CLASS 34. FUSION AND APPLICATIONS OF FISSION AND FUSION

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1 CLASS 34. FUSION AND APPLICATIONS OF FISSION AND FUSION INTRODUCTION One application of fission the nuclear bomb was introduced in the previous chapter. In this chapter, we examine some more peaceful applications of fission, and examine the role of fusion GOALS Explain what a chain reaction is. Explain how a nuclear reactor works and how the chain reaction is controlled Explain the difference between fission and fusion Explain the advantages of generating energy by fusion and why we currently are not taking advantage of fusion as an energy source. Explain how a fusion weapon works and how it differs from a fission weapon. Explain what cold fusion is and why achieving it would be significant FISSION REACTIONS Chain Reactions. We briefly discussed chain reactions in the last section. Figure 34.1 shows the fission reaction for U-235. The end product is two nuclear fragments and a number of neutrons. Each uranium atom that undergoes fission produces neutrons. Neutrons from the first collision (shown on the left of Figure 34.2), generate more fission reactions by hitting other U-235 atoms. neutron For the sake of argument, say that each collision produces three neutrons. Each of those three neutrons goes on to make three collisions, producing 9 neutrons, etc. etc. This is called a chain reaction. The smallest amount of uranium necessary to sustain a chain reaction is called the critical mass. Uranium mined from the ground primarily is U-238. Enriched uranium is uranium that has been purified so that 2%-3% of the uranium atoms are the U-235 isotope. U-235 is the isotope responsible for the majority of the fission reactions. Weapons-grade uranium is 90% or more U-235. When U- 235 it shielded from neutrons, it is a fairly harmless alpha emitter. This means that it is shielded easily and not easily detectable. The decay of one U-235 atom produces on the order of 200 MeV of energy. The fission reaction takes a very short amount of time one the order of picoseconds (10-12 seconds), so the power generated is enormous. A pound of enriched uranium is enough to power a nuclear submarine or aircraft carrier. A pound of uranium which is smaller than a baseball is equal to about a million gallons of Figure 34.2: A chain reaction U-235 U-236 fragments neutrons Figure 34.1: Schematic diagram of how U-235 undergoes fission via a neutron.

2 gasoline or three thousand tons of coal NUCLEAR (FISSION) REACTORS Nuclear (Fission) Reactors. An uncontrolled chain reaction is the key to fission weapon. You want a critical mass of fissile material, so that the reaction will continue and continue, building up huge amounts of energy. If you want to harness this energy to use for power, you have control the rate at which the energy is released. This is the principle behind the nuclear fission reactor. Reactor charge face 3m thick concrete biological shield Hot gas out Steam out Reactor core Cold water in Heat exchanger Uranium fuel rods Boron control rods Steel pressure vessel Cold gas in Graphite moderator Figure 34.3: Schematic of a nuclear reactor. Figure 34.3 shows a schematic of a nuclear reactor. There are many different types of nuclear reactors, but most work on the same basic principles. The core of the reactor contains uranium fuel in the shape of rods. These rods are stuck in a large block of graphite. Graphite slows down neutrons, which is desirable because slow neutrons react better with the uranium than do fast neutrons. The graphite is called a moderator because it moderates the speed of the neutrons. A lot of heat is produced during the reactions, so carbon dioxide gas is blown through the reactor core to absorb heat and move it out of the reactor. The gas passes over tubes containing water, transferring the heat from the gas to the water. The water changes into steam, which can then drive turbines or generators. How is the fission reaction prevented from going out of control? The number of neutrons that are available to react with the U-235 is limited. Boron, hafnium and cadmium are good neutron absorbers, so control rods made of one of these materials are used to limit how much energy is produced. The control rods are lowered into the reactor to decrease the output power. They absorb some of the neutrons (without producing fission) and the smaller number of neutrons means that there are fewer fission events. When the control rods are raised, more neutrons are available to initiate fission and the power output increases. If a reactor is not properly moderated, it is possible for the heat to build up and melt parts of the reactor, which is referred to as a melt-down Potential Problems with Nuclear Reactors. The danger of having a reactor go critical is always present, so safety is an utmost concern. The entire reactor is surrounded by a steel pressure vessel and then by a thick layer of concrete, and often by another steel pressure vessel and even

3 more concrete. Nuclear reactors are designed to withstand being crashed into by a jet plane. The extra layer of containment is critical and was missing in Chernobyl, which is why the accident at Chernobyl in 1986 was of so much concern. The shielding was not sufficient and over 50 tons of radioactive materials was released, which is thirty or forty times the total amount of radioactive material generated by the bombs that were dropped on Hiroshima and Nagasaki. There was widespread contamination of food and water in the area around the reactor. Although reactor design was a problem in the Chernobyl case (and in a number of other existing reactors in the former Soviet Union), the primary causes of nuclear reactor accidents are human error, lack of proper procedures and lack of training. The Three-Mile Island accident (1979) started due to failure of a pump that decreased the flow of water to the reactor. What could have been an easily fixed problem became a catastrophe because the workers at the plant were unprepared to deal with the problem. 21 The other problem with nuclear reactors is radioactive waste. Nuclear waste comes in different strengths Low-level waste includes objects with a low level of radiation, such as contaminated protective clothing. High-level radioactive waste includes materials such as spent fuel rods and moderators. When oil prices were low and regulatory laws were imposed, the cost of nuclear power made it must less attractive compared to petroleum; however, high fuel prices and concerns about the long-term availability of fossil fuels make nuclear power a more attractive alternative; however, no new nuclear reactors have been built in the U.S. since the 1980 s. Issues surrounding nuclear power often are not considered objectively by the public because most people do not understand the process of creating power by nuclear fission and thus cannot make informed arguments. If you compare the number of people killed due to nuclear power to the number of people killed mining coal, you ll find that coal mining is a much more dangerous way to make a living. 22 Nuclear energy proponents argue that, in 11,000 reactor-years worth of operation, there have been only two major accidents and the total loss of life and damage was minimal. They also argue that the risks of nuclear power are comparable to those we routinely accept when we engage in activities such as driving a car or getting on an airplane. The public is ill-equipped to make well-informed decisions about nuclear power because they often don t understand the issues and thus make decisions on an emotional basis. European countries depend much more on nuclear power than the U.S. In France, 78% of the electrical energy is generated by nuclear reactors. In Belgium, the fraction of electrical power from nuclear reactors is 60% and in Sweeden, it is 46%. The United States gets 21% of its electrical power from nuclear reactors FUSION How Fusion Works. Fusion is the opposite of fission: while fission is the splitting of a heavy nucleus into lighter nuclei and the release of energy, fusion occurs when two light nuclei are forced together to form one heavier nucleus. Since the single heavier nucleus has a lower energy that the two separate nuclei, energy is released. The most common fusion reaction is hydrogen fusing to form helium. This is the nuclear reaction that powers the Sun and other stars. Some fusion reactions can be achieved in the laboratory by hitting materials with very-high-energy deuterons. Some of the possible fusion reactions include: 21 has a case history of the three-mile island accident. has the United Nations Scientific Committee on the Effects of Atomic Radiation report on Chernobyl has a list of energy-related accidents, including hydroelectric, coal, nuclear, etc.

4 H + H H + H + 4 MeV H + H H + n+ 3.2 MeV H + H He+ n MeV H + He He+ H MeV where these are written to include the amount of energy that is released by the reaction. The fusion of tritium 3 H and deuterium 2 1 1H (the third equation in A) is one of the most promising reactions for generating power. Deuterium occurs at a rate of about 1 part in 7000 hydrogen atoms g of deuterium reacting with the appropriate amount of tritium would produce energy equivalent to that from about 1140 liters of gasoline. Extracting deuterium from water is relatively cheap. Fusion is an attractive source of power: there are no harmful by-products and there is enough deuterium and tritium in the universe to provide all the power we need. The problem with fusion is that it requires high temperature, high density and time. The hotter atoms are, the faster they move around and the more energy they have when they collide. Two atoms must be pushed together with great force so they can overcome their electrostatic repulsion. 23 There are two ways to achieve this large force: either raise the atoms to very high temperature (as on the Sun), or put them under great pressure (or both). High density is required because the closer atoms are to each other, the more likely it is for collisions to occur. Finally, there must be enough time for a sufficient number of fusion reactions to happen. Magnetic Confinement. High temperature presents a problem. Attaining a temperature of 2 million C precludes the use of any known material to hold the materials. Two alternate forms of confinement are used. Magnetic confinement utilizes plasma: when a material is heated to very high temperature, the electrons are separated from the nuclei and the resulting material is called a plasma. Plasma is made when the electrons and nuclei of the atoms involved are separated, forming a mixture of charged particles. Since the particles are charged, a magnetic field can be used to control the plasma. A special configuration of magnetic field is needed to hold the plasma in place, and the device developed for this purpose is called a torus. The torus applies a magnetic field in a particular configuration that confines the plasma while allowing it to remain at very high temperatures. The second type of confinement is called inertial confinement, and the idea is to simultaneously heat and compress small frozen pellets of deuterium and tritium with energetic laser beams or particle beams. The primary challenge at this point is making very powerful lasers and particle beams. The most common way to confine the hot plasma used to make fusion energy is to use strong magnetic fields. The plasma can't be confined by the material walls, because the plasma is millions of degrees Celsius. (Actually, the problem is the reverse: the vessel walls are so cold, that the cool the plasma and prevent fusion.) Physicists have been working since the 1950s on making better "magnetic bottles," a problem which has been compared to holding jello (the plasma) with rubber bands (the magnetic field). The magnetic field acts like a net to the charged particles of the plasma, preventing the plasma from leaking out. For esoteric reasons, one can't make the magnetic field into a ball which would be the best way to trap the plasma but you can make the magnetic field into a A 23. (See

5 donut shape called a torus. The magnetic "net" traps the plasma better if the magnetic field spirals around the torus, rather than being straight. The helical magnetic field of a fusion reactor traps the plasma and prevents it from contacting the walls of the container. Figure 34.4 is from which is a report on the inertial confinement fusion program at Lawrence Livermore National Laboratory. It explains the idea of inertial confinement as a means of extracting energy from fusion. The National Ignition Facility (NIF) is making the world s most powerful laser to use as the driver for the initial step in the fusion process. The plans at present call for 192 sharply focused laser beams that simultaneously fire and raise the temperature of the materials in the torus. This facility will be used for fusion research, but also for research into the properties of materials and phenomena at very high temperatures, weapons research (i.e. use data to predict and model the behavior of our aging nuclear stockpile without needing full-scale nuclear testing), and astrophysics. The target for inertial fusion compresses a few milligrams of deuterium-tritium fuel to a density approximately thirty times the density of lead. A smaller fraction of the fuel is heated to a temperature over 100-million C, which starts a propagating fusion burn (meaning that the heat released from the first fusion reactions is sufficient to start other fusion reactions). Each milligram that burns inside the target releases 340 MJ, which is equivalent to burning over 10 kilograms of coal. So, if we ve achieved fusion and it s such a great idea, why aren t we using it? To answer this, you Figure 34.4: Inertial fusion principles. need only consider how much electricity they need at NIF to generate those 192 laser beams from

6 the world s most powerful laser. The major challenge is to ignite a small mass of fusion fuel with a minimal amount of energy (which is supplied by a laser or accelerator) so that you get out more energy than you put in. At this point, achieving fusion in a laboratory requires us to supply more energy than we get out, and the reactions last for fractions of a second. Much research remains to be done in this field before we can really consider at as an alternative energy source Fusion in Stars. Stars form from loose clouds of hydrogen gas. If atoms get close enough to each other, their mutual gravitational attraction will form ball that will attract other atoms by gravitational force. The impetus for atoms getting close to each other is often some external force, such as a shock wave. This gets the particles close enough to each other so that the gravitational force can take over. The globule is a loosely correlated, rotating collection of hydrogen atoms. As more hydrogen atoms join the main clump, the gravitational attraction increases, pulling the atoms closer to each other and pulling in more atoms. The condensed cloud of gas atoms is called a protostar. The core of a star provides just the right amount of pressure, temperature, and density for hydrogen to undergo fusion. This agglomeration only happens when the density of the hydrogen atoms gets very large for example, 1 x atoms need to be within several trillion miles of each other to have this happen. Gravitational attraction pulls the atoms into a sphere with a radius of about 1.5 million miles. As the atoms are pulled toward the center, they speed up (i.e. their kinetic energy increases). The interior of the protostar starts heating. This continues for about 10 million years. Over time, the density and the temperature become high enough that nuclear fusion reactions can occur. This leads to heating of the central core and the emission of radiation. In stars, the most common fusion reaction chain starts with hydrogen: H + H H + e+ ν This is a very unlikely reaction: the protons repel each other electrostatically, and have to get within m of each other for fusion to occur. The only way this reaction happens is when the gas is very hot, so that the protons are traveling at high enough speed that they overcome the electrostatic repulsion. Even in very hot circumstances, the reaction doesn t happen frequently. This is good because if it happened a lot, the Sun would have burned itself out by now. The second step of the reaction is for two deuterium atoms to form a helium atom. H + H He + photon The two steps have to happen twice, leaving us with two helium atoms. These helium atoms then fuse according to: H + H He+ H + H We can summarize these steps in a single equation. 24 Remember that photons are where the released energy goes. 4 H He+ 2 e+ 26MeV The Sun converts about 650 million tons of hydrogen to 645 million tons of helium every second via fusion. At the present point, the sun is about 35% hydrogen and 65% helium. We estimate that the Sun has enough hydrogen to continue fusion for about another 5 billion years. 24 Even in this, I left out a step for simplicity. There is annihilation between an electron and an anti-electron (positron) that produces two more photons.

7 Cold Fusion. In 1989, Martin Fleischman of Southampton University and B Stanley Pons of the University of Utah passed an electric current through a solution of lithium in heavy water (water made with deuterium instead of the usual hydrogen) using platinum and palladium electrodes. More heat was produced by the experiment than could have been produced by the electrical current input to the experiment the only possible mechanism that could have produced this heat was fusion but this would be fusion that occurred at room temperature and atmospheric pressure, not the amazing temperatures and pressure found in stars. Instead of submitting their results for publication in a peer-reviewed journal, they held a press conference on March 23, The scientific community was amazed at the results, and many people dropped what they were doing to pursue the new line of research. The problem is that virtually no one could reproduce the results of Pons and Fleischman. Theoretically, the lack of reproducibility should have killed the matter, but there still are people researching cold fusion. Pons and Fleischman still claim they have discovered cold fusion and the failure of others to reproduce their results does not diminish what they have achieved. There sometimes are ugly debates between skeptics and believers about whether such research should be allowed to be published, much less funded. Why are people still interested in funding research in cold fusion when the vast majority of scientists believe that Pons and Fleischman s results were wrong? A large part of the reason is because Pons and Fleischman presented their claims directly to the media: if they had tried to publish the results in a peer-reviewed journal, reviewers would have insisted on more proof. There is a sizeable contingent of people who believe that Pons and Fleischman did achieve cold fusion because there was a lot of media coverage when the discovery was announced, but the media wasn t as interested in the evidence against cold fusion. It took months for scientists to repeat experiments. Once the initial excitement died away, the media wasn t as interested in covering the story. One might be tempted to view this as a humorous episode, but in fact it is not. The research spawned by the initial reports of Pons and Fleischman has cost millions of dollars in private and government-funded research. Graduate students who dropped promising projects to pursue this (at the direction of their advisors) lost years in their pursuit of their degree. There is also the possibility that having been bitten once by this embarrassment, funding agencies and politicians are likely to be extremely wary of future cold fusion claims which might be valid scientists might make. The credibility of scientists among political decision makers and the community at large was severely damaged. A panel estimated that several tens of millions of dollars have been spent in the United States on cold fusion experiments. A couple of private funding agencies continue to provide money for cold fusion, some because they believe that Pons and Fleischman did achieve cold fusion and are being kept quiet due to a conspiracy between the government and the oil companies FUSION WEAPONS The War is Over: The Search for Nuclear Bombs is Not. Although the fission weapons dropped on Japan did end WWII, the resulting political fallout was comparable to the radioactive fallout. The Cold War that followed WWII resulted in a race between the United States and the Soviet Union to develop even more powerful nuclear weapons. Fusion bombs (also called hydrogen bombs or H-bombs ) use an out-of-control fusion reaction the same way that fission bombs do. Fusion bombs have higher kiloton yields and greater efficiency than fission devices. The idea of using a fission device to start nuclear fusion dates back to At the first major conference on the atomic bomb, Edward Teller (whom some have suggested was the model for Dr. Strangelove ), focused discussion on Fermi s idea of a super bomb. There was great opposition to pursuing the idea of the H-bomb (Hydrogen bomb) by most of the leaders of the Manhattan Project. Many had campaigned for the U.S. to demonstrate the power of the bomb and give Japan a chance to surrender before actually dropping the bomb. They had been ignored and many lost faith

8 in the government s ability to ethically handle such power. Opperheimer, the leader of the Manhattan Project, and Hans Bethe argued that the U.S. pursuing such a bomb would only spur the Soviet Union to pursue more weapons. Teller, Lawrence and Luis Alvarez argued that the Soviet Union would develop weapons anyway, and that the U.S. would be behind if they didn t pursue the research. President Truman, in response to the Soviet atomic bomb testing in 1949, approved a crash program to develop a fusion weapon. One of the arguments against the program was that the details of how the bomb would work were uncertain. Mathematician Stanislaw Ulam came up with the idea that the radiation of the fission bomb could first compress the fusion material before igniting it. Teller used this idea to test a smallscale device and then prepared for a true multi-stage bomb test. (This test was called the Teller- Ulam hydrogen bomb test). Some scientists who initially had been against the development (including Oppenheimer and Bethe), changed their opinions and participated in the development, since they felt it was going to go ahead with or without them. The first fusion bomb test was on November 1, 1953 on Eniwetok Atoll in the Marshall Island. The yield was 10.4 megatons (i.e million tons of TNT), which is more than 450 times the power of the bomb dropped on Nagasaki. Liquid deuterium was used as the fusion fuel. The device obliterated the island, leaving a crater more than a mile wide where the island once had been. Truman tried to keep the test secret, since there was an election coming up, but he ended up announcing that the U.S. had developed a successful H-bomb on January 7 th, The Soviet Union exploded its first thermonuclear device in August, 1953 and it was an advance because it was a truly deliverable (i.e. it could be sent out on a rocket) weapon. The U.S. matched them with a deliverable device in The test generated almost two times more power than anticipated and ended up irradiating a Japanese fishing boat (for which we made reparations only a few years ago.) The U.K., France and China all have detonated H-bombs in tests. India claims to have H-bomb technology, but there are some debates over whether they have a true H-bomb or not The Physics behind the Fusion Bomb. We discussed earlier two possible fusion reactions involving 2 H (deuterium) or 3 H (tritium). Fusing atoms requires very high temperatures and pressures. The boosting method places tritium in the center of a fission weapon. The high temperatures and pressures from a fission reaction are used to initiate the fusion reaction. Since tritium ( 3 1H ) is a gas at and above room temperature, it can be loaded in a vessel and the amount of gas used in the reaction could be varied. This technique is widely used and sometimes is referred to dial-a-yield bombs. The disadvantage of this technique is that tritium is not plentiful, it has a short lifetime and it is a gas, which makes it harder to store than a solid.

9 Multistage thermonuclear weapons as shown in Figure are the primary backbone of strategic nuclear weapons today. A fission device is used as a trigger: when the fission device is detonated x- rays are produced. The x-rays are channeled into a cavity that contains the fusion device. Radiation pressure from the x-rays dissolves a container with a thermonuclear fuel such as Li 2 H, (Li 2 H = lithium deuterate). Lithium deuterate is used because combining deuterium with lithium forms a solid, which is more easily stored and handled). This secondary container has a rod of fissile material running through it called a spark plug. Separating the cylinder from the implosion bomb is a shield of uranium-238 and plastic foam that fills the remaining spaces in the bomb casing. When the spark plug begins to fission neutrons change Li 2 H into 3 H, which allows the fusion reaction with 2 H referred to earlier. The heat of the initial reaction sustains the rest of the reaction. The yield can be increased further by adding an exterior blanket of U-238, which doesn t undergo fission when hit with slow neutrons, but does undergo fission when the high energy neutrons from the fusion reaction hits it. This is the technique used in the largest bomb (from the former Soviet Union), which was roughly 60 mt (megaton) The primary difficulties in constructing weapons like this are acquiring enough radioactive material for the fission device, and the overall complexity inherent in the design. Much of the relevant information is classified. Fission device sparkplug fusion device U-238 casing Figure 34.5: Your typical, gardenvariety thermonuclear weapon SUMMARIZE Definitions: Define the following in your own words. Write the symbol used to represent the quantity where appropriate. 1. Chain reaction 2. Critical mass 3. Protostar 4. Plasma 5. Torus Equations: No equations Concepts: Answer the following briefly in your own words. 1. Compare and contrast nuclear fusion and nuclear fission.

10 2. What are the primary differences between the fission reactions used in a weapon and those used in a nuclear reactor? 3. What is the difference between enriched uranium and weapons-grade uranium? 4. What is the source of energy for the Sun? 5. The term critical mass is used outside of science as well as in scientific contexts. What does it mean scientifically? How is it used colloquially? 6. Explain in your own words the basic principles behind a nuclear reactor 7. Why would fusion solve many of the world s energy problems? Why aren t we using it now? 8. What does it mean to say a reactor has a melt-down? 9. How can materials for fusion be contained such they are at such a high temperature that they would melt any known material that might be used? 10. What are the advantages and disadvantages of nuclear power? 11. What are the three conditions that must be satisfied for fusion to occur? Your Understanding 1. What are the three most important points in this chapter? 2. Write three questions you have about the material in this chapter Questions to Think About 1. A company is planning on building a nuclear reactor in your city. What questions do you want to ask them as you try to determine whether to support their effort? 2. If the Sun converts 650 million tons of hydrogen to 645 million tons of helium each second, what happens to the other 5 million tons of hydrogen? 3. Are there nuclear reactors in Nebraska? How many are there, and how old are they?

11 HW Covers Class 34 and is due April 6 th, 2007 PHYS 261 Spring 2007 HW If the Sun converts 650 million tons of hydrogen to 645 million tons of helium each second, what happens to the other 5 million tons of hydrogen? 2. Make a table that compares and contrasts nuclear fusion and nuclear fission. 3. Why would fusion solve many of the world s energy problems? Why aren t we using it now?