CHAPTER 19 Are fission and fusion viable nuclear energy power sources? Contents Fission and fusion Nuclear fission Energy released Achieving a chain reaction Nuclear power plants Generating electricity Nuclear reactors Fast breeder reactors Nuclear wastes Nuclear bombs Fission bombs Bomb design The effects of nuclear weapons Nuclear fusion Fusion reactors Where can we find deuterium and tritium? Fusion reactor designs Environmental impact of a fusion reactor Fusion bombs Conclusion Chapter review Summary Questions Note to students and teachers: This PDF has been provided as an offline solution for times when you do not have internet access or are experiencing connectivity issues. It is not intended to replace your ebook and its suite of resources. While we have tried our best to replicate the online experience offline, this document may not meet Jacaranda's high standards for published material. Please always refer to your ebook for the full and latest version of this title.
CHAPTER 19 Are fission and fusion viable nuclear energy power sources? REMEMBER Before beginning this chapter you should be able to: explain nuclear energy as energy resulting from the conversion of mass into energy using E = mc 2 describe the processes of nuclear fusion and nuclear fission explain, using a binding energy curve, why both fusion and fission are reactions that produce energy.
KEY IDEAS After completing this chapter, you should be able to: explain nuclear fission reactions of 235 U and 239 Pu explain nuclear fusion reactions of proton proton and deuterium tritium describe neutron absorption in 238 U, including formation of 239 Pu explain fission chain reactions including the role of moderators and control rods, and the importance of critical mass describe the energy transfers and transformations in the systems that convert nuclear energy into thermal energy for subsequent power generation compare nuclear fission and fusion as energy sources. Fission and fusion Fission and fusion are two ways of extracting energy from the nucleus of atoms. This chapter investigates both methods. As you will have read in Chapter 7, nuclear energy can be produced in either of two ways: 1. A heavy nucleus splits into two medium-sized nuclei. This nuclear reaction is called fission. 2. Two very light nuclei are forced together to form one nucleus. This reaction is called fusion. For each reaction, this chapter will outline: the reactants, that is, the starting material for the reaction how the reaction is initiated the products at the end of the reaction how plentiful the reactants are how much energy is produced the opportunities and difficulties of the technology being a source of society s energy the environmental impact of the technology. With this information, you will be able to compare the two reactions and their associated technologies. Nuclear fission Chapter 7 described three different fission reactions of uranium-236. The reactants in each case are a neutron and a uranium-235 nucleus, but there are many combinations of products. In Chapter 7 you saw three different reactions with six different products, but about 30 different elements can be produced, two at a time with about 100 isotopes among those 30 elements.
Fission of uranium-236. Distribution of fission fragments by mass number. SAMPLE PROBLEM 19.1 One of the fragments of the fission of uranium-236 has an atomic mass number of about 137. What is the species produced? Solution: Using a periodic table, the element with the mass number of 137 is barium, 56 Ba, which has a relative atomic mass of 137.3. REVISION QUESTION 19.1 a. From the graph above, what is the mass number of the most common fragment of uranium-236 fission? b. Look up the periodic table to find the most likely atomic number of this fragment. c. Uranium-236 has 92 protons. Determine the atomic number of the other fragment in the most common fission reaction. Energy released In chapter 7, Sample problem 7.4 calculated the amount of energy released by the fission of U-236 to produce La-148 and Br-85. The amount of energy released for this reaction is 160 MeV. There are two measures of the energy released that can be used to compare nuclear reactions, in particular fission and fusion reactions. They are:
energy released per nucleon percentage of mass that is transformed into energy. Energy released per nucleon In the fission of U-236, there are obviously a total of 236 protons and neutrons involved. So: energy released per nucleon = = 160 MeV 236 0.68 MeV REVISION QUESTION 19.2 In Chapter 7, Revision question 7.4 asks you to calculate the energy release of two other fission reactions (U-236 to Ba-141 and Kr-92, and U-236 to Xe-140 and Sr-94). Use your answers from Revision question 7.4 to calculate the energy released per nucleon for each reaction, expressed in MeV. Percentage of mass that is transformed into energy The total mass of the fission fragments including the neutrons is less than the mass of the starting U-236 nucleus. This missing mass has appeared as the 160 MeV of kinetic energy of the fragments. It is useful to know how much mass this 160 MeV represents. This was also used in the calculation from Sample problem 7.4, but is worth going over again. The amount of missing mass can be calculate using E = mc 2, but first the energy of 160 MeV needs to be converted into joules. 1 MeV = 1.602 176 10 13 joules. The number comes from the charge on the electron: 1.602 176 10 19 coulomb. The other number required is the speed of light, c = 2.997 924 58 10 8 m/s. Energyreleased inmev = 160MeV Energy released in joules = 160MeV 1.602176 10 joules MeV = 11 2.563482 10 joules Missing mass 11 2.563482 10 = (2.99792458 10 ) 28 = 2.582 26 10 kg 8 2 13 1 The percentage of the original U-236 nucleus converted into energy can now be determined. The mass of the U- 236 nucleus is 3.919 629 10 25 kg. Percentage = 28 2.852 26 10 25 3.919629 10 100 0.073% =
REVISION QUESTION 19.3 a. Use the mass values for U-236, La-148, Br-85 and the neutrons in the table on page XXX in Chapter 7 to calculate the mass difference between the fragments and the U-236 nucleus. Remember this fission reaction release 3 neutrons as products. b. For Revision question 7.4 on page XXX you calculated the energy release of two other fission reactions. Use your answers to calculate the percentage of mass transformed into energy for each reaction. Confirm your answers by using the mass values for the fragments and the U-236 nucleus. Chain reaction There were also two or three neutrons emitted with each fission reaction. This allowed the possibility of a chain reaction with one fission producing two or three others etc. This is what happens if every free neutron goes on to produce another fission. A situation such as this quickly releases an enormous amount of energy. It is called an uncontrolled chain reaction and is what happens in a nuclear bomb.
Achieving a chain reaction There are three factors that make achieving a chain reaction very difficult. 1. Uranium has two main naturally occurring isotopes: uranium-235 (0.7%) and uranium-238 (99.3%). The isotopes responses to an incoming neutron are different and also depend on the speed of the neutron; see table 19.1 below. 2. The high speed of the ejected neutrons: they have kinetic energies about 1 MeV, equivalent to speeds of 1.4 10 7 m s 1. 3. If a chain reaction could be achieved, there needs to be a way of controlling or stopping it. TABLE 19.1 Reactions of uranium isotopes to incoming neutrons Digital doc: Investigation 19.1: Chain reaction with dominoes Model an uncontrolled chain reaction with dominoes. Neutron speed and energy 235 U 238 U Very fast neutrons (5 MeV) Few nuclei fission. Most nuclei fission. Fast neutrons (1 MeV) Many nuclei fission. Very few nuclei fission. Slow neutrons (200 ev) Most nuclei fission. Nearly all nuclei absorb neutrons. Very slow neutrons (0.03 ev) All nuclei fission. All nuclei absorb neutrons. A high-speed neutron from the fission of a uranium-235 nucleus is travelling too slowly to cause a uranium-238 nucleus to split. By the time successive collisions slow the neutrons down to a speed to cause most of the uranium-235 to split, the neutrons will have been gobbled up by the uranium-238 nuclei, which outnumber the uranium-235 nuclei by about 140 to 1 in naturally occuring uranium. Therefore a chain reaction cannot occur in a block of pure natural uranium. Solutions have been developed for each of the three difficulties mentioned. 1. Increase the proportion of the uranium-235 isotope. This process is called enrichment. 2. Slow down the neutrons very quickly. This is done using a moderator. 3. To control the chain reaction, use a material that readily absorbs neutrons and takes them out of the reaction, and that can be quickly inserted at a moment s notice. This is done with control rods. Enriched uranium Natural uranium cannot be used for nuclear bombs or power plants because it contains only small amounts of fissionable uranium-235. The uranium-238 absorbs free neutrons and prevents a sustainable chain reaction from occurring. Enrichment must be carried out to increase the percentage of uranium-235 in the ore. Because uranium-235 and uranium-238 isotopes are chemically identical, the process of separating them is difficult. A number of enrichment methods have been developed, but all are complex and costly. Enriched uranium for nuclear power plants must contain between 1% and 4% uranium-235. For nuclear bombs the percentage of uranium-235 must be closer to 97%, because an uncontrolled chain reaction is required. One enrichment method, called the centrifuge system, uses a rotating cylinder that sends the heavier isotope (uranium-238) in liquid uranium hexafluoride to the outside of the cylinder, where it can be drawn off, while the uranium-235 diffuses to the centre of the cylinder. To effectively enrich the uranium-235, thousands of centrifuges are connected in series.
One centrifuge cylinder Moderators A fast neutron bouncing off a uranium nucleus is like a golf ball bouncing off a basketball. The large difference in mass means that the neutron does not lose much kinetic energy. To slow down quickly, the neutron needs to collide with something of similar mass, like one billiard ball hitting another billiard ball. Also, the moderating material should be a liquid or solid at room temperature, reasonably inexpensive, and not chemically reactive. These constraints mean that ordinary water is commonly used as a moderator. Other possible moderators are carbon (in the form of graphite) and heavy water (water in which the hydrogen atoms are the deuterium isotope, which has one proton and one neutron). Control rods The elements cadmium and boron are deficient in neutrons and readily absorb them. Cadmium boron is usually encased within steel control rods that can be rapidly raised or lowered into the reactor. The first controlled fission reactor was constructed by Enrico Fermi at the University of Chicago in December 1942. He used graphite as the moderator and cadmium for the control rods.
France uses nuclear power for 76% of its electricity. Nuclear power plants Many countries use nuclear power to supply their domestic electric power needs. For three countries it is more than 50%, for another 10 countries it is between 50% and 30%. Overall 11% of the world s electric power is supplied by nuclear energy. Another 17 countries are introducing nuclear reactors in 2015.
Graph showing the percentage of various countries electricity needs supplied by nuclear power in 2014 The amount of pure uranium required to generate this nuclear energy is about 67 000 tonnes per year. Uranium resources are the amount of uranium that is known to exist and can be mined. The amount of uranium resources for various countries is described in the figure below. Uranium resources as in 2013. Data from OECD Nuclear Energy Agency and International Atomic Energy Agency. The world s current resources of uranium, approximately 5.9 megatonnes, would last about 90 years at existing rates of consumption. It is expected that further exploration and higher prices will yield further resources. Generating electricity There are many energy sources that can readily be converted into electricity. In Australia, coal, gas, hydro, electricity and wind are used to generate commercial quantities of electricity. Australia does not have any nuclear power plants that produce electricity for use by the community.
Whatever the energy source used to generate electricity, the techniques are is surprisingly similar. By some means, the energy source is used to turn a turbine. The kinetic energy of the turbine is then converted into electrical energy by a generator. The electrical energy is distributed to the community via a series of transformers and transmission lines. Power plants that use coal are very similar to those using nuclear energy. In both cases, the energy is used to heat water and produce steam, which is then used to turn the turbine. Various methods can be used to turn the turbines that allow electricity to be produced. AS A MATTER OF FACT In 1972 scientists discovered a site in Gabon in, West Africa, which is believed to be the remnant of a natural fission reactor that operated about 2000 million years ago! Scientists suspected that a natural arrangement of uranium ore and water had acted like a modern fission reactor when they noticed lower than expected amounts of uranium-235 in the ore of a mine (0.44% instead of the usual 0.72%). It took some good scientific detective work to solve the puzzle. The main evidence for the existence of a natural reactor was the presence of fission products. The graphs below show the distribution by mass number of the isotopes of the fission product neodymium (Nd). This evidence leaves little doubt that a natural reactor really did exist: (a) and (c) are virtually identical.
Evidence for a natural reactor in Gabon: (a) natural reactor site (b) regular ore (c) spent fuel rods from a nuclear power plant Nuclear reactors There are many different styles of nuclear reactor. The pressurised water reactor (PWR) is the most common. It uses the fission of uranium-235 to produce energy. Generally, nuclear fission reactors have the same basic components. The following figure shows the arrangement of a typical nuclear reactor.
Australia s nuclear reactor Australia currently has one small uranium-235 reactor, OPAL (Open-pool Australian light reactor), which is used for industrial research and the production of medical isotopes, but not for power production. Australia had another reactor, HIFAR (which stands for high flux Australian reactor ), which was closed down in January 2007 and will be decommissioned over the ten years to 2017. OPAL is a 20-megawatt reactor that uses low enriched uranium fuel and is cooled by ordinary water. It has twice the power of HIFAR. The reactor core, which contains the nuclear fuel and the moderator, is about the size of a filing cabinet; it contains 30 kg of uranium, including 6 kg of uranium-235. It is designed to produce a stream of neutrons, which are used to irradiate materials to either identify their structure or to produce radioisotopes for medical or industrial purposes. A nuclear power plant Fast breeder reactors Like coal and gas, the world s supply of uranium-235 is limited. To overcome this problem, fast breeder reactors have been developed. These reactors use plutonium-239 as the energy source. Plutonium-239 is not a naturally occurring isotope; rather, it is produced when uranium-238 captures a neutron and becomes uranium-239. This unstable isotope releases a β particle and becomes neptunium-239. Another β particle is then released and the nucleus becomes plutonium-239, a fissionable isotope. U+ n U Np+β Pu+β 238 1 239 239 239 92 0 92 93 94 Fission reaction for plutonium-239 One possible fission reaction for plutonium-239 is: Pu + n Pu Ce + Kr + 3 n + energy 239 1 240 148 89 1 94 0 94 58 36 0
Nucleus Symbol Mass (kg) Total binding energy (MeV) 240 Plutonium-240 94 3.986 19 10 25 1813.454 956 Cerium-148 148 58 2.456 34 10 25 1219.605 103 Krypton-89 89 36 1.476 51 10 25 766.907 837 1 Neutron 0 1.674 924 10 27 Speed of light, c = 2.997 924 58 10 8 m/s, exactly. 1 MeV = 1.602 176 10 13 joule. REVISION QUESTION 19.4 The fission reaction of plutonium shown above can be analysed in the same way as the fission reaction for uranium on page x in Chapter 7. Use the solution on that page to answer the following questions. a. What is the difference between the binding energy of the plutonium-240 nucleus and the sum of the binding energies of the two fission fragments? b. What is the difference between the mass of the plutonium-240 nucleus and sum of the masses of all the fission fragments, including neutrons? c. Use Einstein s equation E = mc 2 to calculate the energy equivalent of this mass difference in joules and in MeV. Now use the calculations for uranium fission done previously in this chapter as a model to answer two additional questions: d. Calculate the energy released per nucleon in MeV. e. Calculate the percentage of mass transformed into energy. Confirm your answers by using the mass values for the fragments and the Pu-240 nucleus. Fuel rods in a fast breeder reactor consist of 20% plutonium-239 surrounded by 80% natural uranium. As the plutonium undergoes fission, it actually produces more plutonium when the uranium-238 is bombarded with neutrons. In this process more plutonium is produced than is used. This is why they are known as breeder reactors. Uranium- 238 is much more plentiful than uranium-235, so fast breeder reactors are an attractive alternative to conventional reactors. Weblink: Nuclear power plant simulation There is another major difference between reactors that use enriched uranium-235 and fast breeder reactors. Plutonium-239 readily absorbs fast-moving neutrons (hence the name fast breeder). It is therefore not necessary for these reactors to have a moderator to slow down the free neutrons. Generally they use liquid sodium as a coolant. Fast breeder reactors have had a troubled history. France, the UK and the USA built a small number of such reactors during the 1950s and 1960s, but, due to political factors and the relatively low cost of conventional uranium fuel, none of these are currently operational. Russia still has one fast breeder reactor running, and France, Japan, India and China are both planning the construction of new breeder reactors. Fukushima Daiichi nuclear disaster On Friday 11th March 2011, an earthquake, measuring 9.0 on the Richter scale, occurred about 70 km off the east coast of Japan. It was the fourth most powerful earthquake since modern records began in 1900. As well as structural damage on the coast, the earthquake produced a tsunami that reached heights of 40 m and travelled 10 km inland.
Fukushima Daiichi is a complex of six nuclear reactors. It was built on a bluff that was reduced to a height of 10 m above sea level to give solid foundations. On the day of the earthquake only three units (1, 2 and 3) were operating; the others were shut down for maintenance. The earthquake produced a ground acceleration of 5.5 m/s 2 at Unit 2, which was above the design tolerance for that reactor of 4.4 m/s 2. Immediately after the earthquake, the control rods automatically shut down the fission reactions, but there was still heat in the reactor core, sufficient to require active cooling for several days to prevent the fuel rods melting. The tsunami arrived 50 minutes after the earthquake. The tsunami height at Fukushima was 14 m, which flooded the low lying rooms and disabled the emergency diesel generators. The battery operated emergency generators took over, but ran out of power the next day and cooling of the reactor core stopped. Replacement batteries were delayed by poor road conditions. Over the next few days, with increasing reactor temperature, the zirconium coating of the fuel rods reacted with water to produce hydrogen in each of the three reactors. The hydrogen gas vented out of the reactor vessel, mixed with air and exploded in the outer secondary containment building causing a fire. Damage to the roof of the outer containment building due to the hydrogen air explosions in each of the three reactors It is estimated that most of the fuel in each of the reactor cores melted and is now on the concrete floor of the containment vessels. Radioactive material, consisting mainly of iodine-131, caesium-134 and caesium-137, was released from the reactors for following reasons: deliberate venting to reduce gas pressure in the containment vessels deliberate discharge of coolant water into the sea uncontrolled events.
All three radioactive isotopes released undergo beta decay with the following half-lives: iodine-131 8 days caesium-134 2 years caesium-137 30 years. Caesium-137 is the main health concern in decontaminating land around Fukushima. In the first few months following the tsunami, about 8 kg of caesium-137 flowed into the ocean. Fortunately the currents off the coast are rapid and the caesium dispersed to low concentrations. Atmospheric releases of radioisotopes spread around the world, reaching the west coast of the US two days after the explosions. Measurements across the globe suggested that the release of radiation was about 10 40% that of the Chernobyl disaster and covered an area about 10% the size. The World Health Organization (WHO) indicated that people who were evacuated from the area around Fukushima were exposed to so little radiation that radiation-induced health effects are likely to be below detectable levels. However, decommissioning the reactors remains a problem, with radioactivity seeming to still leak into the groundwater. The Fukushima Nuclear Accident Independent Investigation Commission found the nuclear disaster was manmade and that its direct causes were all foreseeable. The report also found that the plant was incapable of withstanding the earthquake and tsunami. The company, the regulators and the government body promoting the nuclear power industry all failed to meet the most basic safety requirements, such as assessing the probability of damage, preparing for containing collateral damage from such a disaster and developing evacuation plans. PHYSICS IN FOCUS Disaster at Chernobyl As in any other industry, accidents have occurred in the nuclear power industry. In most cases the damage has been minimal, and was contained inside the reactor without posing danger to nearby communities. This is due to the large number of safety features incorporated into reactor design. Unfortunately, on 26 April 1986, a major accident happened at Chernobyl, near Kiev then part of the Soviet Union. The Chernobyl nuclear power plant reactors were graphite moderated and water cooled. Unlike those in Western countries, Soviet reactors were sometimes built without a containment vessel, and the Chernobyl reactors were of this kind. On the day before the accident, one of the reactors at the power plant had been reduced to running at about 50% of its usual power output, and the emergency core cooling systems had been switched off, to allow tests to be carried out. The tests were to measure the effectiveness of modifications that had been made to the generators and were not due to concerns about the reactor core. Just after 1 am on 26 April, the operators had to make changes to the system in order to get the reactor to behave in a way that would allow the test to continue. When another change was made to set the system up for the tests, the reactor became more unstable and could no longer be controlled. The operator tried to insert the control rods fully to stop the reaction. In four seconds the power rose to 100 times its normal level and a steam explosion occurred.
The Chernobyl nuclear reactor after the accident The temperature of the core reached about 5000 K, one-third of the fuel was destroyed by the explosion, and about one-tenth of the core (mostly the graphite moderator) was burned, releasing about 4% of the fuel into the atmosphere because there was no containment vessel. Two staff were killed at the time of the accident one was hit by a jet of steam, and the other by a block of concrete. A further 300 reactor staff and emergency workers were treated in hospital, and 29 of them died of acute radiation poisoning. The International Nuclear Safety Group investigated the causes of the Chernobyl disaster. They concluded that there were problems with the design of the reactor and with the test procedures, which contributed to the accident. In addition, investigates felt that operators were not fully aware of safety issues, due to a poor safety culture, both locally and nationally. As a result of the investigation, much has been learned about design and safety of reactors in general, and about Soviet reactors in particular. Nuclear wastes High-level waste Fission reactors do not produce as much waste as coal or gas power plants, but the waste they do produce is highly radioactive. When fission occurs inside the fuel rods, the fission fragments produced are unstable. These fragments then undergo a series of radioactive decays to become more stable. Commonly the fission fragments release β particles in order to increase the number of protons in the nucleus. Some of the isotopes produced have very short half-lives, while others have longer half-lives. Two frequently produced fission fragments and their decay chains are:
Sr Y Zr (stable) 94 94 94 38 39 40 140 140 140 140 140 54 55 56 57 58 Xe Cs Ba La Ce (stable) The used (or spent) fuel rods can be reprocessed. During this procedure any unused fuel is removed from the rods for reuse. Plutonium produced in the fuel rods is also separated. The remnants are classified as high-level waste. While such waste does not take up a lot of space, it is highly radioactive. It has been estimated that highlevel waste will take about 1000 years to return to the same level of radioactivity as the uranium ore that was originally mined, and at least 5 million years before there is no longer any significant radiation. High-level waste must be stored in shielded containers to prevent radiation escaping into the environment. It must also be cooled to stop overheating. Australia does not produce any high-level waste. AS A MATTER OF FACT The USA has so far refused to reprocess spent fuel rods. The reason is concern that the plutonium-239 that is removed during reprocessing may be illegally used to manufacture nuclear weapons. Medium-level waste In addition to the fuel rods, all other material in the reactor core is exposed to a huge amount of radiation. Some of this radiation can be absorbed by the material, which itself becomes radioactive. The fuel containers, pipes, gauges and other reactor components are classified as medium-level waste. This waste does not require cooling, but still needs to be shielded. Radioactive isotopes used in medicine and industry are classified as medium-level waste once their useful life is over. Low-level waste Used protective clothing, water from showers and the cleaning of protective gear, and old plant equipment all make up low-level waste. Often such waste contains levels of radiation that are just above safe levels, or isotopes with very short half-lives or low activities. Sometimes these wastes can be released into the environment after being diluted, or simply stored for a short time. Low-level waste does not require shielding during handling. Such waste has a dose rate of about 2 millisieverts per hour. If the dose rate is above this, then waste is classified as intermediate-level and needs shielding. Waste disposal Because of the long life of high- and medium-level radioactive waste, careful consideration must be given to its storage. At present most waste is planned to be stored underground in geologically stable areas away from underground water. Some of the waste is simply placed in steel storage canisters. Another option is to fuse the waste into glass blocks a process called vitrification. Australian scientists have developed a process by which wastes are encapsulated in an artificial rock dubbed Synroc. In this synthetic substance, the waste is incorporated into the crystal lattice, making it resistant to high temperatures and water. These properties make Synroc ideal for underground storage of nuclear wastes. It can resist the high temperatures present very deep in the Earth s crust, and heat produced by radioactive decay. Underground water supplies will not break down Synroc, thus avoiding the possibility that the waste will contaminate water supplies. Natural rocks with similar composition to Synroc have been known to survive harsh conditions for millions of years. It is hoped that Synroc will do the same.
Storage of nuclear waste deep in the oceans has also been used by some countries. While this is mostly used for lower level wastes, it is still a source of concern for environmentalists because the metal storage canisters will corrode in time, allowing the radioactive contents to leak into the world s oceans. Suggestions have been made that nuclear waste should be shot away from the Earth in rockets, possibly into the Sun s core (although the amount of energy needed to do this makes it prohibitive). It is not known, however, what effect this might have on the stability of the Sun! In addition, the possibility of a rocket carrying nuclear waste exploding in our atmosphere, or crashing back to Earth, is not something most governments would be prepared to risk. Nuclear bombs Fission bombs Immense amounts of energy are available in a very short space of time from the splitting of large nuclei just perfect for making bombs. A nuclear fission bomb is simply a lump of fissionable material in which an uncontrolled chain reaction is allowed to occur. Fortunately, this happens only under fairly specific conditions. An uncontrolled chain reaction The chain reaction To create an uncontrollable chain reaction, a large proportion of fissionable nuclei is necessary. If uranium is the energy source, weapons-grade enriched ore (about 97% uranium-235) is required. This means that there are very few nuclei present that can absorb the free neutrons without undergoing fission themselves. Therefore more of the available neutrons cause an energy release. Critical mass A large ball of weapons-grade material makes a large nuclear bomb, but a small ball of weapons-grade material may not make a small nuclear bomb! The difference lies in the volume-to-surface-area ratio. A small ball has a small ratio of volume to surface area. This means a low proportion of the free neutrons stay inside the ball where
they can initiate a fission. Large balls of weapons-grade material will have a higher ratio of volume to surface area, therefore a higher proportion of the free neutrons stay inside the ball to produce further fissions. Critical mass is the smallest mass of a fissionable substance which, when formed into a sphere, will sustain an uncontrolled chain reaction. This mass varies according to the percentage of fissionable nuclei in the material and the fissionable isotope used. Any mass less than the critical mass is known as subcritical. If the shape of the fissionable material is changed from a sphere to a brick shape, then there is more surface area available for neutrons to escape the material before initiating another fission reaction. A brick shape therefore needs to be larger and heavier before it becomes critical. The sphere is the shape with the smallest surface area for a given volume. TABLE 19.2 Critical mass and diameter of a sphere of fissionable material Fissionable nucleus Critical mass Critical diameter Uranium-235 52 kg 17 cm Plutonium-239 10 kg 10 cm Uranium-233 15 kg 11 cm The critical mass can be reduced by using beryllium around the outside as a neutron reflector. Escaping neutrons are reflected back into the fissionable material, increasing the chances of a fission reaction. The neutrons collide with the beryllium nuclei and a high proportion bounce back. Bomb design All fission bombs rely on two or more subcritical masses being brought together to form a critical mass. The joining of the subcritical masses is usually instigated by the detonation of a small conventional bomb.
This gun-style bomb is similar to Little Boy, the bomb dropped on Hiroshima on 6 August 1945. It uses the fission of enriched uranium-235 as its energy source. This implosion bomb is similar to Fat Man, the bomb dropped on Nagasaki on 10 August 1945. It uses several subcritical pieces of plutonium-239 as its energy source. The effects of nuclear weapons The size of nuclear weapons is measured according to the mass of TNT needed to produce a similar blast. Both bombs dropped on Japan in 1945 were about 20 kiloton bombs. Weapons currently stockpiled around the world are much larger than this, with 20 megaton bombs being common. The devastating damage caused by nuclear weapons can be classified in two ways: immediate and long-term effects. Immediate effects Thermal flash. The high temperatures generated by a nuclear blast cause a flash of thermal (heat) radiation to spread out from ground zero (the centre of the blast). The thermal radiation takes the form of an enormous fireball. A 10 megaton bomb would produce a fireball about 4.3 km in diameter, and the thermal flash emitted would cause second-degree burns up to 32 km away. Eyesight may also be damaged in any creatures looking at the fireball. When exposed to such large amounts of thermal radiation, dry grass and paper may spontaneously ignite, setting fires which may be fanned by the accompanying high winds. Close to ground zero, most substances are melted, burned or exploded.
The immediate after-effects of the nuclear bomb explosion in Hiroshima, Japan, August 1945 Shock wave. Immediately after the detonation of a nuclear bomb, a shock wave spreads out from the centre of the blast. This high-pressure wave moves out at speeds which may be greater than 3000 km h 1. It is estimated that a 10 megaton bomb would irreparably damage houses within a 17.7 km radius, and moderately damage homes up to 24 km away. Electromagnetic pulse. The huge amounts of gamma (γ) radiation emitted by the nuclear explosion can ionise atoms in the air. The numerous free electrons produced form strong electromagnetic fields. These fields are capable of destroying electric and electronic systems, including power distribution systems, telecommunications and computer networks. Data stored electronically would be wiped from memory chips and flash drives. Initial nuclear radiation. Enormous amounts of γ radiation and free neutrons would irradiate everything near the blast. While the radius of damage due to nuclear radiation would not be as great as for the shock wave or thermal blast, the severity of the radiation received by people and animals would cause immediate death in most. Long-term effects Radioactive fallout. In the days and weeks after a nuclear blast, the radioactive products of the fission reaction, floating as dust in the atmosphere, would start to return to Earth. Many of these isotopes have long half-lives, ensuring that their effects would remain for months and years. The fallout from fusion bombs is less than for
fission bombs because radioisotopes are not produced by fusion reactions. (A fusion bomb does still require a small fission bomb to initiate the fusion reaction.) The radiation from the fallout would ensure that any remaining people would continue to receive a greater than normal radiation dose for many years. Nuclear winter. Some scientists have predicted that a so-called nuclear winter may follow large-scale nuclear detonations, such as in a nuclear war. The dust and smoke brought into the atmosphere by the explosions would gather as clouds covering great expanses of the sky. Sunlight would be blocked from the Earth s surface, resulting in lower temperatures and the destruction of plant life until the clouds finally settled. The ozone layer could be damaged, leaving the Earth without protection from the Sun s ultraviolet rays once the dust clouds cleared. Nuclear fusion In Chapter 7 on page xxx, the discovery of nuclear fusion by Marcus Oliphant was described. The three-stage fusion process that occurs in the Sun to convert hydrogen to helium is: H+ H H+ e+ v 1 1 2 0 1 1 1 1 2 1 3 1H+ 1H 2He+ γ 3 3 4 1 1 2 + 2 2 + 1 + 1 He He He H H This process is useful because two of the products in the final stage are the same as reactants in the first stage. The overall effect is that six protons are turned into one helium nucleus with two protons left over. Fusion reactors Stars are giant fusion reactors, and many people have proposed that fusion reactors could be the answer to our energy needs. The fusion of hydrogen isotopes to form helium releases a large amount of energy. (The graph on page xxx shows the large difference in the binding energies of 2 1H and helium.) The practicalities of a fusion reactor, however, are proving difficult. To initiate fusion of hydrogen, the forces of repulsion between the positive nuclei must be overcome. One way to do this is to give the nuclei large amounts of kinetic energy by heating them to extremely high Weblink: temperatures (millions of degrees). It is incredibly difficult to contain material at such temperatures. Currently, scientists are using magnetic Fusion basics fields to contain the hydrogen, while laser beams are used to heat it. This method has proved successful, but for only very short periods of time. Viable production of energy using a fusion reactor is still many years away. A fusion reactor could not feasibly use the same reactions as the Sun. A reactor on Earth would have to use a different reaction, preferably a one-step reaction with only two reactants. Three possible reactions for a terrestrial fusion reactor are displayed below; there are many others. H+ H He+ n 2 3 4 1 1 1 2 0 2 2 3 1 1H+ 1H 1H+ 1H 2 6 4 4 1 + 3 2 + 2 H Li He He The energy changes in these nuclear equations can be investigated to a similar depth as the fission reactions earlier in this chapter.
REVISION QUESTION 19.5 Using data from the table below, for the first reaction between a deuterium and a tritium nucleus, calculate: a. the difference between the binding energy of the products and sum of the binding energies of the reactants b. the difference between the sum of masses of the products and the reactants c. the energy equivalent of this mass difference in joules and MeV. Particle Symbol Mass (kg) Total binding energy (MeV) 4 Helium-4 2 6.665 892 10 27 28.295 673 2 2 Hydrogen-2 1Hor 1D 3.344 494 10 27 2.224 573 3 3 Hydrogen-3 1Hor 1T 5.008 267 10 27 8.481 821 1 Neutron 0 1.674 924 10 27 Using the solutions earlier in this chapter as a model: d. calculate the energy released per nucleon of reactants in MeV e. calculate the percentage of mass transformed into energy. Confirm your answers by using the mass values for the reactants and the products. The stars in Omega Centauri, like all other stars, are giant fusion reactors.
Where can we find deuterium and tritium? Deuterium is a stable isotope of hydrogen. It has one proton and one neutron. In nature, only about one in every 6400 hydrogen atoms is a deuterium atom. However, being twice as heavy as normal hydrogen it is quite easy to separate. In steam, water molecules with two deuterium atoms have a molecular weight of 20 compared to 18 for most water molecules. This 11% difference in mass means that the heavier molecules on average travel slower and can be separated in a distillation process. Water that was almost exclusively deuterium oxide could be made is large quantities by the 1930s. It was predominately used as a moderator in nuclear reactors. It can be easily extracted from sea water, but most industrial processes use glacial ice or mountain water. Tritium is radioactive. It beta decays into helium-3, with a half-life of 4500 days. It is extremely rare; very small amounts are produced by cosmic rays hitting atoms in the atmosphere. It can be produced inside nuclear reactors by using the neutrons to bombard targets such as lithium-6. REVISION QUESTION 19.6 a. What is the decay equation for tritium? b. What is the other product from bombarding a lithium-6 nucleus with a neutron? Fusion reactor designs The key difficulties for fusion reactors are to confine the mixture of deuterium and tritium at a high enough temperature, at a great enough density and for a long enough time for fusion to occur on a sustainable basis. Magnetic confinement In a doughnut-shaped machine a mixture of deuterium and tritium is constricted by a magnetic field. An electric current is passed through the gas heating it up to beyond 10 million degrees Celsius. At these temperatures, electrons are stripped from atoms, and the gas becomes a plasma of ions. An example of magnetic confinement is the Tokamak, which is a Russian designed fusion reactor.
Schematic of a tokamak that contains plasma with a magnetic field. Laser fusion Over 190 laser beams simultaneously emit a strong pulse of light on a small pellet of frozen deuterium and tritium. About 1.8 million joules of energy hits the pellet in billionth-of-a-second pulses and causes fusion. Environmental impact of a fusion reactor The advantages of a fusion reactor over fission are that there is no chain reaction that could possibly run away, and there is no radioactive waste. Any accident is not likely to require the evacuation of the surrounding area. Neutrons will be emitted from the reaction vessel, but they can be stopped by a thick containment vessel. Tritium is radioactive with a reasonably long half-life, but if accidentally released into water, it is impossible to remove and will flow through the planet s water cycle. Fusion bombs The extreme conditions necessary for the fusion of hydrogen can be created by the detonation of a small fission bomb. This means a fusion (or hydrogen) bomb is actually two bombs a small fission bomb which triggers a much larger fusion explosion. Such weapons are known as thermonuclear, because the initial explosion creates the intense heat necessary to overcome the repulsion between positive nuclei and allow them to get close enough for the fusion to occur.
Conclusion Having studied this topic and read this chapter, you are now in a position to compare fission and fusion on each of the following aspects: the reactants, that is, the starting material for the reaction how the reaction is initiated the products at the end of the reaction how plentiful and easily obtained the reactants are how much energy is produced the opportunities and difficulties of the technology being used as a source of society s energy the environmental impact of the technology the risks and benefits for society of using nuclear energy as a power source The thermodynamics chapters also invited you to investigate solar thermal technology as a power source for society, so you may wish to include that technology as part of your comparison. Chapter review Summary Fission reactions occur when a nucleus is split into smaller, more stable fission fragments. If every neutron released in fission is free to initiate more fission reactions, an uncontrolled chain reaction occurs. Controlled chain reactions occur when some of the free neutrons are absorbed by non-fissionable substances. Nuclear reactors use the energy generated by controlled chain reactions to heat water. The steam produced turns the turbines that produce electricity. The fuel in some nuclear reactors is more likely to undergo fission when it absorbs slow-moving neutrons. Moderators are used in these reactors to slow down neutrons. Control rods start and stop the nuclear reaction by absorbing neutrons. The amount of uranium-235 in natural uranium is not enough to sustain a chain reaction. In order for it to be used in some types of nuclear reactors and nuclear weapons, the percentage of uranium-235 needs to be increased to 1 4% for nuclear reactors and 97% for weapons. The fuel in fast breeder reactors undergoes fission when it absorbs fast-moving neutrons. Its fuel does not need to be enriched, because it uses plutonium-239 derived from uranium-238 as the fuel source. The reaction is: U+ n U Np+β Pu+β 238 1 239 239 239 92 0 92 93 94 A critical mass is needed for a sustainable chain reaction. The nuclear fusion reaction between deuterium and tritium is a possible energy source. The processes of nuclear fission and fusion can be compared by a variety of measures including energy released per nucleon and percentage of mass lost. Questions The nucleus 1. a. Define the terms fusion and fission.
b. Which of these reactions occurs in our sun? 2. Why does the splitting of uranium-235 nuclei release energy, but the joining of hydrogen atoms also releases energy? 3. Why is energy released in the process of fusing two small nuclei together? Nuclear fission 4. Explain why a large spherical mass of uranium may be able to sustain a chain reaction while the same mass spread into a flat sheet could not. 5. In what form does the released energy from a nuclear fission reaction appear? 6. Why are neutrons good at initiating nuclear reactions? 7. Describe a chain reaction. Nuclear reactors 8. Make a list of the similarities and differences between the way electricity is produced in a nuclear power plant and the way it is produced in a coal-burning plant. 9. How do control rods allow the fission chain reaction to be controlled? 10. Explain why fast breeder reactors are likely to be the main producers of nuclear power in the future. 11. Enriching uranium is difficult. Why? 12. After the explosion at the Chernobyl reactor, tonnes of lead, sand and boron were dropped into the reactor. Why was boron used? 13. Why are thermal reactors so called? 14. What do control rods control? Nuclear waste 15. What does the phrase reprocessing of spent fuel rods mean? Nuclear weapons 16. How can a nuclear bomb contain sufficient fissionable material to explode, but be transported without exploding? (Use the term critical mass in your answer.) Nuclear fusion 17. In what form does the energy released from a nuclear fusion reaction appear? 18. What are the advantages and disadvantages of fusion power as compared to fission power? 19. Using data from the table below, calculate the following for the two reactions: H+ H H+ H H Li He He 2 2 3 1 1 1 1 1 2 6 4 4 1 + 3 2 + 2 a. The difference between the sum of binding energies of the products and the binding energies of the reactants b. The difference between the sum of masses of the products and the reactants c. The energy equivalent of this mass difference in joules and in MeV
d. The energy released per nucleon of reactants in MeV e. The percentage of mass transformed into energy. Then confirm your answers by using the mass values for the reactants and the products. Particle Symbol Mass (kg) Total binding energy (MeV) Helium-4 4 2 6.665 892 10 27 28.295 673 1 1 Proton 1por 1H 1.678 256 10 27 2 2 Hydrogen-2 1Hor 1D 3.344 494 10 27 2.224 573 3 3 Hydrogen-3 1Hor 1T 5.008 267 10 27 8.481 821 6 Lithium-6 3 9.988 344 10 27 31.994 564