, pebble fuel reactor are the next generation, but he is incorrect in that there have been deaths in power plants. I'm hedging a bit, because the deaths were in SL1, a portable reactor built to be used in place like the Arctic (late 50's). Next, we won't run out of carbon of fuels for a long time. In the US we have hundreds of years of coal and despite what's being said, huge reserves of oil. Not to say we should use it all up. Coal fired power plants can be built to burn relatively clean. Using "fluidized bed" boilers, adding ammonia and limestone, they produce low nox etc. Coal can also be turned to oil.

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