Basic Nuclear Fission

Introduction:

Nuclear fission is the process of splitting atoms, or fissioning them. This page will explain to you the basics of nuclear fission. Before we talk about that, however, I would like to discuss marbles. Everyone's played with marbles at one time or another, right? Well, imagine about 200 marbles lying on a flat surface, all jumbled together, and roughly forming a circle. What would happen if someone took another marble and threw it at them? They would fly all around in different directions and groups, right? That is exactly what happens in nuclear fission. The filled circle is like an atom's nucleus. The marble being thrown is like a "neutron bullet". The only differences are that the marbles are protons and neutrons and the protons and neutrons aren't in a filled circle, but in the actual atom are in the shape of a sphere. Of course, an atom is also a bit more complicated than a pack of marbles.

Choosing the Bullet:

When we spoke about the marble analogy earlier, we said that the marble being thrown is a like a "neutron bullet". But what does this mean, and why not use another type of particle to "throw" at a nucleus to fission it? First, what particles with distinct mass are available to launch at a nucleus? Think back to our lesson on radioactivity. Recall that two particles emitted by radioactive elements are the particle and the neutron. (There are other particles emitted too, but they are generally much smaller than the neutron and the particle.) Recall that the particle is essentially a 4He nucleus. Now, let's review the structure of an atom. Remember that an atomic nucleus is made up of positive protons and neutral neutrons? Because of this, the nucleus carries an overall positive charge. So, if we were to launch another particle with a positive charge at a nucleus, it wouldn't get there. Why wouldn't it get there? The answer lies in magnetism. Have you ever used magnets? If you have, you'd know that two like poles of a magnet repel each other. A positive particle and the positive nucleus would repel each other in the same way. The particle is positive. Why? Well, it's composed of two protons and two neutrons. Its positive protons give it a positive charge. Because it's positive, it would get repelled away from another positive nucleus. So, the only thing left is the neutron. The neutron is electrically neutral and thus would not get repelled from a positive nucleus.

Fissile Isotopes:

Fissile isotopes are isotopes of an element that can be split through fission. Only certain isotopes of certain elements are fissile. For example, one isotope of uranium, 235U, is fissile, while another isotope, 238U, is not. Other examples of fissile elements are 239Pu and 232Th. An important factor affecting whether or not an atom will fission is the speed at which the bombarding neutron is moving. If the neutron is highly energetic (and thus moving very quickly), it can cause fission in some elements that a slower neutron would not. For example, thorium 232 requires a very fast neutron to induce fission. However, uranium 235 needs slower neutrons. If a neutron is too fast, it will pass right through a 235U atom without affecting it at all.

Splitting the Uranium Atom:

Uranium is the principle element used in nuclear reactors and in certain types of atomic bombs. The specific isotope used is 235U. When a stray neutron strikes a 235U nucleus, it is at first absorbed into it. This creates 236U. 236U is unstable and this causes the atom to fission. The fissioning of 236U can produce over twenty different products. However, the products' masses always add up to 236. The following two equations are examples of the different products that can be produced when 235U fissions:

• 235U + 1 neutron 2 neutrons + 92Kr + 142Ba + ENERGY
• 235U + 1 neutron 2 neutrons + 92Sr + 140Xe + ENERGY

Let's discuss those reactions. In each of the above reactions, 1 neutron splits the atom. When the atom is split, 1 additional neutron is released. This is how a chain reaction works. If more 235U is present, those 2 neutrons can cause 2 more atoms to split. Each of those atoms releases 1 more neutron bringing the total neutrons to 4. Those 4 neutrons can strike 4 more 235U atoms, releasing even more neutrons. The chain reaction will continue until all the 235U fuel is spent. This is roughly what happens in an atomic bomb. It is called a runaway nuclear reaction.

In this animation, one can see how the fissioning of each 235U atom (red) releases more neutrons (green) that go on to fission more 235U atoms, thus producing a chain reaction.

Where Does the Energy Come From?:

In the section above we described what happens when an 235U atom fissions. We gave the following equation as an example: 235U + 1 neutron 2 neutrons + 92Kr + 142Ba + ENERGY
You might have been wondering, "Where does the energy come from?". The mass seems to be the same on both sides of the reaction:
235 + 1 = 2 + 92 + 142 = 236
Thus, it seems that no mass is converted into energy. However, this is not entirely correct. The mass of an atom is more than the sum of the individual masses of its protons and neutrons, which is what those numbers represent. Extra mass is a result of the binding energy that holds the protons and neutrons of the nucleus together. Thus, when the uranium atom is split, some of the energy that held it together is released as radiation in the form of heat. Because energy and mass are one and the same, the energy released is also mass released. Therefore, the total mass does decrease a tiny bit during the reaction.

Introduction:

Currently, about half of all nuclear power plants are located in the US. There are many different kinds of nuclear power plants, and we will discuss a few important designs in this text. A nuclear power plant harnesses the energy inside atoms themselves and converts this to electricity. This electricity is used by all of us. By now, you should have an idea of the fission process and how it works. A nuclear power plant uses controlled nuclear fission. In this section, we will explore how a nuclear power plant operates and the manner in which nuclear reactions are controlled.

Uranium Preparation:

Earlier we talked about nuclear fission with 235U. In reality, this will not be the only isotope of uranium present in a nuclear reactor. In naturally occurring uranium deposits, less than one percent of the uranium is 235U. The majority of the uranium is 238U. 238U is not a fissile isotope of uranium. When 238U is struck by a loose neutron, it absorbs the neutron into its nucleus and does not fission. Thus, by absorbing loose neutrons, 238U can prevent a nuclear chain reaction from occurring. This would be a bad thing because if a chain reaction doesn't occur, the nuclear reactions can't sustain themselves, the reactor shuts down, and millions of people are without electrical power. In order for a chain reaction to occur, the pure uranium ore must be refined to raise the concentration of 235U. This is called enrichment and is primarily accomplished through a technique called gaseous diffusion. In this process, the uranium ore is combined with fluorine to create a chemical compound called uranium hexafluoride. The uranium hexafluoride is heated and vaporizes. The heated gas is then pushed through a series of filters. Because some of the uranium hexafluoride contains 238U and some contains 235U, there is a slight difference in the weights of the individual molecules. The molecules of uranium hexafluoride containing 235U are slightly lighter and thus pass more easily through the filters. This creates a quantity of uranium hexafluoride with a higher proportion of 235U. This is collected, the uranium is stripped from it, and the result is an enriched supply of fuel. Usually, nuclear power plants use uranium fuel that is about 4% 235U.

Parts of a Nuclear Reactor - Pressurized Water Reactor (PWR):

Fuel Assembly Containing a Number of Fuel Rods Original Image Used with Permission of the Uranium Institute

A typical nuclear reactor has a few main parts. Inside the "core" where the nuclear reactions take place are the fuel rods and assemblies, the control rods, the moderator, and the coolant. Outside the core are the turbines, the heat exchanger, and part of the cooling system.
The fuel assemblies are collections of fuel rods. These rods are each about 3.5 meters (11.48 feet) long. They are each about a centimeter in diameter. These are grouped into large bundles of a couple hundred rods called fuel assemblies, which are then placed in the reactor core. Inside each fuel rod are hundreds of pellets of uranium fuel stacked end to end.
Also in the core are control rods. These rods have pellets inside that are made of very efficient neutron capturers. An example of such a material is cadmium. These control rods are connected to machines that can raise or lower them in the core. When they are fully lowered into the core, fission can not occur because they absorb free neutrons. However, when they are pulled out of the reactor, fission can start again anytime a stray neutron strikes a 235U atom, thus releasing more neutrons, and starting a chain reaction.
Another component of the reactor is the moderator. The moderator serves to slow down the high speed neutrons "flying" all around the reactor core. If a neutron is moving too fast, and thus is at a high-energy state, it passes right through the 235U nucleus. It must be slowed down to be captured by the nucleus and to induce fission. The most common moderator is water, but sometimes it can be another material.
The job of the coolant is to absorb the heat from the reaction. The most common coolant used in nuclear power plants today is water. In actuality, in many reactor designs the coolant and the moderator are one and the same. The coolant water is heated by the nuclear reactions going on inside the core. However, this heated water does not boil because it is kept at an extremely intense pressure, thus raising its boiling point above the normal 100° Celsius.


Control a nuclear reaction in The Nuclear Reaction Java Applet

The Inside of a Reactor Containment Structure One can see the heavy concrete walls from which the structure is made. Also, a fuel rod transportation canister is in the background (blue arrow). In front of that is the pit where the reactor core would normally reside (red arrow). Photo Used With Permission of Joseph Gonyeau. Original Source: Virtual Nuclear Tourist

The heated water rises up and passes through another part of the reactor, the heat exchanger. The moderator/coolant water is radioactive, so it can not leave the inner reactor containment. Its heat must be transferred to non-radioactive water, which can then be sent out of the reactor shielding. This is done through the heat exchanger, which works by moving the radioactive water through a series of pipes that are wrapped around other pipes. The metallic pipes conduct the heat from the moderator to the normal water. Then, the normal water (now in steam form and intensely hot) moves to the turbine, where electricity is produced.

Three Mile Island, the Site of a Nuclear Accident The steam towers are the large objects in the upper part of the picture. They do not actually house any reactors, and their only purpose is to cool water after it has passed through the turbines. Photo Courtesy Nuclear Regulatory Commission

After the hot water has passed through the turbine, some of its energy is changed into electricity. However, the water is still very hot. It must be cooled somehow. Many nuclear power plants used steam towers to cool this water with air. These are generally the buildings that people associate with nuclear power plants. At reactors that do not have towers, the clean water is purified and dumped into the nearest body of water, and cool water is pumped in to replace it.

PWR Power Plant Schematic Original Image Used with Permission of the Uranium Institute

From Fission to Electricity:

A nuclear power plant produces electricity in almost exactly the same way that a conventional (fossil fuel) power plant does. A conventional power plant burns fuel to create heat. The fuel is generally coal, but oil is also sometimes used. The heat is used to raise the temperature of water, thus causing it to boil. The high temperature and intense pressure steam that results from the boiling of the water turns a turbine, which then generates electricity. A nuclear power plant works the same way, except that the heat used to boil the water is produced by a nuclear fission reaction using 235U as fuel, not the combustion of fossil fuels. A nuclear power plant uses much less fuel than a comparable fossil fuel plant. A rough estimate is that it takes 17,000 kilograms of coal to produce the same amount of electricity as 1 kilogram of nuclear uranium fuel.

Other Types of Reactors:

Although the most common type of reactor is the Pressurized Water Reactor (PWR), many other types of reactors are also used. In the PWR, as we described earlier, there are two main water cycles. One is the water inside the core that is highly radioactive. This water's heat is transferred to other, non-radioactive water inside the second loop. This water is then used to turn a turbine. The second most popular reactor type is the Boiling Water Reactor (BRW). This type of reactor differs from the PWR in that there is only one water cycle. Radioactive water is used to turn the turbine. The major disadvantage of this is that the radioactive nuclides in the water that cause its radioactivity can be transferred to the turbine, thus causing it to become radioactive too. This produces more hazardous material that needs to be disposed of when a reactor is dismantled. However, the BWR also has a few advantages. Its core can be kept at a lower pressure, for example. Another type of reactor is the Heavy Water Reactor (HWR). A HWR uses heavy water as a moderator instead of normal water. Heavy water is water with deuterium, which is an isotope of hydrogen with 1 neutron. Deuterium is heavier than normal hydrogen, which has no neutrons. HWR's come in two types, pressurized and boiling, just like normal "light water" reactors. The advantage of a HWR is that un-enriched uranium fuel can be used. This is because the heavy water is a much more efficient moderator than light water. Thus, more stray neutrons can be slowed down enough to cause fission in 235U. This more efficient moderator makes up for the greater abundance of the neutron-capturing 238U.

Types of Nuclear Waste

Introduction:

One of the greatest problems with nuclear energy is the waste produced. The waste is generally radioactive, and thus toxic. There are also a few different kinds of waste, depending on how it was produced. Nuclear waste is produced in many different ways. There are wastes produced in the reactor core, wastes created as a result of radioactive contamination, and wastes produced as a bi-product of uranium mining, refining, and enrichment. The vast majority (99%) of radiation in nuclear waste is given off from spent fuel rods. However, fuel rods make up a relatively small percentage of the volume of waste. The largest volume of nuclear waste is composed of the leftovers from the mining process. This waste, however, doesn't give off much radiation. Some of the nuclear waste is extremely long-lived, meaning that it lasts a long time without its toxicity decreasing all that much, and some of it is very short-lived. Some types of nuclear waste are considered high-level and some are considered low level. The difference is in the amount of radioactive nuclei in relation to the mass of the waste. If there are a large amount of radioactive nuclei relative to the amount of waste, it is considered high level nuclear waste.

Fission Bi-Products:

When a 235U atom splits, it can produce a number of different products. Many of these are radioactive elements. For example, the following reaction produces 90Sr, which has a half-life of about 29 years. 1 neutron + 235U 2 neutrons + 90Sr + 144Xe
Although its half-life is 29 years, a quantity of 90Sr is not considered safe for 290 years. After 290 years, 10 half-lives would have passed. So, if we started out with half a ton (1000 lbs.) of 90Sr, after 290 years there would be 1000 x (1/2)¹⁰ left. This is about a pound. The rest of the 90Sr would have undergone �^- decay, producing 90Y. 90Y is also radioactive, but is has a very short half-life of about 2.67 days. The 90Y undergoes �^- decay, forming 90Zr, which is a stable, non-radioactive isotope. 90Sr is particularly dangerous because it shares many of the same chemical properties as calcium (Ca), and, if ingested, can take calcium's place in your bones. Then, when 90Sr decays, the radiation released in your body can cause cancer.
This is just one example of a radioactive isotope that is produced from fission. There are hundreds of other fission products, many of which are radioactive. Their half-lives, however, vary greatly from less than a second to many, many years.
The fission products, or fragments, usually remain within the fuel rods of the reactor. When most of the 235U in a fuel rod is spent, the rod must be removed. The radioactive fragments are what make the spent rods toxic. The fission products can be long-lived or short-lived.

Transuranics:

In previous texts we talked about how the 238U in a fuel rod is not fissile and is a neutron absorber. We made the point that because it absorbs neutrons, it stops the chain reaction in a nuclear power plant from running away (and producing a nuclear bomb effect). This is a good thing. However, think about what it means when we say "238U is a neutron absorber". The following reaction expresses that statement: 1 neutron + 238U 239U
When 238U "captures" a neutron, it is added to the original uranium nucleus, producing the radioactive isotope of uranium, 239U. This isotope has a half-life of 23.45 months. It decays, through �^-, into 239Np. 239Np is also radioactive and decays into 239Pu. 239Np has a short half-life of about 2 days. This sequence of decays can be expressed like this:
1 neutron + 238U 239U
239U (�^- decay) 239Np
239Np (�^- decay) 239Pu
239Pu is also radioactive, and has a half-life of approximately 24,000 years. That's a long time!! A lot of 238U is turned into 239Pu through this sequence of decays. 239Pu is called a transuranic element. Any element with a higher atomic number (and thus more protons) than uranium is considered to be transuranic. This applies to all of the elements to the right of uranium in the Periodic Table. In the equations above we showed how a neutron can be captured by a nucleus and, through a series of �^- decays, can produce an isotope with a higher atomic number than the original atom. More than one neutron can be captured. So, for instance, a neutron can be captured again by 239U. This produces 240U, which decays into 240Np. If 240Np captures another neutron, it becomes 241Np, which then decays into 241Pu and then into 241Am, which has a half-life of about 400 years. This sequence of decays and neutron additions can be expressed in the following reactions:
1 neutron + 238U 239U
1 neutron + 239U 240U
240U (�^- decay) 240Np
1 neutron + 240Np 241Np
241Np (�^- decay) 241Pu
241Pu (�^- decay) 241Am
This is only one example of how higher-atomic number transuranic elements can be produced. There are many other pathways involving �^- decay and neutron capture/addition that can produce transuranic elements besides neptunium (Np), plutonium (Pu), and americium (Am).
The transuranic neutron addition products usually remain in the fuel rods, where the original 238U from which they were produced was located. This adds to the rods' toxicity, and makes it harder for them to be disposed. In general, transuranic wastes are long-lived. However, this depends on the isotope produced. The biggest transuranic waste produced is 239Pu. This is an extremely toxic and extremely long-lived compound. 239Pu is fissile. In fact, when a nuclear reactor's fuel rods are almost spent, as much as 30% of the reactor output can come from the fissioning of 239Pu. Thus, the plutonium transuranic "waste" produced in a nuclear reactor can actually be used as fuel. We will discuss more on this later.

Waste from Uranium Mining and Enrichment:

When uranium is mined, it has to be separated from rock. This produces pure uranium ore and "tailings", essentially leftover rock that has had the uranium stripped from it. This rock often still contains radioactive nuclides and is somewhat dangerous. The tailings are generally long-lived, but are considered to be low-level waste. That is, the concentration of radioactive nuclei in them is small, and thus they are not extremely radioactive. As we explained previously, uranium ore is only about .7% 235U. It must be enriched to bring the percentage of 235U up to about 4%. The enrichment process produces a lot of waste. This is because for every gram of enriched uranium fuel produced, there are about 4 grams of 238U waste. 238U is radioactive and has a half-life of 4,468,000,000 years. This means that it is long-lived, but not extremely dangerous. However, some of its "daughter products" are radioactive. Thus, wastes produced as a result of enrichment must be kept in storage. By the way, a "daughter product" is an isotope that results from a decay of another, "parent", isotope. For example, when 238U decays, it produces 234Th, which is very radioactive and has a half-life of about 24 days. The decay can be expressed in the following equation:
238U ( decay) 234Th + particle

Contaminated Stuff:

A major portion of nuclear waste is comprised of spent fuel rods. These contain the fission products and transuranic wastes we mentioned above. However, a lot of other waste is produced in the reactor besides the fuel rods. This occurs as a result of radioactive contamination. A nuclear reactor is extremely hot. This means that the particles inside the reactor are very energetic and are flying around at incredible speeds. Occasionally, an atom that is in a fuel rod can get knocked out. These atoms that get knocked out can be many different types, ranging from fission products to uranium to transuranic elements. Most are radioactive. Atoms that escape the fuel rod careen all over the inside of the reactor core. Eventually these atoms can strike something solid. This is a lot like a bullet hitting a wall. If the wall is small, it might pass through. However, if the wall is big enough, the bullet will smash into the wall and "stick" there. So it is with a nuclear reactor. Occasionally an atom can smash into a structural component of the reactor, implanting itself into it. Because many of the nuclides (fancy term for atomic nucleus) careening about the core of a fission reactor are radioactive, when they smash into a structure and "stick", they make that structure appear to be radioactive. This is because there are many radioactive nuclides embedded in it, which give off radiation. Thus, many of the structural components of a reactor become radioactive over time, as they absorb radioactive nuclei into themselves. Also, many of the pipes and other components of a reactor become radioactive. These must be replaced eventually because over time the extreme radiation inside the reactor weakens them. The biggest problem, however, arises when a nuclear reactor is turned off for good, or "decommissioned". Disposal of the reactor core is a huge problem because it is extremely radioactive. 
 

Nuclear Waste Storage

Introduction:

The major problem of nuclear waste is what to do with it. In fact, one of the biggest (and perhaps the single biggest) expenses of the nuclear power industry could eventually be the storage of nuclear waste. Currently there are several ways in which nuclear waste is stored. Most of these methods are temporary. In most cases a viable long-term solution for waste storage has yet to be found. This is because the time period for storage is so incredibly long, on the order of thousands of years.

Temporary Storage of Fuel Rods:

Fuel Rod Storage Pool Photo Used With Permission of Joseph Gonyeau. Original Source: Virtual Nuclear Tourist

The spent fuel rods from a nuclear reactor are the most radioactive of all nuclear wastes. When all the radiation given off by nuclear waste is tallied, the fuel rods give off 99% of it, in spite of having relatively small volume. There is, as of now, no permanent storage site of spent fuel rods. Temporary storage is being used while a permanent site is searched for and prepared. When the spent fuel rods are removed from the reactor core, they are extremely hot and must be cooled down. Most nuclear power plants have a temporary storage pool next to the reactor. The spent rods are placed in the pool, where they can cool down. The pool is not filled with ordinary water but with boric acid, which helps to absorb some of the radiation given off by the radioactive nuclei inside the spent rods. The spent fuel rods are supposed to stay in the pool for only about 6 months, but, because there is no permanent storage site, they often stay there for years. Many power plants have had to enlarge their pools to make room for more rods. As pools fill, there are major problems. If the rods are placed too close together, the remaining nuclear fuel could go critical, starting a nuclear chain reaction. Thus, the rods must be monitored and it is very important that the pools do not become too crowded. Also, as an additional safety measure, neutron-absorbing materials similar to those used in control rods are placed amongst the fuel rods. Permanent disposal of the spent fuel is becoming more important as the pools become more and more crowded.

Dry Cask Storage Containers Used with Permission of NSP

Another method of temporary storage is now used because of the overcrowding of pools. This is called dry storage (as opposed to "wet" storage which we outlined above). Basically, this entails taking the waste and putting it in reinforced casks or entombing it in concrete bunkers. This is after the waste has already spent about 5 years cooling in a pool. The casks are also usually located close to the reactor site.

Permanent Fuel Storage/Disposal:

There are many ideas about what to do with nuclear waste. The low-level (not extremely radioactive) waste can often be buried near the surface of the earth. It is not very dangerous and usually will have lost most of its radioactivity in a couple hundred years. The high-level waste, comprised mostly of spent fuel rods, is harder to get rid of. There are still plans for its disposal, however. Some of these include burying the waste under the ocean floor, storing it underground, and shooting it into space. The most promising option so far is burying the waste in the ground. This is called "deep geological disposal". Because a spent fuel rod contains material that takes thousands of years to become stable (and non-radioactive), it must be contained for a very long time. If it is not contained, it could come in contact with human population centers and wildlife, posing a great danger to them. Therefore, the waste must be sealed up tightly. Also, if the waste is being stored underground, it must be stored in an area where there is little groundwater flowing through. If ground water does flow through a waste storage site, it could erode the containment canisters and carry waste away into the environment. Additionally, a disposal site must be found with little geological activity. We don't want to put a waste disposal site on top of a fault line, where 1000 years in the future an earthquake will occur, releasing the buried waste into the environment. The waste will probably be encapsulated in large casks designed to withstand corrosion, impacts, radiation, and temperature extremes. Special casks will also have to be used to transfer fuel rods from their holding pools and dry storage areas next to the reactor to the permanent geological storage site.

Aerial View of Yucca Mountain Image Courtesy Yucca Mountain Site Characterization Process

In the US a permanent storage site has been selected at Yucca Mountain, Nevada. Yucca Mountain is in an extremely dry area of Nevada. This minimizes the possibility of water seeping through the rock and corroding the casks. Additionally, if the casks do get corroded, there is not much water flow to carry the nuclear wastes away. The casks will be buried about 1500 feet underground, further preventing the waste from escaping. It is also far from the nearest population center in Las Vegas. While Yucca Mountain is near of a fault line, the fault is believed to be inactive. There are several volcanoes in the vicinity, but scientists believe that they have been dormant for almost a million years and think it unlikely that they will erupt in the next 10,000 years. Naturally, the people in Nevada are opposed to the creation of a nuclear waste repository. They express the common reaction, NIMBY (Not In My Backyard!!). This is because that although most evidence indicates that Yucca Mountain is a suitable place for storage, no one can guarantee that waste will not leak. However, quite a bit of research has already conducted around the Yucca site. Also, work on tunneling into the mountain has been started. The Yucca Mountain Deep Geological Repository is projected to be ready by the year 2010.

The Massive TBM (Tunnel Building Machine) Used to Dig the Tunnels Into Yucca Mountain Image Courtesy Yucca Mountain Site Characterization Process

One of the Storage Tunnels

 

New Nuclear Power Technologies

Introduction:

There are many new waste disposal technologies which could prove to be somewhat of a solution to the problem of nuclear waste.

Reprocessing, The Missing Step:

Although not a new technology, reprocessing can be part of the solution to nuclear waste. When nuclear power was first developed, it was assumed that spent nuclear fuel would go through a process called reprocessing. In reprocessing, one of the major transuranic wastes, 239Pu, is extracted from the spent fuel rods. This 239Pu (plutonium-239) is fissile and can be reused in power plants. The advantages of this process are somewhat obvious: The volume of waste is lessened and more fuel is created for nuclear reactors. However, as with all things, politics can get in the way. In the US plutonium reprocessing was banned because the recovered 239Pu is weapons grade material. If, after reprocessing, the fuel is stolen, it could be used by anyone to construct a nuclear weapon. As of a few years ago, the ban against reprocessing in the US was lifted, but there are still no operating reprocessing plants in the US because of the heavy regulations and the anti-nuclear sentiment of the general public. There are a few countries which do reprocessing, however. France, for instance, regularly reprocesses its spent fuel.

High Temperature Breeder Reactors:

Many of us are familiar from television (and hopefully not from real life experience) of the bar-room game in which a very large man holds a mug of beer on top of his head and challenges people to punch him. If his opponent punches him hard enough, the beer falls off and spills all over the man holding it. The harder the punch, the better chance that the beer will fall off and the puncher will win. Also, the bigger the man is who is getting punched, the harder the punch must be to knock the beer down. You might be wondering why we are talking about a bar-room game. Think of the guy holding the beer as an atom and the guy punching as a neutron. The transuranic elements are bigger than uranium and generally don't fission (get their beer knocked off) in a regular reactor. The neutrons aren't excited enough (don't punch hard enough) to induce fission in them. However, if they are placed in a high-temperature reactor in which the neutrons are much more excited (and carry more punch), there is a much better chance that they will fission. In a reactor being developed by Argonne National Laboratory in the US, almost 100% of the transuranic nuclear wastes produced through neutron capture can be caused to fission. Generally, the fission products created have shorter half-lives and are not as dangerous. This reactor, dubbed EBR-II, uses liquid sodium as a coolant, which means that the internal reactor temperature is much, much hotter than that of a normal PWR reactor, which uses water as a coolant.
Another advantage of EBR-II is that its fuel is not weapons grade quality. When the transuranic wastes are separated from the other wastes in the spent fuel rods, the resultant mix of isotopes can not be used in a bomb. Thus, the mix can be used as fuel for EBR-II without a chance of it getting stolen by a terrorist group for use in an explosive device.
Breeder reactors "breed" fuel. That is, they are designed to create 239Pu from 238U through neutron capture. This "waste" can then be used as fuel.

Nuclear Weapons

Introduction:

Since 1945, when the first nuclear bomb was exploded by the Manhattan Project team in the US, nuclear weapons have proliferated across the globe. Currently, the US has about 7,000 warheads and the nations of the former Soviet Union have approximately 6,000. There are enough nuclear weapons in the world to destroy all civilization as we know it. They are perhaps the most powerful forces that man has ever wielded. Other countries that possess known nuclear capabilities are the United Kingdom, France, the People's Republic of China, Pakistan, and India. When first developed, nuclear weapons were completely strategic weapons. That is, they were not designed to destroy enemy weaponry; they were designed to destroy entire cities. However, there are now small, tactical nuclear weapons in addition to the others. Besides how powerful a nuclear weapon is, there are other differences between them. They can be either a fusion or a fission device, and they can be dropped from an airplane, fired from an artillery gun, or attached to various types of rockets.

The Fission Bomb:

A fission bomb uses an uncontrolled nuclear fission chain reaction to release an enormous amount of energy in a small amount of time. Previously, you read about different ways that a fission chain reaction is controlled in a nuclear reactor. In a bomb all those safeguards are removed. There are no control rods and when uranium is used as the fuel, it is much more enriched than the 4% 235U in nuclear fuel rods. This means that there is less 238U and thus less neutrons are captured. The fissile material (plutonium or uranium) in the bomb is usually surrounded by conventional explosives (non-nuclear). When the bomb is detonated, the conventional explosives are ignited. These explosives are designed to blow inwards, crushing the fissile material they surround. This compaction of the uranium or plutonium increases the chance that a stray neutron will strike a nucleus, inducing fission and the result runaway chain reaction. Once the nuclear material is compressed to the point of criticality (able to undergo a runaway nuclear reaction), a neutron "gun" fires. This "gun" shoots extra neutrons into the critical mass of fuel. This adds a lot of extra neutrons, which increase the chain reaction. As the chain reaction begins to grow exponentially, the inside of the bomb gets hotter and hotter. At a certain point, the pressure and heat are too intense for the casing, and it is ripped apart. Then the bomb explodes into the surrounding area and atmosphere, releasing fall-out and radioactivity into the environment. Additionally, the shock wave from the blast can level immense areas around the bomb. However, this area depends on the strength of the bomb. When the bomb explodes, the particles that made up the bomb are vaporized and enter the atmosphere as a fall-out cloud. The radioactive fall-out that is released consists of the fission products and the transuranic neutron-capture products, just like those in spent fuel rods. The difference is that the reaction in a bomb occurs at a much, much faster rate than that in a power plant.

Thermonuclear Fusion Bombs:

Fusion bombs have two main stages. These are called the "primary" and the "secondary". The primary reaction is a regular fission chain reaction. The radiation from this reaction is used to heat the interior of the bomb to temperatures where a fusion reaction can be sustained. Also, the neutrons produced from the fission reactions are used in the secondary (fusion) part of the reaction. The secondary is composed of lithium-deteuride (deuteride is basically deuterium, which is 2H). The lithium deteuride, under intense heat, splits apart into lithium (6Li) and deuterium ions. The neutrons produced from the primary (fission) reaction react with the 6Li to produce 4He and 3H. This reaction can be expressed in the following equation: 1 neutron + 6Li 4He + 3H
3H + 2H 4He + 1 neutron
A thermonuclear fusion bomb is generally a lot more powerful than a fission bomb. Interestingly, the harmful fall-out from a fusion bomb is generated mostly from the products of the primary (fission) reaction. These are the fission products and the transuranic products, just like in the fall-out of a standard fission bomb.

Aftermath:

If all the nuclear weapons in the world were used, then all of humanity would most like be destroyed. This is for several reasons. Firstly, most major cities would be destroyed by incoming warheads. However, this would leave some areas untouched. Thes e areas would most likely be reached by radioactive fall-out blown by the wind. These would be the immediate repurcussions. Later, the world would go into what is called "Nuclear Winter". Global temperatures would drop significantly, as well as the amount of sunlight received by the earth. This is very similar to what is believed happened to the dinosaurs. It is believed th at a large asteroid collided with the earth, and stirred up a lot of dust into the atmosphere. This blotted out the sun, and plants died. With very few plants to eat, the dinosaurs (and many other animals) went extinct. Nuclear winter would be a lot like this. The only difference is that there the dust would be raised up by impacting nuclear warheads and their explosions. Additionally, the dust would be radioactive. The combination of radioactivity, lack of food, and lowering temperatures cause a Nuclear Holocaust, with the chances of humans surviving it very low.

Nuclear Disasters and Accidents

Introduction:

One of the scariest things about nuclear power is when something goes wrong and an accident occurs. Radiation is released into the environment and people get hurt. Two of the most famous nuclear accidents occurred at the Three Mile Island reactor 2 in the United States and the Chernobyl reactor 4 in the former Soviet Union. In this text we will discuss these two disasters, along with correcting a few common misconceptions about nuclear accidents.

The Myth of a Reactor Explosion:

It is impossible for any PWR or LWR nuclear reactor to explode like an atomic bomb. This is because in order for an uncontrolled chain reaction to occur that is similar to an atom bomb, the uranium fuel must be extremely enriched, much more than the 4% 235U that is present in regular, commercial nuclear reactor fuel. So, if it can't explode, what does happen in a nuclear reactor? The answer is what is called a meltdown. When a meltdown occurs in a reactor, the reactor "melts". That is, the temperature rises in the core so much that the fuel rods actually turn to liquid, like ice turns into water when heated. If the core continued to heat, the reactor would get so hot that the steel walls of the core would also melt. In a complete reactor meltdown, the extremely hot (about 2700� Celsius) molten uranium fuel rods would melt through the bottom of the reactor and actually sink about 50 feet into the earth beneath the power plant. The molten uranium would react with groundwater, producing large explosions of radioactive steam and debris that would affect nearby towns and population centers. In general a nuclear meltdown would occurr if the reactor loses its coolant. This is what occured in the two disasters that we will discuss. Without coolant, the core's temperature would rise, resulting in the meltdown scenario we explained above.
You may be wondering, "Why can't they just drop the control rods in the reactor if it starts to get out of control?". The answer is that they can. The problem is that, even if the control rods are completely dropped in and the nuclear chain reaction stops, the reactor is still extremely hot and will not cool down unless coolant is put back in. The residual heat and the heat produced from the decay of the fission products are enough to drive the core's temperature up even if the nuclear chain reaction stops.

Three Mile Island:

Outside View of the Three Mile Island Nuclear Power Plant Photo Courtesy Nuclear Regulatory Commission

On an island 10 miles from Harrisburg Pennsylvania resides the Three Mile Island Nuclear Power Station. There are two reactors at the plant, dubbed Unit 1 and Unit 2. One of them is inoperable. Unit 2 experienced a partial reactor meltdown on March 28, 1979. A partial nuclear meltdown is when the uranium fuel rods start to liquefy, but they do not fall through the reactor floor and breach the containment systems. The accident which occurred at Unit 2 is considered to be the worst nuclear disaster in US history. Why did it happen? There are many reasons for the accident, but the two main ones are simple human error and the failure of a rather minor valve in the reactor. In the following paragraphs, we will explain how it was possible for the accident to happen and both its psychological and physical effects on the American people. The accident at TMI (Three Mile Island) began at about four in the morning with the failure of one of the valves that controlled coolant flow into the reactor. Because of this, the amount of cool water entering the reactor decreased, and the core temperature rose. When this happened, automatic computerized systems engaged, and the reactor was automatically SCRAMmed. The nuclear chain reaction then stopped. This only slowed the rate at which the core temperature was increasing, however. The temperature was still rising because of residual heat in the reactor and energy released from the decaying fission products in the fuel rods.
Because the pumps removing water from the core were still active, and a valve that controlled the cool water entering the core failed, water was leaving the core, but not coming in. This reduced the amount of coolant in the core. There wasn't enough coolant in the core, so the Emergency Core Cooling System automatically turned on. This should have provided enough extra coolant to make up for the stuck valve, except that the reactor operator, thinking that enough coolant was already in the core, shut it off too early.
There still wasn't enough coolant, so the core's temperature kept increasing. A valve at the top of the core automatically opened to vent some of the steam in the core. This should have helped matters by removing the hot steam, but the valve didn't close properly. Because it didn't close, steam continued to vent from the reactor, further reducing the coolant level. The reactor operators should have known the valve didn't close, but the indicator in the control room was covered by a maintenance tag attached to a nearby switch. Because the operators didn't know that the valve had failed to close, they assumed that the situation was under control, as the core temperature had stopped rising with the first venting of steam from the core. They also thought that the coolant had been replaced in the core, because they didn't know that the pump outlets were closed. A few minutes later the core temperature began to rise again, and the Emergency Core Cooling System automatically switched on. Once again, an operator de-activated it, thinking the situation was under control. In reality, it was not.
Soon, because of the coolant lost through the open valve at the top of the reactor, the core temperature began to rise again. At this point the fuel rods started to collapse from the intense heat inside the core. The operators knew something was wrong, but didn't understand what it was. This was about 5 minutes after the initial valve failure. It took almost 2 hours for someone to figure out that the valve releasing steam at the top of reactor hadn't closed properly. During those 2 hours, precious coolant continued to be released from the reactor a meltdown was underway. At approximately 6AM, an operator discovered the valve at the top of the core was open and closed it.
During the day hydrogen gas began to accumulate inside the reactor and caused an explosion later in the afternoon. This explosion did not damage the containment systems, however. Two days later, the core was still not under operator control. A group of nuclear experts were asked to help evaluate the situation. They figured out that a lot of hydrogen gas had accumulated at the top of the core. This gas could have exploded, like the explosion on the first day of the accident, or it could have displaced the remaining coolant in the reactor, causing a complete nuclear reactor meltdown. No one really knew what to do about the hydrogen build-up. A hydrogen recombiner was used to remove some of the hydrogen, but it was not very effective. However, hydrogen also dissolves in water, which is what the coolant was composed of. Thus, over time the hydrogen that had collected at the top of the core completely dissolved in the coolant. Two weeks later the reactor was brought to a cold shutdown and the accident was over.
No one was directly injured as a result of the accident. However, some radioactive gas and water were vented to the environment around the reactor. At one point, radioactive water was released into the Susquehanna river, which is a source of drinking water for nearby communities. No one is really sure what effects these radioactive releases might have had on people living near the power plant.

Chernobyl:

About 80 miles (130 km) north of Kiev, in what is now the Ukraine, is located the Chernobyl nuclear power plant. At this plant the worst reactor disaster to ever occur took place on April 26, 1986. It happened largely because normal reactor operations were suspended; an experiment was to take place in the reactor. As a result, normal safety guidelines were disregarded, and the accident occurred. However, as with most accidents of this type, it was a result of many small mistakes adding up to create a catastrophe. In the following paragraphs, we will outline just how the event transpired: Early in the day, before the test, the power output of the reactor was dropped in preparation for the upcoming test. Unexpectedly, the reactor's power output dropped way too much, almost to zero. Because of this drop, some control rods were removed to bring the power back up. (As you recall from the fission power text, the more control rods there are in a reactor, the more free neutrons are absorbed and the less fissioning that goes on. So, more control rods means less energy and power output.) The reactor's power output raised up, and all appeared to be normal.
More preparation for the test began later when two pumps were switched on in the cooling system. They increased water flow out of the reactor, and thus removed heat more quickly. They also caused the water level to lower in a component of the reactor called the steam separator. Because of the low level of water in the steam separator, the operator increased the amount of feed water coming into it, in the hopes that the water level would rise. Also, more control rods were taken out of the reactor to raise internal reactor temperature and pressure, also in the hopes that it would cause the water level in the steam separator to rise. The water level in the steam separator began to rise, so the operator adjusted again the flow of feed water by lowering it. This decreased the amount of heat being removed from the reactor core.
Because many control rods had been removed and the amount of heat being taken from the core by the coolant had been reduced, it began to get very hot. Also, there was relatively low pressure in the core because the amount of incoming water had been decreased. Because of the heat and the low pressure, coolant inside the core began to boil to form steam.
The actual test began with the closing of the turbine feed valves. This should have caused an increase in pressure in the cooling system, which in turn would have caused a decrease in steam in the core. This should have lowered the reactivity in the core. Thus, the normal next step when closing the turbine feed valves was to retract more control rods, increasing reactivity in the core. This is what the operator at Chernobyl did. The only problem was that in this case there was no increase in pressure in the cooling system because of the earlier feed water reduction. This meant that there was already a normal amount of steam in the core, even with the turbine feed valves closed. Thus, by retracting more control rods to make up for a reduction in steam that didn't happen, the operator caused too much steam to be produced in the core.
With the surplus of steam, the reactor's power output increased. Soon, even more steam was being produced. The operator realized there was a problem and SCRAMmed the reactor, completely disabling all fission reactions. However, it was too late. The temperature and pressure inside the reactor had already risen dramatically, and the fuel rods had begun to shatter.
After the fuel rods shattered, two explosions occurred as a result of liquid uranium reacting with steam and from fuel vapor expansion (caused by the intense heat). The reactor containment was broken, and the top of the reactor lifted off. With the containment broken, outside air began to enter the reactor. In this particular Soviet reactor, graphite was used as a moderator instead of water. (water was the coolant) As air entered the core, it reacted with the graphite. Graphite is essentially just carbon, so oxygen from the air chemically combined with the carbon to form CO (carbon monoxide). Carbon monoxide is flammable and soon caught fire. The fire emitted extremely radioactive smoke into the area surrounding the reactor. Additionally, the explosion ejected a portion of the reactor fuel into the surrounding atmosphere and countryside. This fuel contained both fission products and transuranic wastes.
During the days following the accident, hundreds of people worked to quell the reactor fire and the escape of radioactive materials. Liquid nitrogen was pumped into the reactor core to cool it down. Helicopters dumped neutron-absorbing materials into the exposed core to prevent it from going critical. Sand and other fire-fighting materials were also dropped into the core to help stop the graphite fire. All in all, over 5000(metric) tons of material were dropped into the core. After the fires were brought under control, construction of what is called "the sarcophagus" began. The word "sarcophagus" is usually used to describe the elaborate coffins the ancient Egyptians used to entomb their dead. In this case, the sarcophagus is a structure erected from about 300,000 metric tons of concrete that surrounds the reactor. It was designed to contain the radioactive waste inside. It has served its purpose well, but, now, ten years after the accident, several flaws have been found in it. Holes have begun to appear in the roof, allowing rainwater to accumulate inside. This water can corrode the structure, further weakening it. Also, birds and other animals have been seen making homes in the sarcophagus. If they should ingest radioactive material, they could spread it around the countryside. Additionally, with time the sarcophagus has become worn down. It is conceivable that an intense event like an earthquake, tornado, or plane crash directly on the sarcophagus could lead to its collapse. This would be catastrophic, as radioactive dust would once again rain down on the surrounding areas. Scientists and engineers are working on ways to repair or replace the structure.
One of the great tragedies of the accident was that the Soviet government tried to cover it up. Clouds of fallout were traveling towards major population centers such as Minsk, and no one was warned. No one outside the Soviet Union knew about the accident until two days later, when scientists in Sweden detected massive amount of radiation being blown from the east.
The effects of the disaster at Chernobyl were very widespread. The World Health Organization (WHO) found that the radiation release from the Chernobyl accident was 200 times that of the Hiroshima and Nagasaki nuclear bombs combined. The fallout was also far-reaching. For a time, radiation levels in a Scotland were 10,000 times the norm. 30 lives were directly lost during the accident or within a few months after it. Many of these lives were those of the workers trying to put out the graphite fire and were lost from radiation poisoning. The radiation released has also had long-term effects on the cancer incidence rate of the surrounding population. According to the Ukrainian Radiological Institute over 2500 deaths resulted from the Chernobyl incident. The WHO has found a significant increase in cancer in the surrounding area. For example, in 1986 (the year of the accident), 2 cases of childhood thyroid cancer occurred in the Gomel administrative district of the Ukraine (this is the region around the plant). In 1993 there were 42 cases, which is 21 times the rate in 1986. The rate of thyroid cancer is particularly high after the Chernobyl accident because much of the radiation was emitted in the form iodine-131, which collects in the thyroid gland, especially in young children. Other cancer incidence rates didn't seem to be affected. For example, leukemia was no more prevalent after the accident than before.
What caused the accident? This is a very hard question to answer. The obvious one is operator error. The operator was not very familiar with the reactor and hadn't been trained enough. Additionally, when the accident occurred, normal safety rules were not being followed because they were running a test. For example, regulations required that at least 15 control rods always remain in the reactor. When the explosion occurred, less than 10 were present. This happened because many of the rods were removed to raise power output. This was one of the direct causes of the accident. Also, the reactor itself was not designed well and was prone to abrupt and massive power surges.

Nuclear Reaction Applet

Instructions

This java applet simulates a nuclear reaction in which 235U is the fissile isotope. The chain reaction can be controlled by moving the "neutron absorbers" slider up and down. This slider dynamically changes the number of neutron absorbers present as the applet is running, just like an operator at a nuclear power plant lowers and raises the control rods. When a neutron hits the fissile 235U, an average of 2.5 neutrons are released; sometimes 2, sometimes 3. You can also specify the inital amount of 235U; but you must hit the RESET button for this change to take effect.
Start - Starts the reaction
Stop - Pauses the reaction
Reset - Reconfigures reaction
Enable Sound - Actives sound which occurrs everytime a U235 atom fissions (the simulation may not produce the sound 100% of time due to large amount of atoms). This option may slow down the speed of the applet.

NOTE: When the applet is paused (STOP or RESET has been hit), moving the slider up and down does not have its desired effect. You can move it while the reaction is running. In addition, you must hit RESET for the change in initial 235U amount to take effect.