Presentation on theme: "Session I.4.6 Part I Review of Fundamentals"— Presentation transcript:
1Session I.4.6 Part I Review of Fundamentals Module 4 Sources of RadiationSession 6 Basic Reactor Physics TheoryIAEA Post Graduate Educational CourseRadiation Protection and Safety of Radiation Sources
2Overview In this session we will discuss fission and fusion reactions We will also discuss criticality
4Fission Sample fission reactions: 235U + n 141Ba + 92Kr + 3n MeV235U + n 94Zr + 139La + 3n MeVThe fission reaction in U‑235 produces fission products such as Ba, Kr, Sr, Cs, I and Xe with atomic masses distributed around 95 and 135. Examples of typical reaction products are listed in this slide.In these equations, the number of nucleons (protons + neutrons) is conserved, e.g = A small loss in atomic mass is the source of the energy released. Both the barium and krypton isotopes subsequently decay and form more stable isotopes of neodymium and yttrium, with the emission of several electrons from the nucleus (beta decays). It is the beta decays, with some associated gamma rays, which make the fission products highly radioactive although the radioactivity decreases with time.The total energy released in fission varies with the precise break up. For 235U it averages about 200 MeV or 3.2 x 10‑11 joule.For Pu‑239 it averages about 210 MeV per fission. This is the total available energy released consisting of kinetic energy values (Ek) of the fission fragments plus neutron, gamma and delayed energy releases which add about 30 MeV.This contrasts with 4 eV or 6.5 x 10‑19 J per molecule of carbon dioxide released in the combustion of carbon in fossil fuels.
5Fission Follows neutron capture Thermal neutrons fission 233U, 235U, 239Pu which have odd number of neutronsFor isotopes with even number of neutrons, the incident neutron must have energy above about 1 MeVFission may take place in any of the heavy nuclei after capture of a neutron. However, low‑energy (slow or thermal) neutrons are able to cause fission only in those isotopes of uranium and plutonium whose nuclei contain odd numbers of neutrons (e.g. U‑233, U‑235, and Pu‑239).For nuclei containing an even number of neutrons, fission can only occur if the incident neutrons have energy above about one million electron volts (1 MeV).
6FissionThe probability that fission or any other neutron‑induced reaction will occur is described by the cross‑section for that reaction. The cross‑section may be imagined as an area surrounding the target nucleus and within which the incoming neutron must pass if the reaction is to take place. The fission cross sections increase greatly as the neutron velocity decreases. In nuclei with an odd‑number of neutrons, such as U‑235, the fission cross‑section becomes very large at thermal energies.As you can see on this graph, both scales are logarithmic.
7Fission Neutron Name/Title Energy (eV) Cold Neutrons 0 < 0.025 Thermal Neutrons0.025Epithermal Neutrons0.025 < 0.4Cadmium Neutrons0.4 < 0.6Epicadmium Neutrons0.6 < 1Slow Neutrons1 < 10Resonance Neutrons10 < 300Intermediate Neutrons300 < 1,000,000Fast Neutrons1,000,000 < 20,000,000Relativistic Neutrons>20,000,000A neutron is said to have thermal energy when it has slowed down to be in thermal equilibrium with its surroundings. In other words, the kinetic energy of the neutrons is similar to the kinetic energy of the surrounding atoms due to their random thermal motion.As can be seen from this table, neutrons can range in energy from 2.5 x 10-2 eV to over 2 x 107 eV, nine orders of magnitude.To promote fission in reactions where thermal neutrons are required, moderators such as water are used to slow the neutrons down.
8FissionThe fission chain reaction must be initiated by introducing some neutrons into the fissile material. This can be done by inserting an Am-Be, Pu-Be, Po-Be or Ra-Be source. The Am, Pu, Po or Ra emits alpha particles which release neutrons from the beryllium as it turns to carbon‑12.Using U‑235 in a thermal reactor as an example, when a neutron is captured, the total energy is distributed amongst the 236 nucleons (protons & neutrons) now present in the compound nucleus. This nucleus is relatively unstable and it is likely to break into two fragments of around half the mass. These fragments are nuclei found near the middle of the Periodic Table and the probabilistic nature of the break‑up leads to several hundred possible combinations as seen in this figure. Creation of the fission fragments is followed almost instantaneously by emission of a number of neutrons (typically 2 or 3, average 2.5), which enable the chain reaction to be sustained.
9transuranics from neutron capture fission productsandtransuranics from neutron captureThe graph shows the decay of fission products after removal from a reactor.
10Fission Source of energy released during fission: Kinetic energy of fission fragmentsGamma raysKinetic energy of neutrons emittedPromptDelayedAbout 85% of the energy released is initially the kinetic energy of the fission fragments. However, in solid fuel they can only travel a microscopic distance, so their energy becomes converted into heat. The balance of the energy comes from gamma rays emitted during or immediately following the fission process and from the kinetic energy of the neutrons. Some of the latter are immediate (so‑called prompt neutrons), but a small proportion (0.7% for U‑235, 0.2% for Pu‑239) are delayed, as these are associated with the radioactive decay of certain fission products. The longest delayed neutron group has a half‑life of about 56 seconds.
11Criticality Neutrons ejected during fission equal neutrons producing more fissions+ neutrons absorbed+ neutrons lost from systemCriticality is constant if balance exists. Fission rate (power) can be changed by varying the number of neutrons absorbed and/or controlling the number lostThe delayed neutron release is the crucial factor enabling a chain reacting system (or reactor) to be controllable and to be able to be held precisely critical. At criticality the chain reacting system is exactly in balance, such that the number of neutrons produced in fissions remains constant. This number of neutrons may be completely accounted for by the sum of those causing further fissions, those otherwise absorbed, and those leaking out of the system. Under these circumstances the power generated by the system remains constant. To raise or lower the power, the balance must be changed (using the control system) so that the number of neutrons present (and hence the rate of power generation) is either reduced or increased. The control system is used to restore the balance when the desired new power level is attained.
12Criticality Multiplication Factor (4 factor formula) Nf+1 Keff = Nf Nf+1 is the number of neutrons produced in the “f+1” generation by the Nf neutrons of the previous “f” generation
13CriticalitySub-Critical (keff < 1) – more neutrons lost by escape from system and/or non-fission absorption by impurities or “poisons” than produced by fission.Critical (keff = 1) – one neutron per fission available to produce another fissionSuper-Critical (keff > 1) – rate of fission neutron production exceeds rate of loss
14CriticalityKeff depends on the availability of neutrons with the required energy and the availability of fissile atomsAs a result, keff depends on composition, arrangement and size of fissile materialIf assembly is infinitely large, no neutrons are lost and Keff = L x k , where L is the non-leakage probability and K depends on 4 factors
15Criticality Let’s follow “n” fission neutrons through their life cycle η is mean number of neutrons emitted per absorption in uraniumif “n” fission neutrons are captured,n x η fission neutrons will be produced
16Criticalityν is the mean number of neutrons emitted per fission which depends on the fuel (2.5 for 235U and 3 for 239Pu)not every uranium absorption results in fission (238U can absorb thermal neutrons without fission)mean number of fission neutrons per absorption is less than ν
17Sample CalculationFor 100% enrichment of 235U, = What is for natural uranium? = x faf = (N5 x f 5)a = (N5 x a 5) + (N8 x a 8) = macroscopic cross section = microscopic cross section= average number of neutrons per fissionN = atoms per cm3a = absorption (5 = 235U)f = fission (8 = 238U)
18Sample Calculation f = a N8 = 2.5 for 235U For natural U, = 139 N5 (N5 x f 5)(N5 x a 5) + (N8 x a 8)(f 5)(a 5) + ( x a 8)N8N5=f 5 = 549 barnsa 5 = 650 barnsa 8 = 2.8 barns = 2.5 for 235UN8N5For natural U, = 139 = x 2.5 = 1.32 for natural U549650 + (139 x 2.8)
19Criticality238U has a small cross section for fission by fast neutrons (not thermal) = 0.29 barnsFast fission factor isε =total number of fission neutronsnumber of thermal fission neutrons
20Criticality ε depends on 3 factors: ratio of moderator to fuel ratio of inelastic scattering cross section to fission cross sectiongeometrical relationship between fuel and moderatorcapture of n thermal neutrons will producen x x ε fission neutrons
21CriticalityFor unmoderated pure U metal, ε = 1.29 which is the maximum valueFor homogenous fuel (such as a solution)ε is very close to 1
22Criticalitywhile fast neutrons are being slowed, they may be captured by 238U without fissionresonance for this capture occurs between 5 and 200 eVp is the probability that a neutron will escape this resonance capture
23Criticalityp = resonance escape probability (fraction of fast, fission produced neutrons that become thermalized)p depends on ratio of moderator to fuelfor high ratio of moderator to fuel, p 1for a low ratio of moderator to fuel, p smallfor pure unmoderated natural U, p = 0
24Criticalityfrom the original n thermal neutrons we have n x x ε x p thermal neutronssome of the thermal neutrons are absorbed by non-fuel atomssome are absorbed by 235U without fission (only 84% of thermal neutrons absorbed by 235U cause fission)
25Criticalityf = thermal utilization factor which is the fraction of all the thermal neutrons which are absorbed by the fuel (all of the U)the total number of new neutrons produced by the original n thermal neutrons is n x x ε x p x f
26Criticality aU f = aU + aM + ap aU = macroscopic cross section for UaM = macroscopic cross section for moderatorap = macroscopic cross section for other stuff
27Criticality K = = = εpf Nf+1 Nf nεpf n depends only on the fuel ε, p and f depend on the composition and arrangement of the fuelε varies from 1.29 for unmoderated U to almost 1 for homogeneous dispersion of fuel and moderatorp is about 0.8 to 1 (for pure 235U, p = 1)
28Reactivity and Reactor Control in a nuclear reactor the factors are combined to produce a controlled, sustained chain reactionexcess reactivity (k) is an increase in the multiplication factor (k) above 1k = k - 1
29Reactivity and Reactor Control For n neutrons in one generation, the number of additional neutrons in the next generation is nkIf the lifetime of a neutron generation is L sec, the time rate of change of neutrons isdndtnkL=
30Reactivity and Reactor Control dndtnkL=when integrated from no to n we getnnokL= etThe reactor period (T) is the time during which the neutrons (power level) increase by factor of “e”
31Reactivity and Reactor Control kL=1Tor the reactor period is:kLT =The mean lifetime of a neutron (birth to absorption) in pure 235U is about sec
32Reactivity and Reactor Control The mean lifetime of a neutron (birth to absorption) in pure 235U is about secAssume excess reactivity = 0.1% (k = 0.001)0.001T = = 1 secAnd the power level would increase by each second
33Reactivity and Reactor Control If k is increased to 0.5%0.0050.001T = = 0.2 secThe power level increase each second would betTnno= e = e = 15010.2This would be hard to control
34Reactivity and Reactor Control The actual mean generation time in a reactor is much greater than because % of fission neutrons are delayed from 0.3 seconds to 80 seconds. This delay permits the reactor to be controlled.
35Reactivity and Reactor Control Delayed NeutronsGroupni (%)Ti (sec)ni x Ti10.02670.330.00920.07370.880.06530.25263.310.83640.12558.971.12550.140132.784.59260.021180.391.688ni =nixTi = 8.315
36Reactivity and Reactor Control The mean generation time for all fission neutrons isdelayedpromptT = = = secniTini( x 0.001)100If k the reactor is prompt critical – the reaction can be sustained by prompt neutrons aloneIf k < the reactor is delayed critical – the delayed neutrons are needed to sustain the reaction
37Sample Calculation n no = e = 1.012 0.001 0.084 T = = 84 sec n no What is the reactor period and the increase in power level in 1 second for a generation time of sec for excess reactivity (k) of 0.1% and 0.5%For k = 0.001184nno= e = 1.0120.0010.084T = = 84 secFor k = 0.005116.8nno= e = 1.060.0050.084T = = 16.8 secThis would be hard to control
38Reactivity and Reactor Control Excess reactivity is measured in units of “dollars” and “cents” (one dollar = 100 cents) and “inhours” (inverse hours)One “dollar” worth of reactivity will cause the reactor to go prompt criticalOne “inhour” is the amount of excess reactivity which results in a reactor period of 1 hour
39Fission Control of Fission Fission typically releases 2-3 neutron (average 2.5)One is needed to sustain the chain reaction at a steady level of controlled criticalityThe other 1.5 leak from the core region or are absorbed in non‑fission reactionsFission of U‑235 nuclei typically releases 2 or 3 neutrons with an average of about 2.5. One of these neutrons is needed to sustain the chain reaction at a steady level of controlled criticality; on average, the other 1.5 leak from the core region or are absorbed in non‑fission reactions.
40Fission Control of Fission Boron or cadmium control rods absorb neutronsWhen slightly withdrawn the number of neutrons available for fission exceeds unity and the power level increasesWhen the power reaches the desired level, the control rods are returned to the critical positionNeutron‑absorbing control rods are used to adjust the power output of a reactor. These typically use boron and/or cadmium (both are strong neutron absorbers) and are inserted among the fuel assemblies. When they are slightly withdrawn from their position at criticality, the number of neutrons available for ongoing fission exceeds unity (i.e. criticality is exceeded) and the power level increases. When the power reaches the desired level, the control rods are returned to the critical position and the power stabilizes.
41Fission Control of Fission ability to control the reaction is due to presence of delayed neutronswithout delayed neutrons, change in the critical balance of the chain reaction would lead to a virtually instantaneous and uncontrollable rise or fall in the neutron populationsafe design and operation of a reactor sets strict limits on departures from criticalityThe ability to control the chain reaction is entirely due to the presence of the small proportion of delayed neutrons arising from fission. Without these, any change in the critical balance of the chain reaction would lead to a virtually instantaneous and uncontrollable rise or fall in the neutron population. It is also relevant to note that safe design and operation of a reactor sets very strict limits on the extent to which departures from criticality are permitted. These limits are built in to the overall design.
42Fission Control of Fission fission neutrons initially fast (energy above 1 MeV)fission in 235U most readily caused by slow neutrons (energy about 0.02 eV)moderator slows fast neutrons by elastic collisionsFor natural (unenriched) U only graphite and “heavy” water suitable moderatorsFor enriched uranium “light” water may be usedNeutrons released in fission are initially fast (velocity about 107 m/sec, or energy above 1 MeV), but fission in U‑235 is most readily caused by slow neutrons (velocity about 103 m/sec, or energy about 0.02 eV). A moderator material comprising light atoms thus surrounds the fuel rods in a reactor. Without absorbing too many, it must slow down the neutrons in elastic collisions (compare it with collisions between billiard balls on an atomic scale). In a reactor using natural (unenriched) uranium the only suitable moderators are graphite and heavy water (these have low levels of unwanted neutron absorption). With enriched uranium (i.e. increased concentration of U‑235), ordinary (light) water may be used as moderator. (Water is also commonly used as a coolant, to remove the heat and generate steam.)
43Fission Control of Fission commercial power reactors are designed to have negative temperature and void coefficientsif temperature too high or excessive boiling occurs rate of fission, and hence temperature, are reduced238U absorbs more neutrons as the temperature rises, pushing neutron balance towards subcriticalsteam within water moderator reduces its density which pushes neutron balance towards subcriticalCommercial power reactors are usually designed to have negative temperature and void coefficients. The significance of this is that if the temperature should rise beyond its normal operating level, or if boiling should occur beyond an acceptable level, the balance of the chain reaction is affected so as to reduce the rate of fission and hence reduce the temperature. One mechanism involved is the Doppler effect, whereby U‑238 absorbs more neutrons as the temperature rises, thereby pushing the neutron balance towards subcritical. Another mechanism in light water reactors is that the formation of steam within the water moderator reduces its density and hence its moderating effect, and this again tilts the neutron balance towards subcritical.
44Fission Control of Fission fuel gradually accumulates fission products and transuranic elements which increases neutron absorption (control system has to compensate)after about three years, fuel is replaced due to:build‑up in neutron absorptionmetallurgical changes as a consequence of the constant neutron bombardmentfuel burn‑up is effectively limited to about half of the fissile materialWhile fuel is in use in the reactor, it is gradually accumulating fission products and transuranic elements which cause additional neutron absorption. The control system has to be adjusted to compensate for the increased absorption. When the fuel has been in the reactor for three years or so, this build‑up in absorption, along with the metallurgical changes induced by the constant bombardment of the fuel materials, dictates that the fuel should be replaced. This effectively limits the burn‑up to about half of the fissile material, and the fuel assemblies must then be removed and replaced with fresh fuel.
45Fission SummaryNote that the 235U atom at the bottom of the picture can’t capture the fast neutron, however, the atom at the top can capture the neutron after it has been moderated.
46FusionIn 1920 Arthur Eddington suggested that the energy of the sun and stars was a product of the fusion of hydrogen atoms into heliumIn the core of the sun at temperatures of 10‑15 million degrees Celsius, hydrogen is converted to heliumSince the 1950's, great progress has been made in nuclear fusion research: however, the only practical application of fusion technology to date has been the "hydrogen" or thermonuclear bombNuclear Fusion is the energy‑producing process which takes place continuously in the sun and stars. In the core of the sun at temperatures of 10‑15 million degrees Celsius, Hydrogen is converted to Helium providing enough energy to sustain life on earth.The dream of harvesting energy from the same reaction that powers our sun has been around since 1920, when Arthur Eddington suggested that the energy of the sun and stars was a product of the fusion of hydrogen atoms into helium. Since the 1950's, great progress has been made in nuclear fusion research. However, the only practical application of fusion technology to date has been the "hydrogen" or thermonuclear bomb.
47Fusion Fusion has an almost unlimited potential The hydrogen isotopes in one gallon of water have the fusion energy equivalent of 300 gallons of gasolineA fusion power plant would have no greenhouse gas emissions and would not generate high level radioactive wasteExperts predict the world is still at least 50 years and billions of dollars away from having fusion generated electricity largely due to the enormous size and complexity of a fusion reactorResearchers stress that nuclear fusion has an almost unlimited potential to supply electricity. The hydrogen isotopes in one gallon of water have the fusion energy equivalent of 300 gallons of gasoline. A nuclear fusion power plant would also have no greenhouse gas emissions, and would generate none of the long lived, high level radioactive waste associated with conventional nuclear fission power plants.Despite its theoretical potential, leading experts predict that the world is still at least 50 years and billions of research dollars away from having electricity generated from nuclear fusion. This is largely due to the enormous size and complexity of a reactor that would be capable of sustaining nuclear fusion.
48Fusion Hydrogen atoms merged to create helium Helium mass is slightly less (1%) than the original mass with the difference being given off as energyRather than using hydrogen atoms, it is easier to promote fusion by using two isotopes of hydrogen, deuterium and tritiumDeuterium is a naturally occurring isotope of hydrogen which has one extra neutronOne hydrogen atom in 6700 occurs as deuterium and can be separated from the restNuclear fusion involves the binding together of hydrogen atoms, creating helium. The total mass of the final products is slightly less, one percent, than the original mass, with the difference being given off as energy. If this energy can be captured, it could be used to generate electricity.Rather than using normal hydrogen atoms, it has been found that it is easier to promote fusion by using two isotopes of hydrogen, deuterium and tritium. Isotopes are forms of the same chemical element which have the same number of protons in their nuclei, but a different number of neutrons. Deuterium is a naturally occurring isotope of hydrogen which has one extra neutron.One hydrogen atom in 6700 occurs as deuterium and can be separated from the rest.
49FusionDeuterium and tritium combine to produce helium and energy.
50Fusion Fusion A 56 Fission A 56 Atomic Mass Number 250 200 150 100 25020015010050-2-4-6-8-10Energy per Nucleon (MeV)Energy release per nucleon is larger for A = 2 or3 (deuterium or tritium) than for A = 235 (uranium).
51FusionTritium is very rare because it is naturally radioactive and decays quicklyTritium can be made by bombarding the naturally occurring element lithium with neutronsTritium could be created by having a "blanket" made of lithium surrounding a fusion containment vessel (this would result in a breeder reactor)Fusion can only be accomplished at temperatures typical of the centre of stars, (~100 million degrees Celsius)Tritium has two extra neutrons and is very rare, because it is naturally radioactive and decays quickly. Tritium can be manufactured by bombarding the naturally occurring element lithium with neutrons from either a fission or fusion reactor. Current thinking is that tritium would be created by having a "blanket" made of lithium surrounding a containment vessel. A reactor such as this, which "breeds" its own fuel, is called a breeder reactor.Unlike nuclear fission used in conventional nuclear power plants, there is no convenient method of starting a nuclear fusion reaction. Fusion can only be accomplished at temperatures typical of the centre of stars, about 100 million degrees Celsius.
52FusionThe fusion components exist in the form of a plasma, where atoms are broken down into electrons and nucleiNo known solid material could withstand the temperatures involved in nuclear fusion, so that a powerful confinement system is required to keep the plasma away from the walls of the vessel in which it is containedAt such temperatures, the fusion components exist in the form of a plasma, where atoms are broken down into electrons and nuclei. No known solid material could withstand the temperatures involved in nuclear fusion. Therefore, a powerful confinement system is required to keep the plasma away from the walls of the vessel in which it is contained.
53Fusion FuelsDeuterium can be extracted from water (If all the world's electricity were to be provided by fusion, deuterium would last for millions of years)Tritium does not occur naturally and will be manufactured from lithium within the machineLithium, the lightest metal, is plentiful in the earth's crust (if all the world's electricity were to be provided by fusion, known reserves would last for at least 1000 years)Even though fusion occurs between Deuterium and Tritium, the consumables are Deuterium and LithiumFuelsDeuterium is abundant as it can be extracted from all forms of water. If all the world's electricity were to be provided by fusion power stations, Deuterium supplies would last for millions of years.Tritium does not occur naturally and will be manufactured from Lithium within the machine.Lithium, the lightest metal, is plentiful in the earth's crust. If all the world's electricity were to be provided by fusion, known reserves would last for at least 1000 years.Once the reaction is established, even though it occurs between Deuterium and Tritium, the consumables are Deuterium and Lithium.
54Fusion FuelsFor example, 10 grams of Deuterium which can be extracted from 500 litres of water and 15g of Tritium produced from 30g of Lithium would produce enough fuel for the lifetime electricity needs of an average person in an industrialised countryQuantitiesFor example, 10 grams of Deuterium which can be extracted from 500 litres of water and 15g of Tritium produced from 30g of Lithium would produce enough fuel for the lifetime electricity needs of an average person in an industrialised country.
55Current Fusion Research Fusion research was big news in 1989 when it was reported that scientists had achieved fusion at room temperatures with simple equipmentUnfortunately, the scientists involved could not prove their claims and the experiments were not repeatableCurrently, two methods of confining the hot plasma are being studied around the world, "magnetic confinement" and "inertial confinement"Current ResearchFusion research was big news in 1989 when it was reported that scientists had achieved fusion at room temperatures with simple equipment. Unfortunately, the scientists involved could not prove their claims. Therefore, present day fusion researchers still cannot avoid their greatest barrier, the ultra hot temperatures required for sustaining nuclear fusion. There are currently two methods of confining the hot plasma which are being studied around the world, "magnetic confinement" and "inertial confinement".
56Current Fusion Research Magnetic confinement has the greatest potential and most research is now based on the "TOKOMAK" system (Tokomak is an acronym for the Russian words "torroidal magnetic chamber“)The tokomak system was developed in the former U.S.S.R. and has been taken to an advanced stage in the ITER project whose construction began in 2007A torroidal magnetic chamber is a doughnut-shaped steel structure in which the fusion plasma is confined by means of powerful coils of super‑conducting material which create a strong magnetic fieldMost experts feel that magnetic confinement has the greatest potential and most of the recent research on fusion reactors has been based on the "TOKOMAK" system. Tokomak is an acronym for the Russian words "torroidal magnetic chamber". The tokomak system was developed in the former U.S.S.R. and has been studied extensively in a number of countries. Work began in 2007 on the ITER project at the Cadarache site in France. This is an ambitious project aimed at taking the Tokomak system from the level of scientific research to an level closer to a power-producing reactor. The design intent is that 500 MW of power will be produced for every 50 MW of power input. It is a multi-billion euro project funded by the EU, India, Japan, China, Russia, South Korea and the USA.A torroidal magnetic chamber is a doughnut-shaped steel structure in which the fusion plasma is confined by means of powerful coils of super‑conducting material which create a strong magnetic field.
57Current Fusion Research The other method is inertial confinement (small amounts of a deuterium‑tritium mixture are rapidly heated to extremely high temperatures with a high powered laser beam or a beam of charged particles)Very high-power lasers are needed in the inertial confinement method. Biggest test facility is NIF. Less advanced than magnetic confinement.To be useful the energy produced must be many times that required to sustain the reactionEven the most optimistic researchers feel this will not be achieved before second half of the centuryThe other method of confining a fusion plasma is inertial confinement, where small amounts of a deuterium‑tritium mixture are rapidly heated to extremely high temperatures with a high powered laser beam or a beam of charged particles. Very high power lasers are needed, and work on inertial confinement is not as far advanced as that on magnetic confinement. The largest test facility is the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory.In November 1991, the British‑based Tokomak (JET) reported that it had achieved break even conditions. This occurs when the energy given off by the fusion reaction is equal to the energy input required to sustain the reaction. In order for a fusion reaction to generate useful amounts of electricity, the energy given off must be many times greater than that required to sustain the reaction. Even the most optimistic researchers feel that it will be the second half of the century before this stage is reached.
58Fusion Power PlantsA fusion reactor capable of generating 1000 MW of electricity would be very large and complexWhile fission reactors can be made small enough to be used in submarines or satellites, the minimum size of a fusion reactor would be similar to that of today's largest commercial nuclear plantsThe difficult part is creating a sustainable fusion reaction - capturing the energy to generate electricity is very similar to a fission reactorA 1000 MW fusion generator would consume only 150 kg of deuterium and 400 kg of lithium annuallyFusion Power PlantsA full scale fusion reactor capable of generating 1000 MW (1 MW = 1 million watts) of electricity, comparable to conventional nuclear power plants, would be a very large and complex machine. While fission reactors can be made small enough to be used in submarines or satellites, the minimum size and output of a fusion reactor would be similar to that of today's largest nuclear plants.Although a fusion reactor capable of generating electricity has never been built, the difficult part is creating a sustainable fusion reaction. Capturing the energy given off by the reaction in the form of heat and transforming the heat to electricity is very similar to generating electricity from a conventional fission reactor.A 1000 MW fusion generator would have a yearly fuel consumption of only 150 kg ofdeuterium and 400 kg of lithium.
59Advantages of FusionThe fuels required for fusion reactors, deuterium and lithium, are so abundant that the potential for fusion is virtually unlimitedOil and gas fired power plants as well as nuclear plants relying on uranium will eventually run into fuel shortages as these non‑renewable resources are consumedUnlike fossil plants, fusion reactors have no emission of carbon dioxide (contributor to global warming) or sulphur dioxide (cause of acid rain)Advantages of FusionNuclear fusion, if it can be developed, would have several advantages over conventional fossil fuel and nuclear fission power plants. The fuels required for fusion reactors, deuterium and lithium, are so abundant that the potential for fusion is virtually unlimited. Oil and gas fired power plants as well as nuclear plants relying on uranium will eventually run into fuel shortages as these non‑renewable resources are consumed. Like conventional nuclear plants, fusion reactors have no emission of carbon dioxide, the major contributor to global warming or sulphur dioxide, the main cause of acid rain. Fossil fuel power plants burning coal, oil and natural gas are large contributors to global warming and acid rain.
60Advantages of FusionBarriers to the widespread use of nuclear power have been public concern over operational safety, and the disposal of radioactive wasteAccidents such as Chernobyl are virtually impossible with a fusion reactor because only a small amount of fuel is in the reactor at any timeIt is also so extremely difficult to sustain a fusion reaction that, should anything go wrong, the reaction would invariably stopOne of the barriers to the widespread use of conventional nuclear power plants has been public concern over operational safety, and the disposal of radioactive waste. Major accidents, such as Chernobyl, are virtually impossible with a fusion reactor because only a small amount of fuel is in the reactor at any time. It is also so extremely difficult to sustain a fusion reaction, that should anything go wrong, the reaction would invariably stop.
61Advantages of FusionLong lived highly radioactive wastes are generated by conventional nuclear plantsRadioactive wastes generated by a fusion reactor are the walls of the vessel exposed to neutronsAlthough the quantity of radioactive waste produced by a fusion reactor might be slightly greater than that from a conventional nuclear plant, the wastes would have low levels of short lived radiation, decaying almost completely within 100 years.Long lived highly radioactive wastes are generated by conventional nuclear plants; these must be safely disposed of and represent a hazard to living things for thousands of years. The radioactive wastes generated by a fusion reactor are simply the walls of the containment vessel which have been exposed to neutrons. Although the quantity of radioactive waste produced by a fusion reactor might be slightly greater than that from a conventional nuclear plant, the wastes would have low levels of short lived radiation, decaying almost completely within 100 years.
62Disadvantages of Fusion The major disadvantages of nuclear fusion are the vast amounts of time and money which will have to be expended before any electricity is generated, even assuming that the technical/materials problems can eventually be solvedDevelopment of other electricity supply technologies such as photovoltaic cells, which convert sunlight directly into electricity, could potentially eliminate the need for fusion before it is operationalThe major disadvantages of nuclear fusion are the vast amounts of time and money which will be required before any electricity is generated by fusion. Fusion produces no greenhouse gases but will not be able to contribute to reducing carbon dioxide emissions until well after the middle of the century. If the world does nothing and waits for fusion as the solution to the global warming problem, it may well be too late. Development of other electricity supply technologies, such as photovoltaic cells which convert sunlight directly into electricity, could also potentially eliminate the need for fusion before it is operational.
63Where to Get More Information Cember, H., Johnson, T. E., Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2008)Martin, A., Harbison, S. A., Beach, K., Cole, P., An Introduction to Radiation Protection, 6th Edition, Hodder Arnold, London (2012)Glasstone, S., Sesonske, A., Nuclear Reactor Engineering, 4th Edition, Dordrecht:Kluwer Academic Publishers (1995)Friedberg, J., Plasma Physics and Fusion Energy, Cambridge University Press, Cambridge (2007)