Karen Hudes seems to think Fukushima was intentional. Skip to timestamp 53:02.
How a nuclear plant works. Click image to enlarge in new tab.
As Japan marks the third anniversary of the earthquake, tsunami and nuclear disaster, Tepco is struggling to find a solution for hundreds of thousands of tonnes of contaminated water
A worker at the Fukushima Daiichi plant looks at tanks, under construction, to store radioactive water. Photograph: Toru Hanai/AP
A senior adviser to the operator of the wrecked Fukushima Daiichinuclear power plant has told the firm that it may have no choice but to eventually dump hundreds of thousands of tonnes of contaminated waterinto the Pacific Ocean.
Speaking to reporters who were on a rare visit to the plant on the eve of the third anniversary of the March 2011 earthquake, tsunami and nuclear disaster, Dale Klein said Tokyo Electric Power [Tepco] had yet to reassure the public over the handling of water leaks that continue to frustrate efforts to clean up the site.
“The one issue that keeps me awake at night is Tepco’s long-term strategy for water management,” said Klein, a former chairman of the US nuclear regulatory commission who now leads Tepco’s nuclear reform committee.
“Storing massive amounts of water on-site is not sustainable. A controlled release is much safer than keeping the water on-site.
“Tepco is making progress on water management but I’m not satisfied yet. It’s frustrating that the company takes four or five steps forward, then two back. And every time you have a leakage it contributes to a lack of trust. There’s room for improvement on all fronts.”
Tepco’s failure to manage the buildup of contaminated water came to light last summer, when it admitted that at least 300 tonnes of tainted water were leaking into the sea every day.
That revelation was followed by a string of incidents involving spills from poorly assembled storage tanks, prompting the government to commit about $500m (£300m) into measures to contain the water.
They include the construction of an underground frozen wall to prevent groundwater mixing with contaminated coolant water, which becomes tainted after coming into contact with melted nuclear fuel deep inside the damaged reactors.
Tepco confirmed that it would activate an experimental wall at a test site at the plant on Tuesday. If the test is successful, the firm plans to build a similar structure almost 2km in length around four damaged reactors next year, although some experts have questioned its ability to use the technology on such a large scale.
Klein, too, voiced scepticism over the frozen wall solution, and suggested that the controlled release of treated water into the Pacific was preferable to storing huge quantities of it on site.
But Tepco, the government and nuclear regulators would have to win the support of local fishermen, and the release of even treated water would almost certainly draw a furious response from China and South Korea.
“It’s a very emotional issue,” Klein said. “But Tepco and the government will have to articulate their position to other people. For me, the water issue is more about policy than science.”
Tepco is pinning its hopes on technology that can remove dozens of dangerous radionuclides, apart from tritium, internal exposure to which has been linked to a greater risk of developing cancer.
Klein, however, said tritium does not pose the same threat to heath as bone-settling strontium and caesium, and can be diluted to safe levels before it is released into the sea.
The Fukushima Daiichi plant’s manager, Akira Ono, said the firm had no plans to release contaminated water into the Pacific, but agreed that decommissioning would remain on hold until the problem was solved.
“The most pressing issue for us is the contaminated water, rather than decommissioning,” he said.
“Unless we address this issue the public will not be assured and the evacuees will not be able to return home.
“We are in a positive frame of mind over decommissioning the plant over the next 30 to 40 years, But we have to take utmost care every step of the way because errors can cause a lot of trouble for a lot of people.”
Currently about 400 tonnes of groundwater is streaming into the reactor basements from the hills behind the plant each day. The plant has accumulated about 300,000 tonnes of contaminated water, which is being stored in 1,200 tanks occupying a large swath of the Fukushima Daiichi site.
Eventually Tepco hopes to have enough space to store 800,000 tonnes, but fears are rising that it will run out of space sometime next year because it can’t keep up with the flow of toxic water.
Fukushima three years on
For visitors and workers alike, the journey to the plant begins at J-Village, a former training complex for the Japanese football team that now serves as the Fukushima cleanup’s logistical base.
During the 20-minute bus ride through neighbourhoods still bearing the scars of the earthquake and tsunami, there were signs that decontamination work is making modest progress.
Atmospheric radiation levels are falling, leading the authorities to partially lift evacuation orders in neighbourhoods on the edge of the evacuation zone.
Some of Fukushima’s 100,000-plus nuclear evacuees are now permitted to return to their homes during the day, but radiation levels are still too high for them to make a permanent return.
In the town of Naraha, where atmospheric radiation hovered around 2 microsieverts an hour on Monday – the official decontamination target is 0.23 microsieverts an hour – large black bags filled with radioactive soil cover fields once used for agriculture, where they will remain until agreement can be reached on a permanent disposal site.
Part of a railway line running along to coast is due to reopen in the spring, although stretches of track that pass through the most contaminated areas are expected to remain closed for years.
Inside Fukushima Daiichi, reporters were reminded of the frantic attempts by a small group of Tepco engineers to save the plant from an even greater catastrophe in the hours after its power supply was knocked out by a towering tsunami three years ago.
In the control room for reactors 1 and 2, both of which suffered meltdowns, one worker’s attempts to record water levels by scribbling them on to a disabled control panel are still visible.
None of the unnamed men, who had to work by torchlight, are still at the site: some have retired, but most had to leave because they quickly reached their lifetime radiation dose limit.
“It is difficult to describe what that time was like for those workers,” said Kenichiro Matsui, a Tepco official. “They worked tirelessly to save the reactors. They had a real sense of mission.”
How Japan’s Nuclear Crisis Works
Introduction to How Japan’s Nuclear Crisis Works
Different people have different opinions of the nuclear power industry. Some see nuclear power as an important green technology that emits no carbon dioxide while producing huge amounts of reliable electricity. They point to an admirable safety record that spans more than two decades.
Others see nuclear power as an inherently dangerous technology that poses a threat to any community located near a nuclear power plant. They point to accidents like the Three Mile Island incident and the Chernobyl explosion as proof of how badly things can go wrong.
In either case, commercial nuclear reactors are a fact of life in many parts of the developed world. Because they do make use of a radioactive fuel source, these reactors are designed and built to the highest standards of the engineering profession, with the perceived ability to handle nearly anything that nature or mankind can dish out. Earthquakes? No problem. Hurricanes? No problem. Direct strikes by jumbo jets? No problem. Terrorist attacks? No problem. Strength is built in, and layers of redundancy are meant to handle any operational abnormality.
Shortly after an earthquake hit Japan on March 11, 2011, however, those perceptions of safety began rapidly changing. Explosions rocked several different reactors in Japan, even though initial reports indicated that there were no problems from the quake itself. Fires broke out at the Onagawa plant, and there were explosions at the Fukushima Daiichi plant.
So what went wrong? How can such well-designed, highly redundant systems fail so catastrophically? Let’s take a look.
This diagram shows all the parts of a nuclear reactor.
© 2011 HowStuffWorks.com
Understanding a Nuclear Reactor
If you have read How Nuclear Reactors Work, you are familiar with the basic idea behind a nuclear power plant. At a high level, these plants are quite simple. Nuclear fuel, which in modern commercial nuclear power plants comes in the form of enriched uranium, naturally produces heat as uranium atoms split (see the Nuclear Fission section of How Nuclear Bombs Work for details). The heat is used to boil water and produce steam. The steam drives a steam turbine, which spins a generator to create electricity. These plants are large and generally able to produce something on the order of a gigawatt of electricity at full power.
In order for the output of a nuclear power plant to be adjustable, the uranium fuel is formed into pellets approximately the size of a Tootsie Roll. These pellets are stacked end-on-end in long metal tubes called fuel rods. The rods are arranged into bundles, and bundles are arranged in the core of the reactor. Control rods fit between the fuel rods and are able to absorb neutrons. If the control rods are fully inserted into the core, the reactor is said to be shut down. The uranium will produce the lowest amount of heat possible (but will still produce heat). If the control rods are pulled out of the core as far as possible, the core produces its maximum heat. Think about the heat produced by a 100-watt incandescent light bulb. These bulbs get quite hot — hot enough to bake a cupcake in an Easy Bake oven. Now imagine a 1,000,000,000-watt light bulb. That is the kind of heat coming out of a reactor core at full power.
The reactors that failed in Japan are Mark 1 boiling water reactors designed by General Electric in the 1960s. This is one of the earlier reactor designs, in which the uranium fuel boils water that directly drives the steam turbine. This design was later replaced by pressurized water reactors because of safety concerns surrounding the Mark 1 design. As we have seen, those safety concerns turned into safety failures in Japan. Let’s take a look at the fatal flaw that led to disaster.
The Fatal Flaw in Boiling Water Nuclear Reactors
A boiling water reactor has an Achilles heel — a fatal flaw — that is invisible under normal operating conditions and most failure scenarios. The flaw has to do with the cooling system.
A boiling water reactor boils water: That’s obvious and simple enough. It’s a technology that goes back more than a century to the earliest steam engines. As the water boils, it creates a huge amount of pressure — the pressure that will be used to spin the steam turbine. The boiling water also keeps the reactor core at a safe temperature. When it exits the steam turbine, the steam is cooled and condensed to be reused over and over again in a closed loop. The water is recirculated through the system with electric pumps.
The design’s vulnerability comes into play if the electric pumps lose power. Without a fresh supply of water in the boiler, the water continues boiling off, and the water level starts falling. If enough water boils off, the fuel rods are exposed and they overheat. At some point, even with the control rods fully inserted, there is enough heat to melt the nuclear fuel. This is where the term meltdown comes from. Tons of melting uranium flows to the bottom of the pressure vessel. At that point, it’s catastrophic. In the worst case, the molten fuel penetrates the pressure vessel gets released into the environment.
Because of this known vulnerability, there is huge redundancy around the pumps and their supply of electricity. There are several sets of redundant pumps, and there are redundant power supplies. Power can come from the power grid. If that fails, there are several layers of backup diesel generators. If they fail, there is a backup battery system. With all of this redundancy, it seems like the vulnerability is completely covered. There is no way for the fatal flaw to ever be exposed.
Unfortunately, shortly after the earthquake, the worst-case scenario unfolded.
The worst-case scenario in Japan’s nuclear crisis would be a meltdown and a massive release of nuclear radiation into the environment.
© iStockphoto.com/caracterdesign
The Worst-case Scenario in Japan’s Nuclear Crisis
The nuclear power plants in Japan weathered the earthquake itself without difficulty. The four plants nearest the quake’s epicenter shut down automatically, meaning that the control rods were fully inserted into their reactor cores and the plants stopped producing power. This is normal operating procedure for these plants, but it meant that the first source of electricity for the cooling pumps was gone. That isn’t a problem because the plant could get power from the power grid to run the pumps.
However, the power grid became unstable and it shut down as well. The second source of electricity for the cooling pumps was gone. That brought the backup diesel generators into play. Diesel generators are a robust and time-tested way to generate electricity, so there were no worries.
But then the tsunami hit. And unfortunately, the tsunami was far larger than anyone had planned for. If the backup diesel generators had been higher off the ground, designed to run while submerged in water or protected from deep water in some way, the crisis could have been averted. Unfortunately, the unexpected water levels from the tsunami caused the generators to fail.
This left the last layer of redundancy — batteries — to operate the pumps. The batteries performed as expected, but they were sized to last for only a few hours. The assumption, apparently, was that electricity would become available from another source fairly quickly.
Although operators did truck in new generators, they could not be hooked up in time, and the coolant pumps ran out of electricity. The fatal flaw in the boiling water design — thought to be impossible to uncover through so many layers of redundancy — had nonetheless become exposed. With it exposed, the next step in the process led to catastrophe.
Explosions at Japan’s Nuclear Power Plants
With the batteries dead, the coolant pumps failed. With no fresh coolant flowing into the reactor core, the water that kept it cool began boiling off. As the water boiled away, the tops of the fuel rods were exposed, and the metal tubes holding the uranium fuel pellets overheated and cracked. The cracks allowed water to enter the tubes and get to the fuel pellets, where it began generating hydrogen gas. The process is called thermolysis — if you get water hot enough, it breaks down into its constituent hydrogen and oxygen atoms.
Hydrogen is a highly explosive gas — recall the Hindenburg explosion, in which the Hindenburg was full of hydrogen gas. In Japan’s nuclear plants, pressure from the hydrogen built up, and the gas had to be vented. Unfortunately, so much hydrogen vented so quickly that it exploded inside the reactor building. This same chain of events unfolded in several different reactors.
The explosions did not rupture the pressure vessels holding the nuclear cores, nor did they release any significant amounts of radiation. These were simple hydrogen explosions, not nuclear explosions. The explosions damaged the concrete and steel buildings surrounding the pressure vessels.
The explosions also indicated that things had gotten out of control. If water were to continue boiling off, a meltdown would be almost assured.
So operators decided to flood the reactors with seawater. This is a last-ditch effort to control the situation, since seawater completely ruins a reactor, but it’s better than a meltdown. In addition, the seawater was mixed with boron to act something like a liquid version of the control rods. Boron absorbs neutrons and is one of the main constituents in the control rods.
Tricastin Nuclear Power Plant is one of 59 French plants that provide 75 percent of the country’s electricity.
David McGlynn/ Photographer’s Choice RF/Getty Images
Next Steps in Japan’s Nuclear Crisis
The nuclear incidents in Japan are described as Level 6 INES events (International Nuclear and Radiological Event Scale). Three Mile Island was a Level 5 event. Chernobyl was a Level 7 event, and that is the top of the event scale
. Obviously, it’s a serious situation.
Japan has lost a significant portion of its electrical generating capacity. Approximately a third of Japan’s electricity comes from nuclear power plants, and about half of that capacity has been lost (approximately 20 percent of total generating capacity)
. That capacity will need to be replaced in some way.
At 40 years old, these reactors are nearing the end of their design lifespans anyway. One alternative is to simply rebuild the plants. The two problems with this approach are that it will be a very lengthy process — possibly taking a decade or more — and the general public in Japan may have no appetite for new nuclear reactors. It is still too early to tell.
There are a number of Mark 1 reactors in the United States. It is certain that they will be decommissioned or altered to take advantage of the lessons learned in Japan. Other reactors may also be altered as needed.
The nuclear industry was hoping for a renaissance of nuclear power in the United States now that more than three decades have elapsed since the Three Mile Island incident shut down new nuclear plant construction in the United States. The events in Japan may stop this renaissance. Or they may spur research in other, possibly safer, nuclear technologies.
Lots More Information
Related Articles
- How Nuclear Radiation Works
- How Nuclear Power Works
- How Radiation Works
- 5 Biggest Nuclear Reactors
- How Nuclear Bombs Work
- How Earthquakes Work
- How Tsunamis Work
- Fact or Fiction: Earthquake Quiz
Sources
- Fisher, Jenna. “Japan’s nuclear crisis: A timeline of key events.” Christian Science Monitor. 3/15/2011 (3/15/2011)http://www.csmonitor.com/World/Asia-Pacific/2011/0315/Japan-s-nuclear-crisis-A-timeline-of-key-events
- Izzo, Phil. “Economists React: If Worst Happens in Japan, All Bets Are Off” Wall Street Journal. 3/15/2011 (3/15/2011) http://blogs.wsj.com/economics/2011/03/15/economists-react-if-worst-happens-in-japan-all-bets-are-off/
- Reuters. “UPDATE 1-French nuclear agency now rates Japan accident at 6” 3/15/2011 (3/15/2011)http://www.reuters.com/article/2011/03/15/japan-quake-nuclear-france-idUSLDE72E2M920110315
How Nuclear Radiation Works
Introduction to How Nuclear Radiation Works
You’ve probably heard people talk about radiation both in fiction and in real life. For example, when the Enterprise approaches a star on “Star Trek,” a member of the crew might warn about an increase in radiation levels. In Tom Clancy’s book “The Hunt for Red October,” a Russian submarine has a nuclear reactor accident with radiation leakage that forces the crew to abandon ship. At Three Mile Island and Chernobyl, nuclear power plants released radioactive substances into the atmosphere during nuclear accidents. And in the aftermath of the March 2011 earthquake and tsunami that struck Japan, a nuclear crisis raised fears about radiation and questions about the safety of nuclear power.
Nuclear radiation can be both extremely beneficial and extremely dangerous. It just depends on how you use it. X-ray machines, some types of sterilization equipment and nuclear power plants all use nuclear radiation — but so do nuclear weapons. Nuclear materials (that is, substances that emit nuclear radiation) are fairly common and have found their way into our normal vocabularies in many different ways. You have probably heard (and used) many of the following terms:
- Uranium
- Plutonium
- Alpha rays
- Beta rays
- Gamma rays
- X-rays
- Cosmic rays
- Radiation
- Nuclear power
- Nuclear bombs
- Nuclear waste
- Nuclear fallout
- Nuclear fission
- Neutron bombs
- Half-life
- Radon gas
- Ionization smoke detectors
- Carbon-14 dating
All of these terms are related by the fact that they all have something to do with nuclear elements, either natural or man-made. But what exactly is radiation? Why is it so dangerous? In this article, we will look at nuclear radiation so that you can understand exactly what it is and how it affects your life on a daily basis.
Click image to watch video.
In this figure, the yellow particles are orbital electrons, the blue particles are neutrons and the red particles are protons.
The “Nuclear” in “Nuclear Radiation”
Let’s start at the beginning and understand where the word “nuclear” in “nuclear radiation” comes from. Here is something you should already feel comfortable with: Everything is made of atoms. Atoms bind together into molecules. So a water molecule is made from two hydrogen atoms and one oxygen atom bound together into a single unit. Because we learn about atoms and molecules in elementary school, we understand and feel comfortable with them. In nature, any atom you find will be one of 92 types of atoms, also known as elements. So every substance on Earth — metal, plastics, hair, clothing, leaves, glass — is made up of combinations of the 92 atoms that are found in nature. The Periodic Table of Elements you see in chemistry class is a list of the elements found in nature plus a number of man-made elements.
Inside every atom are three subatomic particles: protons, neutrons and electrons. Protons and neutrons bind together to form the nucleus of the atom, while the electrons surround and orbit the nucleus. Protons and electrons have opposite charges and therefore attract one another (electrons are negative and protons are positive, and opposite charges attract), and in most cases the number of electrons and protons are the same for an atom (making the atom neutral in charge). The neutrons are neutral. Their purpose in the nucleus is to bind protons together. Because the protons all have the same charge and would naturally repel one another, the neutrons act as “glue” to hold the protons tightly together in the nucleus.
The number of protons in the nucleus determines the behavior of an atom. For example, if you combine 13 protons with 14 neutrons to create a nucleus and then spin 13 electrons around that nucleus, what you have is an aluminum atom. If you group millions of aluminum atoms together you get a substance that is aluminum — you can form aluminum cans, aluminum foil and aluminum siding out of it. All aluminum that you find in nature is called aluminum-27. The “27” is the atomic mass number — the sum of the number of neutrons and protons in the nucleus. If you take an atom of aluminum and put it in a bottle and come back in several million years, it will still be an atom of aluminum. Aluminum-27 is therefore called a stable atom. Up to about 100 years ago, it was thought that all atoms were stable like this.
Many atoms come in different forms. For example, copper has two stable forms: copper-63 (making up about 70 percent of all natural copper) and copper-65 (making up about 30 percent). The two forms are called isotopes. Atoms of both isotopes of copper have 29 protons, but a copper-63 atom has 34 neutrons while a copper-65 atom has 36 neutrons. Both isotopes act and look the same, and both are stable.
The part that was not understood until about 100 years ago is that certain elements have isotopes that are radioactive. In some elements, all of the isotopes are radioactive. Hydrogen is a good example of an element with multiple isotopes, one of which is radioactive. Normal hydrogen, or hydrogen-1, has one proton and no neutrons (because there is only one proton in the nucleus, there is no need for the binding effects of neutrons). There is another isotope, hydrogen-2 (also known as deuterium), that has one proton and one neutron. Deuterium is very rare in nature (making up about 0.015 percent of all hydrogen), and although it acts like hydrogen-1 (for example, you can make water out of it) it turns out it is different enough from hydrogen-1 in that it is toxic in high concentrations. The deuterium isotope of hydrogen is stable. A third isotope, hydrogen-3 (also known as tritium), has one proton and two neutrons. It turns out this isotope is unstable. That is, if you have a container full of tritium and come back in a million years, you will find that it has all turned into helium-3 (two protons, one neutron), which is stable. The process by which it turns into helium is called radioactive decay.
Certain elements are naturally radioactive in all of their isotopes. Uranium is the best example of such an element and is the heaviest naturally occurring radioactive element. There are eight other naturally radioactive elements: polonium, astatine, radon, francium, radium, actinium, thorium and protactinium. All other man-made elements heavier than uranium are radioactive as well.
Radioactive Decay
Radioactive decay is a natural process. An atom of a radioactive isotope will spontaneously decay into another element through one of three common processes:
- Alpha decay
- Beta decay
- Spontaneous fission
In the process, four different kinds of radioactive rays are produced:
- Alpha rays
- Beta rays
- Gamma rays
- Neutron rays
Americium-241, a radioactive element best known for its use in smoke detectors, is a good example of an element that undergoes alpha decay. An americium-241 atom will spontaneously throw off an alpha particle. An alpha particle is made up of two protons and two neutrons bound together, which is the equivalent of a helium-4 nucleus. In the process of emitting the alpha particle, the americium-241 atom becomes a neptunium-237 atom. The alpha particle leaves the scene at a high velocity — perhaps 10,000 miles per second (16,000 km/sec).
If you were looking at an individual americium-241 atom, it would be impossible to predict when it would throw off an alpha particle. However, if you have a large collection of americium atoms, then the rate of decay becomes quite predictable. For americium-241, it is known that half of the atoms decay in 458 years. Therefore, 458 years is the half-life of americium-241. Every radioactive element has a different half-life, ranging from fractions of a second to millions of years, depending on the specific isotope. For example, americium-243 has a half-life of 7,370 years.
Tritium (hydrogen-3) is a good example of an element that undergoes beta decay. In beta decay, a neutron in the nucleus spontaneously turns into a proton, an electron, and a third particle called an antineutrino. The nucleus ejects the electron and antineutrino, while the proton remains in the nucleus. The ejected electron is referred to as a beta particle. The nucleus loses one neutron and gains one proton. Therefore, a hydrogen-3 atom undergoing beta decay becomes a helium-3 atom.
In spontaneous fission, an atom actually splits instead of throwing off an alpha or beta particle. The word “fission” means “splitting.” A heavy atom like fermium-256 undergoes spontaneous fission about 97 percent of the time when it decays, and in the process, it becomes two atoms. For example, one fermium-256 atom may become a xenon-140 and a palladium-112 atom, and in the process it will eject four neutrons (known as “prompt neutrons” because they are ejected at the moment of fission). These neutrons can be absorbed by other atoms and cause nuclear reactions, such as decay or fission, or they can collide with other atoms, like billiard balls, and cause gamma rays to be emitted.
Neutron radiation can be used to make nonradioactive atoms become radioactive; this has practical applications in nuclear medicine. Neutron radiation is also made from nuclear reactors in power plants and nuclear-powered ships and in particle accelerators, devices used to study subatomic physics.
In many cases, a nucleus that has undergone alpha decay, beta decay or spontaneous fission will be highly energetic and therefore unstable. It will eliminate its extra energy as an electromagnetic pulse known as a gamma ray. Gamma rays are like X-rays in that they penetrate matter, but they are more energetic than X-rays. Gamma rays are made of energy, not moving particles like alpha and beta particles.
While on the subject of various rays, there are also cosmic rays bombarding the Earth at all times. Cosmic rays originate from the sun and also from things like exploding stars. The majority of cosmic rays (perhaps 85 percent) are protons traveling near the speed of light, while perhaps 12 percent are alpha particles traveling very quickly. It is the speed of the particles, by the way, that gives them their ability to penetrate matter. When they hit the atmosphere, they collide with atoms in the atmosphere in various ways to form secondary cosmic rays that have less energy. These secondary cosmic rays then collide with other things on Earth, including humans. We get hit with secondary cosmic rays all of the time, but we are not injured because these secondary rays have lower energy than primary cosmic rays. Primary cosmic rays are a danger to astronauts in outer space.
A “Natural” Danger
Although they are “natural” in the sense that radioactive atoms naturally decay and radioactive elements are a part of nature, all radioactive emissions are dangerous to living things. Alpha particles, beta particles, neutrons, gamma rays and cosmic rays are all known as ionizing radiation, meaning that when these rays interact with an atom they can knock off an orbital electron. The loss of an electron can cause problems, including everything from cell death to genetic mutations (leading tocancer), in any living thing.
Because alpha particles are large, they cannot penetrate very far into matter. They cannot penetrate a sheet of paper, for example, so when they are outside the body they have no effect on people. If you eat or inhale atoms that emit alpha particles, however, the alpha particles can cause quite a bit of damage inside your body.
Beta particles penetrate a bit more deeply, but again are only dangerous if eaten or inhaled; beta particles can be stopped by a sheet of aluminum foil or Plexiglas. Gamma rays, like X-rays, are stopped by lead.
Neutrons, because they lack charge, penetrate very deeply, and are best stopped by extremely thick layers of concrete or liquids like water or fuel oil. Gamma rays and neutrons, because they are so penetrating, can have severe effects on the cells of humans and other animals. You may have heard at some point of a nuclear device called a neutron bomb. The whole idea of this bomb is to optimize the production of neutrons and gamma rays so that the bomb has its maximum effect on living things.
As we have seen, radioactivity is “natural,” and we all contain things like radioactive carbon-14. There are also a number of man-made nuclear elements in the environment that are harmful. Nuclear radiation has powerful benefits, such as nuclear power to generate electricity and nuclear medicine to detect and treat disease, as well as significant dangers.
Introduction to How Nuclear Power Works
The nuclear power plant stands on the border between humanity’s greatest hopes and its deepest fears for the future.
On one hand, atomic energy offers a clean energy alternative that frees us from the shackles of fossil fuel dependence. On the other, it summons images of disaster: quake-ruptured Japanese power plants belching radioactive steam, the dead zone surrounding Chernobyl’s concrete sarcophagus.
But what happens inside a nuclear power plant to bring such marvel and misery into being? Imagine following a volt of electricity back through the wall socket, all the way through miles of power lines to the nuclear reactor that generated it. You’d encounter the generator that produces the spark and the turbine that turns it. Next, you’d find the jet of steam that turns the turbine and finally the radioactive uranium bundle that heats water into steam. Welcome to the nuclear reactor core.
The water in the reactor also serves as a coolant for the radioactive material, preventing it from overheating and melting down. In March 2011, viewers around the world became well acquainted with this reality as Japanese citizens fled by the tens of thousands from the area surrounding the Fukushima-Daiichi nuclear facility after the most powerful earthquake on record and the ensuing tsunami inflicted serious damage on the plant and several of its reactor units. Among other events, water drained from the reactor core, which in turn made it impossible to control core temperatures. This resulted in overheating and a partial nuclear meltdown
.
As of March 1, 2011, there were 443 operating nuclear power reactors spread across the planet in 47 different countries
. In 2009 alone, atomic energy accounted for 14 percent of the world’s electrical production. Break that down to the individual country and the percentage skyrockets as high as 76.2 percent for Lithuania and 75.2 for France
. In the United States, 104 nuclear power plants supply 20 percent of the electricity overall, with some states benefiting more than others.
In this article, we’ll look at just how a nuclear reactor functions inside a power plant, as well as the atomic reaction that releases all that crucial heat.
WHAT ABOUT PLUTONIUM?
Uranium-235 isn’t the only possible fuel for a power plant. Another fissionable material is plutonium-239. Plutonium-239 is created by bombarding U-238 with neutrons, a common occurrence in a nuclear reactor.
Nuclear Fission: The Heart of the Reactor
Despite all the cosmic energy that the word “nuclear” invokes, power plants that depend on atomic energy don’t operate that differently from a typical coal-burning power plant. Both heat water into pressurized steam, which drives a turbine generator. The key difference between the two plants is the method of heating the water.
While older plants burn fossil fuels, nuclear plants depend on the heat that occurs during nuclear fission, when one atom splits into two and releases energy. Nuclear fission happens naturally every day. Uranium, for example, constantly undergoes spontaneous fission at a very slow rate. This is why the element emits radiation, and why it’s a natural choice for the induced fissionthat nuclear power plants require.
Uranium is a common element on Earth and has existed since the planet formed. While there are several varieties of uranium, uranium-235 (U-235) is the one most important to the production of both nuclear power and nuclear bombs.
U-235 decays naturally by alpha radiation: It throws off an alpha particle, or two neutrons and two protons bound together. It’s also one of the few elements that can undergo induced fission. Fire a free neutron into a U-235 nucleus and the nucleus will absorb the neutron, become unstable and split immediately. See How Nuclear Radiation Works for complete details.
The animation to the right shows a uranium-235 nucleus with a neutron approaching from the top. As soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off two or three new neutrons (the number of ejected neutrons depends on how the U-235 atom splits). The process of capturing the neutron and splitting happens very quickly.
The decay of a single U-235 atom releases approximately 200 MeV (million electron volts). That may not seem like much, but there are lots of uranium atoms in a pound (0.45 kilograms) of uranium. So many, in fact, that a pound of highly enriched uranium as used to power a nuclear submarine is equal to about a million gallons of gasoline.
The splitting of an atom releases an incredible amount of heat and gamma radiation, or radiation made of high-energy photons. The two atoms that result from the fission later release beta radiation (superfast electrons) and gamma radiation of their own, too.
But for all of this to work, scientists have to first enrich a sample of uranium so that it contains 2 to 3 percent more U-235. Three-percent enrichment is sufficient for nuclear power plants, but weapons-grade uranium is composed of at least 90 percent U-235.
Stick with us. We’ll head inside the power plant and investigate the reactor next.
This diagram shows all the parts of a nuclear reactor. Take a tour of the inside of a nuclear power plant.
© 2011 HowStuffWorks.com
Inside a Nuclear Power Plant
In order to turn nuclear fission into electrical energy, nuclear power plant operators have to control the energy given off by the enriched uranium and allow it to heat water into steam.
Enriched uranium typically is formed into inch-long (2.5-centimeter-long) pellets, each with approximately the same diameter as a dime. Next, the pellets are arranged into long rods, and the rods are collected together into bundles. The bundles are submerged in water inside a pressure vessel. The water acts as a coolant. Left to its own devices, the uranium would eventually overheat and melt.
To prevent overheating, control rods made of a material that absorbs neutrons are inserted into the uranium bundle using a mechanism that can raise or lower them. Raising and lowering the control rods allow operators to control the rate of the nuclear reaction. When an operator wants the uranium core to produce more heat, the control rods are lifted out of the uranium bundle (thus absorbing fewer neutrons). To reduce heat, they are lowered into the uranium bundle. The rods can also be lowered completely into the uranium bundle to shut the reactor down in the event of an accident or to change the fuel.
The uranium bundle acts as an extremely high-energy source of heat. It heats the water and turns it to steam. The steam drives a turbine, which spins a generator to produce power. Humans have been harnessing the expansion of water into steam for hundreds of years. To learn more about the properties involved, read How Steam Technology Works.
In some nuclear power plants, the steam from the reactor goes through a secondary, intermediate heat exchanger to convert another loop of water to steam, which drives the turbine. The advantage to this design is that the radioactive water/steam never contacts the turbine. Also, in some reactors, the coolant fluid in contact with the reactor core is gas (carbon dioxide) or liquid metal (sodium, potassium); these types of reactors allow the core to be operated at higher temperatures.
Given all the radioactive elements inside a nuclear power plant, it shouldn’t come as a surprise that there’s a little more to a plant’s outside than you’d find at a coalpower plant. In the next section, we’ll explore the various protective barriers between you and the atomic heart of the plant.
As you can tell by looking at this photograph of Germany’s Brokdorf nuclear plant, concrete plays an important role in containing radioactive materials.
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Outside a Nuclear Power Plant
Once you get past the reactor itself, there’s very little difference between a nuclear power plant and a coal-fired or oil-fired power plant, except for the source of the heat used to createsteam. But as that source can emit harmful levels of radiation, extra precautions are required.
A concrete liner typically houses the reactor’s pressure vessel and acts as a radiation shield. That liner, in turn, is housed within a much larger steel containment vessel. This vessel contains the reactor core, as well as the equipment plant workers use to refuel and maintain the reactor. The steel containment vessel serves as a barrier to prevent leakage of any radioactive gases or fluids from the plant.
An outer concrete building serves as the final layer, protecting the steel containment vessel. This concrete structure is designed to be strong enough to survive the kind of massive damage that might result from earthquakes or a crashing jet airliner. These secondary containment structures are necessary to prevent the escape of radiation/radioactive steam in the event of an accident. The absence of secondary containment structures in Russian nuclear power plants allowed radioactive material to escape in Chernobyl.
Workers in the control room at the nuclear power plant can monitor the nuclear reactor and take action if something goes wrong. Nuclear facilities also typically feature security perimeters and added personnel to help protect sensitive materials.
As you probably know, nuclear power has its share of critics, as well as its supporters. On the next page, we’ll take a quick look at some of the pros and cons of splitting an atom to keep everyone’s TVs and toasters running.
This storage facility near the site of the Chernobyl Nuclear Power Plant currently houses nuclear waste.
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Pros and Cons of Nuclear Power
What’s nuclear power’s biggest advantage? It doesn’t depend on fossil fuels and isn’t affected by fluctuating oil and gas prices. Coal and natural gas power plants emit carbon dioxide into the atmosphere, which contributes to climate change. With nuclear power plants, CO2emissions are minimal.
According to the Nuclear Energy Institute, the power produced by the world’s nuclear plants would normally produce 2 billion metric tons of CO2 per year if they depended on fossil fuels. In fact, a properly functioning nuclear power plant actually releases less radioactivity into the atmosphere than a coal-fired power plant
Plus, all this comes with a far lighter fuel requirement. Nuclear fission produces roughly a million times more energy per unit weight than fossil fuel alternatives
And then there are the negatives. Historically, mining and purifying uranium hasn’t been a very clean process. Even transporting nuclear fuel to and from plants poses a contamination risk. And once the fuel is spent, you can’t just throw it in the city dump. It’s still radioactive and potentially deadly.
On average, a nuclear power plant annually generates 20 metric tons of used nuclear fuel, classified as high-level radioactive waste. When you take into account every nuclear plant on Earth, the combined total climbs to roughly 2,000 metric tons a year
All of this waste emits radiation and heat, meaning that it will eventually corrode any container that holds it. It can also prove lethal to nearby life forms. As if this weren’t bad enough, nuclear power plants produce a great deal of low-level radioactive waste in the form of radiated parts and equipment.
Over time, spent nuclear fuel decays to safe radioactive levels, but this process takes tens of thousands of years. Even low-level radioactive waste requires centuries to reach acceptable levels. Currently, the nuclear industry lets waste cool for years before mixing it with glass and storing it in massive cooled, concrete structures. This waste has to be maintained, monitored and guarded to prevent the materials from falling into the wrong hands. All of these services and added materials costmoney — on top of the high costs required to build a plant.
A glimpse of the aftermath from the largest earthquake in history and the ensuing tsunami that tore Japan apart and led to its nuclear catastrophe.
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Nuclear Catastrophe and Reactor Shutdown
Remember, at the heart of every nuclear reactor is a controlled environment of radioactivity and induced fission. When this environment spins out of control, the results can be catastrophic.
For many years, the Chernobyl disaster stood as a prime worst-case example of nuclear malfunction. In 1986, the Ukrainian nuclear reactor exploded, spewing 50 tons of radioactive material into the surrounding area, contaminating millions of acres of forest. The disaster forced the evacuation of at least 30,000 people, and eventually caused thousands to die from cancer and other illnesses
Chernobyl was poorly designed and improperly operated. The plant required constant human attention to keep the reactor from malfunctioning. Meanwhile, modern plants require constant supervision to keep from shutting down. Yet even a well-designed nuclear power plant is susceptible to natural disaster.
On Friday, March 11, 2011, Japan suffered the largest earthquake in modern history. A programmed response at the country’s Fukushima-Daiichi nuclear facility immediately descended all of the reactor’s control rods, shutting down all fission reactions within ten minutes. Unfortunately, however, you can’t shut down all radioactivity with the flip of a switch.
As we explored on the previous page, nuclear waste continues to generate heat years after its initial run in a power plant. Similarly, within the first few hours after a nuclear reactor shuts down, it continues to generate heat from the decay process.
The March 2011 quake manifested a deadly tsunami, which destroyed the backup diesel generators that powered the water coolant pumps and that the facility had turned to after it couldn’t get power from Japan’s grid. These pumps circulate water through the reactor to remove decay heat. Uncirculated, both the water temperature and water pressure inside the reactor continued to rise. Furthermore, the reactor radiation began to split the water into oxygen and volatile hydrogen. The resulting hydrogen explosions breached the reactor building’s steel containment panels.
Simply put, the Fukushima-Daiichi facility had many countermeasures in place to shut down operations in the event of severe seismic activity. They just didn’t count on losing power to their coolant pumps.
Plants such as Japan’s Fukushima-Daiichi facility, Russia’s Chernobyl and the United States’ Three Mile Island remain a black eye for the nuclear power industry, often overshadowing some of the environmental advantages the technology has to offer. You can read more about exactly what happened in How Japan’s Nuclear Crisis Works.
