Chapter 18: Consequences of Nuclear Energy Use

Chapter 18: Consequences of Nuclear Energy Use Goals of Period 18 Section 18.1: To discuss the generation of electricity by nuclear reactors Section 18.2: To describe the design of nuclear fission reactors Section 18.3: To compare types of nuclear reactors Section 18.4: To discuss safety and economic issues of nuclear power Section 18.5: To describe nuclear fusion This period addresses the practical use of nuclear energy. The physics and some of the technology of fission reactors are discussed at a level needed to understand the practical problems involved. These problems include reactor safety, nuclear waste, and the economic need for fuel reprocessing. The physics of nuclear fusion is discussed briefly. 18.1 Generation of Electric Power by Nuclear Reactors In the U.S., about 9% of generated electricity comes from nuclear power stations. Many European countries rely more heavily on nuclear power. France, which has few fossil fuel resources, now generates 76% of its electricity from fission reactors. In this section we describe how a fission reactor works, and how the underlying physics of nuclear fission gives rise to some serious practical problems. In a nuclear power station, electric power is generated in the same way as in coal or oil power plants: water is heated to produce steam, and expanding steam turns a fan-like turbine. The turbine then turns an electric generator to produce electric power. The essential difference is that in a nuclear power plant, the water is heated by nuclear rather than by chemical reactions. Fission and Chain Reactions The heavy element uranium, which has 92 protons (Z=92), has no stable 238 isotopes, but has two isotopes which are nearly stable: U-238 ( 92 U ) with a half-life of 235 4.5 billion years, and U-235 ( 92 U ) with a half-life of 0.7 billion years. These uranium 4 isotopes decay mainly by alpha emission of a helium nuclei ( 2 He ). However, uranium- 235 sometimes decays by splitting into two smaller nuclei of roughly equal size, called fission fragments, plus two or three free neutrons. This decay is called spontaneous fission. The neutrons that are emitted when a uranium-235 nucleus fissions are capable of causing other uranium-235 nuclei to fission. This process is called induced fission. Thus an exothermic chain reaction is possible, provided that enough of the released neutrons cause new fissions to keep the reaction going. Uranium-235 is the only nucleus known to decay by spontaneous fission. 163

The Fission Process in Nuclear Reactors Next, we describe several technical details that are very important in the design, safety and economics of fission reactors. 1. Why is the speed of neutrons important? Slow neutrons are most easily captured. The neutrons released by uranium fission are moving at high speed. However, slow neutrons have a much greater chance of being captured by a uranium-235 nucleus, and thus of causing another fission, than do fast neutrons. (The nuclear force has more time to act on a slowly-moving neutron.) In reactors, fission neutrons are slowed down, or moderated, by allowing them to make billiard-ball collisions with light nuclei such as hydrogen or carbon. 2. What isotopes of uranium are used? Slow neutrons don't make U-238 fission. When a U-235 nucleus captures a slow neutron, the nucleus fissions. However, when a U-238 nucleus captures a slow neutron, it does not fission but instead forms U-239, which then beta decays to plutonium-239. Thus U-238 cannot produce a chain reaction. Unfortunately, only 0.7% of naturally-occurring uranium is U-235, and 99.3% is U-238. Therefore only a tiny fraction of natural uranium is directly useful. Furthermore, since U-238 absorbs slow neutrons and therefore removes them from circulation, it actually inhibits the chain reaction in U-235 if the natural mix of U-235 and U-238 is used. For this reason it is necessary to process naturallyoccurring uranium to enrich its content of U-235 from 0.7% to about 3% for use in most kinds of nuclear reactors. This enrichment process is both difficult and expensive and adds to the cost of nuclear energy. A much larger degree of U-235 enrichment is necessary for nuclear weapons. The difficulty of the enrichment process is one of the key factors in limiting the spread of nuclear weapons. 3. What controls the rate of nuclear fission? Neutrons can be absorbed before they cause fissions. It is desirable to be able to absorb neutrons in a controlled way in order to govern the rate of heat generation by the chain reaction in a reactor. For example, one needs a way to turn off the chain reaction. Some materials, notably cadmium and boron, are very efficient neutron absorbers. Such neutron sponges are incorporated into reactors in adjustable control rods. Other materials present in a reactor can also absorb slow neutrons; these include U- 238 in the fuel, and ordinary hydrogen ( 1 1 H), which is often present in cooling water. 4. Why do fissioning nuclei emit neutrons? Uranium-235 has 143 neutrons and only 92 protons, while light stable nuclei such as carbon-12 have equal numbers of protons and neutrons. This high neutron/proton ratio in uranium and other very heavy elements is necessary for stability because of the large electrical repulsion among all the protons. When an uranium nucleus fissions, the fission fragments have a Z (number of protons) about half as large, typically around 46. For this smaller Z, stability requires a more equal neutron/proton ratio. The fission 164

fragments are thus much too neutron-rich to be stable. They adjust this ratio by converting neutrons to protons in a series of beta decays, initially of very short half-lives of a fraction of a second, and then with progressively longer half-lives of up to hundreds or even thousands of years. The radioactivity of fission fragments has two important consequences. a) Because of the initial decays of very short half-life, the material in the reactor continues to produce large amounts of heat for minutes or hours even after fission has stopped. This has important implications for reactor safety, as we shall see. b) Daughters of the fission fragments remain radioactive for hundreds or thousands of years. Where do we keep them? This is the very difficult problem of nuclear waste disposal. 18.2 Design of Fission Reactors Conventional nuclear fission reactors contain five essential components: rods, moderator, control rods, coolant, and a containment system. fuel Fuel rods The uranium fuel must be packaged in a form which can withstand high temperatures, and which allows easy removal of the very radioactive spent fuel after it has been used. A common form is hollow metal rods containing ceramic pellets of uranium oxide. Moderator Materials commonly used as the moderator to slow down the neutrons are water and graphite (carbon). Fuel rods are usually inserted into the moderator material in the reactor core. Control rods Control rods are made of neutron-absorbing material such as boron or cadmium. They are moved in and out of the reactor core to decrease or increase the rate at which the fission chain reaction proceeds. Coolant The coolant carries heat away from the fuel to generate steam to turn the turbines, and also prevents the fuel rods from getting too hot. In some designs, water acts as both moderator and coolant. A crucial safety issue in reactor design is how the reactor behaves when the flow of coolant is interrupted or lost. Because the heat generated by beta decays of fission fragments continues for some time after fission has been turned off, loss of coolant can allow the fuel rods to get hot enough to melt. This hot, extremely radioactive material could then melt through the floor of the reactor into the earth below, causing extensive contamination of ground water. There is also a danger of chemical explosion and fire due to the uncontrolled heat. 165

Containment system The purpose of the containment system is to prevent leakage of radioactive substances into the air both during normal operation and in the event of an accident. Most reactor designs employ a double containment system: a heavy steel pressure vessel surrounds the reactor core, and a second layer of thick steel-reinforced concrete provides additional protection in case of a chemical explosion. 18.3 Types of Nuclear Reactors and Their Safety All major sources of energy exact some price in human lives. Construction workers are killed building dams for hydroelectric power. Coal miners die of black lung disease, and more people are killed from the effects of air pollution from fossil-fuel power plants. With nuclear energy, the nature of the risk is different: nuclear power plants in normal operation are quite safe, and release negligible amounts of radiation. However, there is always a very small chance of a very big accident, which could release large amounts of radioactive material into the environment. This risk can be greatly reduced by proper design and safety measures. However, such measures significantly increase the cost and construction time of nuclear power plants. In this section we look at some safety aspects of reactor design. Graphite moderated, water cooled reactors (Chernobyl type) The 1986 accident at the Chernobyl nuclear power station in the Ukraine resulted in the short-term deaths of 31 people. It has been estimated that 5,000 to 20,000 additional cancer deaths will eventually occur in Europe as a result of the radiation released by this accident. The immediate cause of the Chernobyl accident was a tragic series of human errors, but its root cause was poor reactor design. The fatal flaws in this Chernobyl design were the nearly complete lack of a containment system, and the combination of graphite moderator and water coolant, which resulted in increased power output when the coolant was lost. This is like having a car that automatically speeds up when something goes wrong! The reason for this unfortunate behavior was this: The moderator was made of blocks of graphite, into which the fuel and control rods were inserted. Cooling water flowed past the fuel rods in grooves in the graphite. This water played little role in slowing neutrons, but did absorb some of them. When the reactor core overheated due to human error, the water suddenly boiled to steam. Since steam is less dense than water, fewer neutrons were absorbed, thus increasing the fission rate. The result was a runaway chain reaction which suddenly generated an enormous amount of heat. The resulting high temperature caused the steam to dissociate into hydrogen and oxygen, which recombined explosively and blew the lid and floor off the reactor core. The graphite moderator then caught fire and burned for days, releasing large amounts of radioactive material in the smoke. The cloud of radioactivity then drifted over much of Europe, where it was deposited in an uneven fashion by rain. No new 166

reactors are being built with this design, and no active nuclear power plants outside the former Soviet Bloc are built this way. Water-moderated, water-cooled reactors Most nuclear power stations in North America, western Europe and Japan use reactors in which water serves as both the moderator and the coolant. The strong feature of this design is that if the water flow is interrupted, the chain reaction shuts down because neutrons are no longer moderated. A runaway chain reaction such as occurred at Chernobyl is therefore excluded. However, meltdown is still possible due to the remaining heat generated by the decaying fission fragments. Safety design for such reactors emphasizes several independent cooling systems which can be turned on if one or more fails, and massive multilayer containment systems. In 1979, an accident occurred at a water-moderated reactor at the Three Mile Island power station near Harrisburg, Pennsylvania. This accident was caused by a coincidence of human error and the failure of the both primary and backup cooling systems. The reactor core was badly damaged by the resulting heat, but the containment system remained intact, and no major release of radioactive material occurred. Part of the core melted, but the molten material did not melt through to the earth. It is a matter of debate whether the damage was limited entirely by successful design, or involved some measure of luck in restoring cooling capacity in the nick of time. On March 11, 2011, an earthquake off the coast of Japan produced a tsunami wave that inundated the Fukushima nuclear power plant. Following the earthquake, the three working reactors automatically shut down. Emergency generators were used to pump coolant through the reactors. However, the tsunami water breached a floodwall and filled the basements of the reactor buildings. The emergency generators and the electronic controls for the reactor cooling system were located in these basements. These systems failed due to water damage. Battery backup systems were able to pump some coolant to the reactors for a few days, but this was not enough to prevent the reactors from overheating and produce a meltdown of three reactor cores. The resulting high temperatures caused a release of hydrogen gas as water was heated into steam and the steam dissociated into hydrogen and oxygen gas. Chemical reactions between the hydrogen gas and air caused a series of explosions that destroyed the reactor buildings and released radiation into the atmosphere. In an unsuccessful attempt to prevent the complete meltdown of the reactor cores, the reactors were flooded with sea water. Later this radioactive sea water was returned to the ocean. Radioactive isotopes from the explosion have spread around the globe. While the nuclear accident at the Fukushima plant was a meltdown, rather than an uncontrolled chain reaction, the radiation released may equal the radiation released from the Chernobyl accident of 1986. Graphite moderated, gas-cooled reactors An apparently safer reactor design has been tested in Britain and Europe. This design uses a graphite moderator and a gas coolant. The idea is to make the graphite moderator massive enough to absorb all the heat from decaying fission fragments 167

without getting dangerously hot. These pebble bed reactors have successfully survived tests in which all cooling was turned off for hours. However, the early versions of these pebble bed reactors were plagued with problems and high costs. Modified versions of this reactor design have been licensed to China and South Africa. Breeder Reactors Although uranium ore is relatively plentiful, only 0.7% of natural uranium is the fissionable isotope U-235. The reactors now in operation will exhaust the world's known supply of U-235 in about 100 years. This supply will run out sooner if more U-235 reactors are built. However, there is an alternative: the plutonium which is created from U-238 by neutron absorption in reactors is also fissionable. Plutonium is created in small amounts in ordinary reactors, but can be created in large quantities in specially-designed reactors called breeder reactors. Breeders operate on a fuel which is about 15% plutonium and 85% U-238. They not only produce electric power, but also convert much of the U-238 to more plutonium to be used as fuel. In this way more than half of the plentiful U-238 isotope in uranium ore can eventually be utilized. Other fuel cycles than U-238 to plutonium also exist. For example, breeders of a different design convert naturally occurring thorium to fissionable U-233. This cycle is sometimes referred to as burning rocks. The design of a breeder reactor is quite different from that of a reactor designed for U-235. Fast neutrons rather than slow ones are needed, so there is no moderator. The operating temperature is much higher, and the coolant is often liquid sodium rather than water. A different set of safety problems must be faced. In addition, the plutonium must be extracted chemically from very radioactive spent fuel, and transported from one site to another. Plutonium is chemically toxic and can be made into crude nuclear weapons by renegade nations or by terrorist groups. The use of plutonium thus presents safety and security problems which must be solved. Breeder reactor development has not been actively pursued in the U.S. since about 1980. Other countries have moved ahead, notably France. Because of the small supply of U-235, the lack of a breeder reactor program implies a limited future role for nuclear energy in the U.S. 18.4 Other Reactor Safety and Economic Issues The spent fuel rods from a nuclear reactor are extremely radioactive due to the fission fragments, which comprise about 5% of the material in the rods. Most of the rest of the spent fuel rods, about 94%, is unmodified U-238, while most of the remaining 1% is plutonium formed from U-238 and unused U-235. These spent fuel rods remain radioactive for thousands of years. Since the early 1980's, it has been official U.S. policy that these very radioactive high-level wastes should be stored in a single safe location deep underground, later designated provisionally as Yucca Mountain in Nevada. Crash proof containers for 168

transportation of these wastes have been designed and successfully tested. However, citizen protests ( not in my backyard ) and some evidence for past upwelling of groundwater at the Yucca Mountain site have delayed implementation indefinitely. Meanwhile, nearly all the spent fuel rods ever produced from U.S. reactors are still stored temporarily at the reactor sites. These temporary sites are far from ideal: many are near populated areas and many are located near rivers. A related problem is what to do with old nuclear plants. Radiation damage eventually weakens the structural materials of nuclear reactors, making them unsafe. The estimated safe lifetime of a reactor is about 30 years. Some U.S. reactors have now been in operation for at least this long. Do you seal up retired nuclear plants and leave them where they are? Or do you tear them down and put the radioactive portions somewhere else? 18.5 Nuclear Fusion Nuclear fusion is the energy source which fuels the stars, including our sun. The concept of fusion reactors is very attractive, both because of the high energy yield (0.6% of the mass is converted to energy), and because deuterium fuel is available in almost unlimited amounts in seawater. However, the technical problems in producing a sustained fusion reaction are far more difficult than those involved in fission. The reason for this is that in order to fuse, two positively-charged nuclei must be pushed together against the repulsive electrical force until they are within the short range of the nuclear force. No such problem exists for neutron-induced fission because the neutron has no electric charge. The nuclei can be brought close to each other by making the fuel very hot, so that violent collisions between rapidly-moving atoms occur. These violent collisions not only ionize the atoms into a plasma of free electrons and nuclei, but also occasionally result in collisions in which the two nuclei get close enough together to fuse. The problem is that the temperature required is nearly a million degrees, much too hot for any material bottle to hold or confine the reactants. (In stars, the hot reacting gas is confined near the center of the star by the star's very strong gravity.) Gradual progress has been made with systems which use magnetic forces to confine the plasma, and also with an approach in which pellets of fuel are bombarded from several directions by intense laser beams. However even the most optimistic estimates place commerciallyviable fusion reactors many decades into the future. There will be safety problems with fusion, also, which have only begun to be faced. For example, although the final fusion product, helium-4, is not radioactive like fission fragments are, for practical reasons the fusion reactions will probably be accomplished in steps, producing large amounts of radioactive light nuclei such as tritium ( 3 1 H), which are easily incorporated into peoples' bodies. In addition, most of the energy release in fusion reactions is in the form of fast neutrons, which will create additional radioactive isotopes when they are absorbed in the reactor structure. 169

Energy from Matter and Antimatter Annihilation Is it possible to convert 100% of mass to energy, rather than the 0.6% achievable in fusion? The answer is yes. As mentioned in Chapter 16, it is an experimental fact that the electron, proton and neutron all have antiparticles, or antimatter twins. When a particle and its antiparticle (such as a proton and an antiproton) meet, they annihilate each other: both disappear in a burst of energy, mainly in the form of photons. The conversion of mass to energy is 100%. However, antiprotons are not an untapped energy source like the deuterium in seawater. The universe seems to consist almost entirely of matter, with antimatter cropping up only occasionally, as in beta-minus decay in which an anti-electron is emitted. There are no antiprotons to be mined. They can be created by particle accelerators as antiproton-proton pairs, but this requires the input of at least as much energy as the mass-energy contained in the antiproton and proton which are created. Period 18 Summary 18.1: Nuclear reactors generate electricity by heating water to produce steam. Pressure from the steam turns turbines. Water is heated with the thermal energy given off during the fission of radioactive isotopes uranium-238 and uranium-235. U-238 and U-235 usually decay by alpha emission of helium nuclei. U-235 may also decay by spontaneous fission, splitting into two smaller nuclei, called fission fragments, plus two or three free neutrons. These free neutrons are capable of inducing fission by causing other uranium nuclei to fission. This may produce an exothermic chain reaction. The free neutrons are slowed down by a moderator to make the neutrons more likely to be captured and cause another fission. For use in reactors, naturally occurring uranium ore must be processed to enrich its content of U-235. 18.2: Nuclear fission reactors contain five components: 1) Fuel rods of radioactive material, which usually contain ceramic-coated pellets of uranium oxide; 2) a moderator, into which the fuel rods are inserted, to slow down the neutrons (water and graphite are common moderators); 3) control rods made of neutron-absorbing material such as boron or cadmium which, when inserted, decrease the rate of fission; 4) a coolant that transfers thermal energy from the reactor to generate steam and prevents the fuel rods from getting too hot; and 5) a containment system to prevent leakage of radioactive substances into the environment. 170

Period 18 Summary, Continued 18.3: The types of nuclear reactors and their safety issues are: Graphite moderated, water cooled reactors (the Chernobyl type), which could result in a runaway chain reaction if the water coolant is lost; Water moderated, water cooled reactors, which could experience a meltdown if the water coolant is lost but not a runaway chain reaction since water also serves as the moderator that slows the neutrons; Graphite moderated, gas-cooled reactors, which have sufficient moderator to absorb all of the heat from the decaying fission fragments and prevent a meltdown or runaway chain reaction; and Breeder reactors use a fuel containing 15% plutonium and 85% U-238. One byproduct of breeder reactors is more plutonium. Breeder reactors use fast neutrons, so no moderator is needed. Since they operate at higher temperatures than conventional nuclear reactors, liquid sodium is commonly used as a coolant. Breeder reactor technology has not been pursued in the U.S., in part because spent fuel rods of plutonium could be converted into crude nuclear weapons. 18.4 Used fuel rods from reactors are extremely radioactive due to the fission fragments they contain. A decades-long plan for a radioactive waste repository in Nevada have been suspended. Currently, spent fuel rods are stored at the reactor sites. 18.5: Nuclear fusion reactors using the abundant non-radioactive fuel deuterium are an attractive alternative to fission reactors if the challenging technical problems involved in pushing two positively-charged nuclei together to fuse could be overcome. If matter and antimatter particles meet, they annihilate each other in a burst of energy, mainly in the form of photons, in which all of the matter is converted into energy. However, since the Universe seems to consist almost entirely of matter, this does not appear to be a reasonable energy source. 171