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Nuclear Fission 'S and Fusion t is nts

Mk he discovery of radioactivity in 1896 sparked much interest among many Tkinds of people. Some people thought it was no more than a scientific curiosity, some an thought it would be a cure for medical ailments, and a few thought . it might turn out to be a source of plentiful to homes, ilm factories, and up cities at night. Radioactivity does release energy, but has not become a sub— 3hic stantial source of energy for . On a small scale it powers Ii small energy sources in spacecraft, and makes a sample of radium warm. On a large scale it melts rocks and is the source of within Earth. In 1939, just at the beginning of World War II, a I by was discovered that released much more energy per if than radioactivity, and had the potential to be used for both and power production. This was the splitting of the atom, or . A very different nuclear reaction, , can also release huge amounts of energy Both nuclear fission and nuclear fusion produce vastly more energy per kilogram of than any chemi- cal reaction, and even more than most other nuclear reactions. The awesome release of this energy in atomic and ush- ered in the present "nuclear age." Out of the ashes of despair brought about by these bombs, hope grew that could be used for peaceful purposes—that the awesome energy of nuclear reactions could be used for domestic power instead of for arsenals of war. What exactly are nuclear fission and nuclear fusion? How do they differ? What is the that underlies why so much energy is released by these reactions? The answers to these questions are what this chapter is about.

629 40.1 Nuclear Fission

Biology students know that living tissue grows by the division of THE NUCLEAR IS cells. The splitting in half of living cells is called fission. In a similar DOMINANT way, the splitting of atomic nuclei is called nuclear fission. Nuclear fission involves the delicate balance between the attrac- tion of nuclear strong and the repulsion of electrical forces within the nucleus. In all known nuclei the nuclear strong forces dominate. In , however, this domination is tenuous. If the CRITICAL DEFORMATION uranium nucleus is stretched into an elongated shape (Figure 40.1), the eleciiical forces may push it into an even more elongated shape. If the elongation passes a critical point, electrical forces overwhelm nuclear strong forces, and the nucleus splits. This is nuclear fission. THE ELECTRICAL FORCE IS The absorption of a by a uranium nucleus supplies DOMINANT enough energy to cause such an elongation. The resulting fission process may produce many different combinations of smaller nuclei. Figure 40.1 A A typical example is Nuclear deformation to fission when repelling electrical forces dominate over attracting nuclear forces.

I n + 235u 91Kr .1. 142Ba 3(in) 0 92 36 56 0 The energy that is released by the fission of one U-235 atom is enormous—about seven million times the energy released by the of one TNT . This energy is mainly in the form of of the fission fragments, with some energy given to ejected , and the rest to gamma . Note that one neutron starts the fission of the uranium atom, and, in this example, three more neutrons are produced when the uranium fissions. Between two and three neutrons are produced in most nuclear fission reactions. These new neutrons can, in turn, cause the fissioning of two or three other nuclei, releasing from four to nine more neutrons. If each of these succeeds in splitting just one atom, the next step in the reaction will produce between 8 and 27 neutrons, and so on. This makes a chain reaction (Figure 40.2). Why do chain reactions not occur in naturally occurring ura- nium ore deposits? They would if all uranium atoms fissioned so eas- ily. Fission occurs mainly for the rare 11-235, which makes up only 0.7% of the uranium in pure uranium . When the preva- lent isotope 11-238 absorbs neutrons from fission, it does not undergo fission. So a chain reaction can be snuffed out by the neutron-absorbing U-238. It is rare for uranium deposits in to spontaneously undergo a chain reaction. If a chain reaction occurred in a chunk of pure U-235 the size of 2 Laboratory Manual 99 a baseball, an enormous explosion would likely result. If the chain reaction were started in a smaller chunk of pure U-235, however, no

630 Chapter 40 Nuclear Fission and Fusion Figure 40.2 A chain reaction. (Here, only two emitted neutrons per reaction \ 'Ise re/ are shown.) aeL ti ,ersor a It t

/ 0 NEUTRON

ty 5U-235 NUCLEUS 1.

FISSION FRAGMENT lei. explosion would occur. Why? Because a neutron ejected by a fission event travels a certain average distance through the material before it encounters another uranium nucleus and triggers another fission event. If the piece of uranium is too small, a neutron is likely to escape through the surface before it "finds" another nucleus. On the average, fewer than one neutron per fission will be available to trigger more fission, and the chain reaction will die out. In a bigger piece, a neutron can move farther through the material before reaching a surface. Then more than one neutron from each fission GU 235 Ou238 event, on the average, will be available to trigger more fission (Figure 40.4).The chain reaction will build up to enormous energy* Figure 40.3 • Only 1 part in 140 of naturally NEUTRONS TRIGGER occurring uranium is U-235. NEUTRONS ESCAPE MORE REACTIONS ( SURFACE

Le in Figure 40.4 A four The exaggerated view shows that a chain reaction in a small piece of pure leak from the surface too easily. In a larger one U-235 dies out, because neutrons piece, a chain reaction builds up because neutrons are more likely to trigger additional fission events than to escape through the surface. L- The critical is the amount of mass for which each fission 3 eas- event produces, on the average, one additional fission event. It is es up just enough to "hold even." A subcritical mass is one in which the va- chain reaction dies out. A supercritical mass is one in which the chain reaction builds up explosively

ure * Another way to understand this is geometrically. Recallthe concept of scaling in Chapter 18. 2e 0f Small pieces of material have more surface area relative to than large pieces (there am is more skin on a kilogram of small potatoes than on a single 1-kilogram large potato). The ?x, no larger the piece of fission , the less surface area it has relative to its volume.

631 SHORT PATH THROUGH In Figure 40.5 there are two pieces of pure 11-235, each of them sub- EACH PIECE critical. Neutrons readily reach a surface and escape before a sizable chain reaction builds up. But if the pieces are joined together, there will be more distance available for neutron travel and a greater likelihood of their triggering fission before escaping through the surface. If the com- bined mass is supercritical, we have a nuclear fission . LONGER The construction of a uranium fission bomb is not a formidable PATH task. The difficulty is separating enough 11-235 from the more abun- dant 11-238. It took scientists and engineers more than two years to extract enough 11-235 from to make Figure 40.5 A the bomb that was detonated over in 1945. Uranium iso- Each piece is subcritical. The tope separation is still a difficult, expensive process today. average path of a neutron in each piece is short enough that a neu- tron is likely to escape. When the TNT 70 DRIVE URANIUM PIECES pieces are combined, the average QU ICK LY TOGETHER path of a neutron is greater, and there is less chance that a neu- tron will escape. The combina- tion may be supercritical.

RADIOACTIVE SOURCE SUBCRITICAL PIECES THAT SUPPLIES FREE OF URANIUM NEUTRONS

Figure 40.6 A Simplified diagram of an idealized uranium fission bomb. (In an actual "gun- type" , only one of the two pieces of uranium is fired toward the other one, which is the "target.")

• Questions 1. What is nuclear fission? 2. What is a chain reaction? 3. Five kilograms of U-235 broken up into small separated chunks is subcritical, but if the chunks are put together in a ball shape, it is supercritical. Why?

• Answers

1. Nuclear fission is the splitting of the . When a heavy nucleus such as the 13-235 nucleus splits into two main parts, there is a large release of energy. 2 A chain reaction is a self-sustaining reaction that, once started, continues because one reaction event triggers one or more additional reaction events. 3. Five kilograms of U-235 in small chunks will not support a sustained reaction because the path for a neutron in each chunk is so short that the neutron is likely to escape through the surface without causing fission. When the chunks are brought together, the average neutron path within the material is much longer and a neutron is likely to cause fission rather than escape.

632 Chapter 40 Nuclear Fission and Fusion 40.2 The Nuclear Fission Reactor

A better use for uranium than for bombs is for power reactors. About 21% of in the United States is generated by nuclear fis- sion reactors. These reactors are simply nuclear furnaces, which (like furnaces) do nothing more elegant than boil to produce steam for a turbine (Figure 40.7). The greatest practical difference is the amount of fuel involved. One kilogram of uranium fuel, less than the size of a baseball, yields more energy than 30 freight-car loads of .

SuPERNEAki) Fioure REACTOR WATER Diagram of a nuclear fission power plant.

WATER RUMPS

A reactor contains three main components: the combined with a moderator to slow down neutrons, the control rods, and water used to transfer heat from the reactor to the genera- tor. The nuclear fuel is uranium, with its fissionable isotope U-235 enriched to about 3%. The moderator may be , a pure form of , or it may be water. Because the U-235 is so highly diluted with U-238, an explosion like that of a nuclear bomb is not possible. Control rods that can be moved in and out of the reactor control the "multiplication" of neutrons, that is, how many neutrons from each fission event are available to trigger additional fission events. The control rods are made of a material, usually the metal or , that readily absorbs neutrons. Heated water around the nuclear fuel is kept under high pressure and thus brought to a high temperature without boiling. It transfers heat to a second, lower- pressure water system, which operates the electric generator in a

Question What is the function of the control rods in a ?

• Answer Control rods absorb more neutrons when they are pushed.into the reactor and fewer neu- trons when they are pulled out of the reactor. They thereby control the number of neutrons that participate in a chain reaction.

633 conventional fashion. In this design two separate water systems are used so that no radioactivity can reach the turbine. A major drawback to fission power is the waste products of fission. Recall that light atomic nuclei are most stable when composed of equal numbers of and neutrons, and that heavy nuclei need more neutrons than protons for stability So there are more neutrons than protons in uranium-143 neutrons compared with 92 protons in 11-235, for example. When uranium fissions into two medium-weight elements, the ratio of neutrons to protons in the product nuclei is greater than for medium-weight stable nuclei. These fission products are said to be "neutron rich." They are radioactive, most with very short half-lives, but some with half-lives of thousands of years. Safely dispos- ing of these waste products requires special storage casks and proce- dures, and is subject to a developing that is less than ideal.

40.3

When a neutron is absorbed by a 11-238 nucleus, no fission results. The nucleus that is created, 11-239, emits a instead and becomes an isotope of the first beyond uranium— the transuranic element called (named after the first discovered from the application of Newton's law of gravitation).* This isotope, Np-239, in turn, very soon emits a and becomes an isotope of plutonium (named after Pluto, the second planet to be discovered via Newton's law). This isotope, Pu-239, like U-235, will undergo fission when it captures a neutron.

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URANIUM- 238 — URANIUM-239 — NEPTUNIUM - 239 — PLUTONIUM- 239

Figure 40.8 After U-238 absorbs a neutron, it emits a beta particle (and an antineutrino, not shown), which means that a neutron in the nucleus becomes a . The atom is no longer uranium, but neptunium. After the neptunium atom emits a beta particle it becomes plutonium.

The half-life of neptunium 239 is only 2.3 days, while the half-life of plutonium 239 is about 24 000 years. Since plutonium is an ele- ment distinct from uranium, it can be separated from uranium by ordinary chemical methods. Unlike the difficult process of separat- ing U-235 from 11-238, it is relatively easy to separate plutonium from uranium.

* At this writing, transuranic elements extend to 118. See the of the elements, Figure 17.11.

634 Chapter 40 Nuclear Fission and Fusion The element plutonium is chemically a poison in the same sense ,11 as are and arsenic. It attacks the nervous system and can cause 1 7 paralysis. Death can follow if the dose is sufficiently large. Fortu- nately, plutonium does not remain in its elemental form for long because it rapidly combines with oxygen to form three compounds, PuO, PuO2, and Pu203, all of which are chemically relatively benign. They will not dissolve in water or in biological systems. These pluto- nium compounds do not attack the nervous system and have been found to be biologically harmless. Plutonium in any form, however, is radioactively toxic. It is more toxic than uranium, although less toxic than radium. Pu-239 emits high-energy alpha , which kill cells rather than simply dis- rupting them and leading to mutations. Interestingly enpugh, dam- aged cells rather than dead cells contribute to cancer, which is why plutonium ranks low as a cancer-producing substance. The greatest danger that plutonium presents to humans is its potential for use in nuclear fission bombs. Its usefulness is in breeder reactors.

are employe 40.4 The tiClealiDOWer acUity. They must When small amounts of Pu-239 are mixed with U-238 in a reactor, Pve.1"sdlid understand- n).* the fissioning of plutonium liberates neutrons that convert the ing Pf,thd process of abundant, nonfissionable U-238 into more of the fissionable Pu-239. nuclear fisslon and chain This process not only produces useful energy, it also "breeds" more reactions as well as the properties of radioactive ce fission fuel. A reactor with this fuel is a breeder reactor. Using a breeder reactor is like filling a gas tank in a car with water, adding materials. some , then driving the car, and having more gasoline after plant technicians monitor the trip than at the beginning, at the expense of common water! the processes at the After the initial high costs of building such a device, this is a very power plant and are economical method of producing vast amounts of energy. After a few trained to recognize problems and to immedi- years of operation, breeder-reactor power utilities breed twice as ately follow containment much fuel as they start with. procedures in the event First, it supplies plentiful elec- Fission power has several benefits. of an emergency. tricity Second, it conserves the many billions of tons of coal, oil, and that every year are literally turned to heat and smoke, and which in the long run may be far more precious as sources of organic than as sources of heat. Third, it eliminates the megatons

f-life

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dic Figure 40.9 A Pu-235, like U-235, undergoes fission when it captures a neutron. 635 of sulfur oxides and other poisons that are put into the air each year by the burning of these . The drawbacks include the problems of storing radioactive wastes, the production of plutonium and the danger of nuclear proliferation, low-level release of radioactive materials into the air and groundwater, and the risk of an accidental release of large amounts of radioactivity. Reasoned judgment is not made by considering only the benefits or the drawbacks of fission power. You must also compare its bene- fits and its drawbacks with those of alternate power sources. All power sources have drawbacks of some kind. The benefits versus the drawbacks of fission power is a subject of much debate.

II Question In a breeder reactor, what material is "bred"? What is this material bred from?

40.5 Mass-Energy Equivalence

The key to understanding why a great deal of energy is released in nuclear reactions has to do with the equivalence of mass and energy Recall from our study of special relativity in Chapter 16 that mass and energy are essentially the same—they are two sides of the same coin. Mass is like a super storage battery It stores energy—vast quantities of energy—which can be released if and when the mass decreases. If you stacked up 238 bricks, the mass of the stack would be Figure 40.10 A equal to the sum of the of the bricks. Is the mass of a U-238 is required to pull a nucleus equal to the sum of the masses of the 238 that from an atomic nucleus. make it up? Like so much ruled by relativity, the answer isn't obvious. This work goes into mass energy. To find the answer, we consider the work that would be required to separate all the nucleons from a nucleus. Recall that work, which transfers energy, is equal to the product of force and distance. Imagine that you can reach into a U-238 nucleus and, pulling with a force even greater than the attractive , remove one nucleon. That would require considerable work. Then keep repeating the process until you end up with 238 nucleons, stationary and well separated. What happened to all the work done? You started with one stationary nucleus containing 238 particles and ended with 238 separate stationary particles. The work done shows up as mass energy. The separated nucleons have a total mass greater than the mass of the original nucleus. The extra mass, multiplied by the square of the , is exactly equal to your energy input: A.E= Amc2. One way to interpret this mass change is to say that a nucleon inside a nucleus has less mass than its rest mass outside the nucleus. How much less depends on which nucleus. The mass difference is

• Answer

Fissionable Pu-239 is bred from nonfissionable U-238.

636 Chapter 40 Nuclear Fission and Fusion .„_r— ARE DIRECTED INTO THE ENTRANCE LINK TO TECHNOLOGY SLIT AT A VELOCITY REGULATED BY ENTRANCE ELECTRICAL AND MAGNETIC FIELDS IN AN SLIT GUN ASSEMBLY (NOT SHOWN) to rge SINGLE PROTONS STRIKE HERE Ills CARBON -12 IONS STRIKE HERE U-235 IONS STRIKE HERE U-238 IONS STRIKE HERE THIS IC ni A PI-10704P is nt-lic THE POL FILM ELECTRO

MWi, Ht otamitt9„,z/e.mnsliw,crtfti‘ev„x:pt, Figure 40.11 A :.%•191Yes.;..b.teeirl#-PagaP.Yeiit The mass spectrometer. Ions of a fixed speed are directed into the semicircu- dfla6agtOneragitiolinit lar "drum," where they are swept into semicircular paths by a strong mag- drift Agi:nst,p,Vp.w of air rgy netic field. Because of inertia, heavier ions are swept into curves of larger , toward and radii and lighter ions are swept into curves of smaller radii. The radius of the positivei*~' Te,harged deteatdr. The oin. curve is directly proportional to the mass of the ion. heavier the negOtive, on, r ies 6 . Idib br it will take to S. related to the "" of the nucleus. For uranium, the mass "reach the detector. difference is about 0.7%, or 7 parts in a thousand. The 0.7% reduced Ina body scan a per- . ns nucleon mass in uranium indicates the binding energy of the son stands momentarily, - nucleus, or how much work it would take to disassemble the atom. in an enclosed region dous. The standard nucleus by which others are compared is carbon- where puffs of air Ito 12, which has a mass of exactly 12.000 00 units.* In these units, a impinge on the body. The proton outside the nucleus has a mass of 1.007 28, a neutron has a air is then analyzed by the ct of mass of 1.008 66, and an has a mass of 0.000 55. The masses same technique. rus of the pieces that make up the carbon atom-6 protons, 6 neutrons, orce, and 6 —add up to 12.0989, about 0.8% more than the mass I keep of a C-12 atom. That difference indicates the binding energy of the ary C-12 nucleus. We will see shortly that binding energy is greatest in -ted the nucleus of . vith The masses of ions of of various elements can be accu- $s rately measured with a mass spectrometer (Figure 40.11). This impor- e mass tant device uses a magnetic field to deflect ions into circular arcs. of the The ions entering the device all have the same speed. The greater the inertia (mass) of the ion, the more it resists deflection, and the on greater the radius of its curved path. In this way the nuclear masses cleus. can be compared as the magnetic force sweeps heavier ions into e is larger arcs and fighter ions into smaller arcs.

These units are called units and are the units for atomic mass used in .

637 Figure A somewhat atomic mass 638 graph NUCLEAR MASS increases number. ATOMIC that 40.12 Chapter exaggerated. shows with The NUMBER 40 how curvature increasing nuclear Nuclear is Fission with sive through you there by obtain different the atom. anything. per occurs binding nucleus heavier pioLon other increases. nucleus Figure The value is halfway mass nucleus graph. fragments nium nucleons and greatest the A mass A nucleon as would graph From MASS / NUCLEON increasing are graph of for more Fusion nucleus. None nucleus number atomic the at Most has nuclei a uranium 40.13 would energy does. between splits the proportionally average of the nucleon.) Progressing shows are for the For nuclear is get Cie important your of from heaviest of important, the less element combined graph number Beyond elements the in is the are the Iron gieutzs: take of atomic per A that split lightest whole two, than effective is hydrogen uranium nucleons nuclear average effectively proton's mass shown holds nucleon more you nuclei. the into Fe the iron. beyond the graph iron, increases. P. number mass class more nuclei, mass note lighter in - can per 1 - masses its work mass nuclei masses masses 0/Y\ in (The mass and the mass This in the through is see nucleon, nucleons results Figure that smaller. by per neutrons the the hydrogen, less uranium hcr hydrogen than vertical as average per per IC the why means per The of nucleon of is the for least expected particular of than lower binding the nucleon nucleon 40.12. number from lc uranium person.) iron NUMBER nucleons energy slope the mass is The simply scale for more in fi that the nucleus. effective it is ssion elements atomic on and the the the iron, The low is not curves —elements per nuclei covers energy when is in than pulling the tightly nucleus. of masses The more divide plot heavier (Figure a point fragments released and graph in nucleon iron. people constant horizontal When number. mass atomic pulling graph slightly iq of only the has from —it massive than apart of of the nuclear slopes of 40.13). than (If same an about in a isn't the are of this when in nucleons hydrogen for nuclear indicates you hydrogen any nuclei. lie intermediate your apart nucleons because If the an more graph scale iron, all decrease bound upward a atoms. set about 1% To divided mass other iron a uranium fi nuclei. class, ura- ssion of any of A mas- the mass of in the the to the It in i Figure 40.14 trd The mass of a uranium nucleus rnas is greater than the combined 2 MORE MASS PER ie masses of the fission fragments NUCLEON: LOOSER (plus any ejected neutrons). The Is. BINDING difference in the masses is the ss energy released in the fission LESS MASS PER process. mass NUCLEON: TIGHTER ided BINDING [ass, the I A en nd to Fe Li Ls in ATOMIC NUMBER mass is multiplied by the speed of light squared, it is equal to the any energy yielded by each uranium nucleus that undergoes fission. her You can think of the mass-per-nucleon graph as an energy valley ons that starts at hydrogen (the highest point) and drops steeply to the , the lowest point (iron), and then rises gradually to uranium. Iron is at the bottom of the energy valley, which is the place with the greatest bind- ing energy per nucleon. Any nuclear transformation that moves nuclei toward iron releases energy Heavier nuclei move toward iron by divid- ing—nuclear fission. A drawback is the fission fragments, which are radioactive because of their greater-than-normal number of neutrons. A more promising source of energy is to be found when lighter- than-iron nuclei move toward iron by combining—as indicated on the left side of the energy valley. ENERGY Figure 40.15 RELEASED MASS DIFFERENCE The mass of a nucleus is not equal to the sum of the masses of its parts. Fission fragments of a heavy nucleus (including gisraces ejected neutrons) have less total SPEED OF LIGHT mass than the nucleus. Why is there a difference in mass?

40.6 Nuclear Fusion nuclei. It mediate Inspection of the graph of Figure 40.13 will show that the steepest of the % part of the energy hill is from hydrogen to iron. Energy is gained as light nuclei fuse, or combine, rather than split apart. This process is nuclear fusion, the opposite of nuclear fission. Whereas energy is a ura- released when heavy nuclei split apart in the fission process, energy uranium is released when light nuclei fuse together. After fusion, the total bout mass of the light nuclei formed in the fusion process is less than the ale of the total mass of the nuclei that fused (Figure 40.16). ; fission Atomic nuclei are positively charged. For fusion to occur, they et of normally must collide at very high speed in order to overcome electri- ecrease in cal repulsion. The required speeds correspond to the extremely high

639 PROTON HAS MORE MASS BY ITSELF... g me

A NUCLEUS

Fe ATOMIC NUMBER

Figure 40.16 A temperatures found in the center of the and other . Fusion (Left) The mass of a single pro- brought about by high temperatures is called — ton is more than the mass per that is, the welding together of atomic nuclei by high temperature. In nucleon in a helium-4 nucleus. the hot central part of the sun, approximately 657 million tons of When protons fuse to form hydrogen are converted into 653 million tons of helium each second. helium, mass is reduced and The missing 4 million tons of mass is discharged as . energy is released. (Right) Two protons and two neutrons have Such reactions are, quite literally, nuclear burning. more total mass when they are Thermonuclear fusion is analogous to ordinary chemical com- free than when they are com- bustion. In both chemical and nuclear burning, a high temperature bined in a helium nucleus. starts the reaction; the release of energy by the reaction maintains a high enough temperature to spread the fire. The net result of the is a combination of atoms into more tightly bound molecules. In nuclear reactions, the net result is more tightly bound nuclei. The difference between chemical and nuclear burning is essentially one of scale.

• Ouestions 1. First it was stated that nuclear energy is released when atoms split apart. Now it is stated that nuclear energy is released when atoms combine. Is this a contradiction? How can energy be released by opposite processes? 2. To get energy from the element iron, should iron be fissioned or fused?

II Answers

1. Energy is released only in a nuclear reaction in which the mass per nucleon decreases. Light nuclei, such as hydrogen, lose mass and release energy when they combine (fuse) to form heavier nuclei. Heavy nuclei, such as uranium, lose mass and release energy when they split to become lighter nuclei. For energy release, "Lose Mass" is the name of the game—any game. 1 Explore ,4 2 Develop 3 Apply 2. Iron will release no energy at all, because it is at the very bottom of the energy valley. 2 Concept-Development If fdsedwith something else, it climbs the right side of the hill and gains mass. If fis- Practice Book 40-1 sioned, it climbs the left side of the hill and gains mass. In gaining mass, it absorbs energy instead of releasing energy.

640 Chapter 40 Nuclear Fission and Fusion 40.7 Controlling Nuclear Fusion

Producing thermonuclear fusion reactions under controlled condi- tions requires temperatures of hundreds of millions of degrees. Producing and sustaining such high temperatures along with rea- sonable is the goal of much current research. There are a variety of techniques for attaining high temperatures. No matter how the temperature is produced, a problem is that all materials melt and vaporize at the temperatures required for fusion. The solution to this problem is to confine the reaction in a nonmaterial container. Acagnctic field is nonntatoritil, can exist at any tcmperaturc, and can exert powerful forces on charged particles in . "Magnetic walls" of sufficient strength provide a kind of magnetic straitjacket for hot ionized gases called plasmas. Magnetic compres- sion further the to fusion temperatures. L— In

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Figure 40.17 A A magnetic bottle used for containing plasmas for fusion research.

At a temperature of about a million degrees, some nuclei are moving fast enough to overcome electrical repulsion and slam together, but the energy output is much smaller than the energy used to heat the plasma. Even at 100 million degrees, more energy must be put into the plasma than will be given off by fusion. At about 350 mil- lion degrees, the fusion reactions will produce enough energy to be eases. self-sustaining. At this ignition temperature, nuclear burning yields a sustained power output without further input of energy. A steady 350 feeding of nuclei is all that is needed to produce continuous power. " is Fusion has already been achieved in several devices, but insta- bilities in the plasma have thus far prevented a sustained reaction. A alley. big problem is devising a field system that will hold the plasma in a f fis- stable and sustained position while an airiple number of nuclei fuse. .rbs A variety of magnetic confinement devices are the subject of much present-day research. 641 Another promising approach bypasses magnetic confinement alto- gether with high-energy . One technique is to aim an array of beams at a common point and drop solid pellets composed of frozen hydrogen isotopes through the synchronous cross fire (Figure 40.18). Other fusion schemes involve the bombardment of fuel pellets not by laser light but by beams of electrons, light ions, and heavy ions. As this book goes to press we are still looking forward to the great "Break-Even Day" when one of the variety of fusion schemes will sustain a yield of at least as much energy as is required to initiate it. is nearly ideal. Fusion reactors cannot become "supercritical" and get out of control because fusion requires no critical there is no air pollution because the only Figure 40.18 A mass. Furthermore, product Fusion with multiple laser beams. of the thermonuclear combustion is helium (good for children's bal- Pellets of frozen are loons). Except for some radioactivity in the inner chamber of the fusion rhythmically dropped into syn- device because of high-energy neutrons, the by-products of fusion are chronized laser cross fire. not radioactive. Disposal of is not a major peoblem. According to plan, the resulting heat will be carried off by molten to produce steam.

Figure 40.19 le• Pellet chamber at Lawrence Livermore Laboratory. The laser source is , the most power- ful laser in the world, which directs 10 beams into the target region.

The fuel for nuclear fusion is hydrogen—in particular, its heavier isotopes, deuterium (H-2) and (H-3). Hydrogen is the most plentiful element in the . The thermonuclear reaction that occurs most readily at an achievable temperature is the so-called + D—T reaction, in which a deuterium nucleus and a tritium nucleus fuse. Both of these isotopes are found in ordinary water. For exam- Zu .1. 2 u 7_ 3. 3 us I 1" l in 2 . m -r O n ple, 30 liters of seawater contains 1 gram of deuterium, which when fused releases as much energy as 10 000 liters of gasoline or 80 tons of TNT. Natural tritium is much scarcer, but given enough to get H + 304 42 n made in a fission reactor), a controlled thermonu- He started (it can be clear reactor will breed it from deuterium in ample quantities. Because of the abundance of fusion fuel, the amount of energy that Figure 40.20 A can be released in a controlled manner is virtually unlimited. Fusion reactions of hydrogen iso- The development of fusion power has been slow and difficult, topes that will be used for con- already extending over nearly fifty years. It is one of the biggest sci- trolled thermonuclear power (the every reactions in the sun are differ- entific and engineering challenges that we face. Yet there is ent). Most of the energy released reason to believe that it will be achieved and will be a primary is carried by the lighter-weight energy source for future generations. particles, protons and neutrons, Humans may one day travel to the stars in ships fueled by the which fly off at high speeds. same energy that makes the stars shine.

642 Chapter 40 Nuclear Fission and Fusion Chapter Assessment

"I\ In nuclear fusion, hydrogen nuclei fuse to form For: Study and Review helium nuclei, releasing large amounts of energy. EGo Online Visit: PHSchool.com • After fusion, the total mass of the products is pHschoacom Web Code: csd-6400 less than the mass of the nuclei that fused. • Thermonuclear fusion occurs at the high temperatures found in the center of the sun * conr,,ar, and other stars.

Nuclear fission, the splitting of atomic nuclei, • Sustained fusion under controlled conditions occurs when the repelling electrical forces in is the goal of continuing research. the nucleus overpower the attracting nuclear strong forces. • Fission is initiated by the absorption of a neu- Key Terms tron by a nucleus. It can spread in a chain breeder reactor (40.4) nuclear fusion (40.6) reaction in which neutrons ejected in one fis- sion event trigger fission in other nuclei. chain reaction (40.1) thermonuclear (40.1) fusion (40.6) • A critical mass of a fissionable element is nuclear fission (40.1) required for sustained fission to occur. • In a subcritical mass, the chain reaction dies out. In a supercritical mass, the chain reac- tion builds up explosively. Review Questions Check Concepts • In a breeder reactor, extra neutrons from a 1. What is the role of electrical forces in nuclear small amount of fissionable plutonium-239 fission? (40.1) are absorbed by nonfissionable uranium-238, 2. What is the role of a neutron in nuclear fission? converting it into plutonium-239. (40.1) vier • Fission reactors are efficient energy producers 3. Of what use are the neutrons that are produced 3t but generate radioactive wastes. when a nucleus undergoes fission? (40.1) It Nuclei of the lightest elements have the most 4. Why does a chain reaction not occur in ura- mass per nucleon, the nucleus of iron has the is nium ore? (40.1) least mass per nucleon, and the heaviest ele- ments have intermediate mass per nucleon. 5. a. Which isotope of uranium is most common? ten ms • When fission occurs, the total mass of the fis- b. Which isotope of uranium will fission? (40.1) sion fragments (including ejected neutrons) is 6. Jason sticks a dozen two-inch nails randomly nu- less than the mass of the fissioning nucleus. into an apple. Jane sticks half a dozen of the The missing mass is equivalent to the great same size nails randomly into each piece of hat energy released. an apple cut in half. Who will see more points of nails sticking out? (40.1) it' sci-

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643 7. Which has greater average path length inside the material-two separate pieces of uranium Think and Explain Think Critically or the same pieces stuck together? (40.1) 26. Why does a neutron often make a better 8. Which will leak more neutrons-two separate nuclear bullet than a proton? pieces of uranium or the same pieces stuck together? (40.1) 27. Why does a chain reaction die out in small pieces of fissionable fuel, but not in large 9. Will a supercritical chain reaction be more pieces? likely in two separate pieces of U-235 or in the same pieces stuck together? (40.1) 28. If a piece of uranium is flattened into a pan- cake shape, will this make a supercritical chain 10. What controls the chain reaction in a nuclear reaction more or less likely? Why? reactor? (40.2) 29. Why are there no appreciable deposits of 11. Are the fission tragments nom a nuclear reac- plutonium in Earth's crust? tor light, medium, or heavy elements? (40.2) 30. Your tutor says atomic nuclei are converted 12. Why are the fission-fragment elements to energy in a nuclear reaction. Why should radioactive? (40.2) you seek a new tutor? 13. What happens when U-238 absorbs a neutron? 31. Is the mass of an atomic nucleus greater or (40.3) less than the total mass of the nucleons that 14. How can plutonium be created? (40.3) compose it? 15. Is plutonium an isotope of uranium or is it a 32. The energy release of nuclear fission is fled completely different element? (40.3) to the fact that the mass per nucleon of medium-weight nuclei is about 0.1% less 16. What is the effect of putting a little Pu-239 than the mass per nucleon of the heaviest with a lot of U-238 in a reactor? (40.4) nuclei. What would be the effect on energy 17. Is the mass per nucleon of a nucleus greater release if the 0.1% figure were 1%? than, less than, or the same as the mass of a 33. To predict the approximate energy release of nucleon outside a nucleus? (40.5) either a fission or a fusion reaction, explain 18. What device can be used to measure the rela- how a physicist makes use of the curve of tive masses of ions of isotopes? (40.5) Figure 40.13, or a table of nuclear masses, and the equation AE, Amc2. 19. What is the primary difference in the graphs shown in Figures 40.12 and 40.13? (40.5) 34. Which process would release energy from gold-fission or fusion? From carbon? From 20. What becomes of the loss in mass of nuclei iron? when heavy atoms split? (40.5) 35. If a uranium nucleus were to split into three 21. Why does helium not yield energy if fissioned? pieces of approximately the same size instead (40.5) of two, would more energy or less energy be 22. Why does uranium not yield energy if fused released? Defend your answer in terms of with something else? (40.6) Figure 40.13. 23. Why does iron not yield energy if fused with something else or fissioned? (40.6) 24. What becomes of the loss in mass when light atoms fuse to become heavier ones? (40.6) 25. Why are fusion reactors not a present-day reality like fission reactors? (40.7)

644 Chapter 40 Nuclear Fission and Fusion