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NUCLEAR POWER

Nuclear Power Station, Diablo Canyon, CA. NUCLEAR POWER. Introduction. Nuclear energy is the conversion of mass energy in the nuclei of atoms into heat energy of the material containing the nuclei which undergoing a nuclear reaction.

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NUCLEAR POWER

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  1. Nuclear Power Station, Diablo Canyon, CA NUCLEAR POWER

  2. Introduction • Nuclear energy is the conversion of mass energy in the nuclei of atoms into heat energy of the material containing the nuclei which undergoing a nuclear reaction. • The magnitude of this energy conversion is about 108 times greater than that for chemical reactions (100s MeV compared to eV) • Fusion energy in the deuterium in 1 gallon of water = 300 gal gas • Some nuclear reactions occur spontaneously in elements that are termed radioactive • Two other nuclear reactions have also been studied - fission and fusion • Fission can be controlled to release energy on demand • The fusion reaction has yet to be harnessed for controlled energy release • Both fission and fusion reactions are the basis of weapons capable of enormous destruction due to the huge energy release • This legacy has greatly influenced public acceptance of nuclear power

  3. Electron Nucleus NOT TO SCALE Atomic Structure • NOT TO SCALE! • Nucleus is ~ 20,000 times smaller than the electron orbit diameter • Electrons have negative charge • Nucleus has positive charge • Most of mass is in the nucleus

  4. Protons Neutrons The Nucleus (1) • The nucleus consists of a group of particles called protons and neutrons • Protons have positive charge • Neutrons have no charge • In a neutral atom the number of protons equals the number of orbiting electrons so the net charge is zero. • The number of neutrons approximately equals the number of protons • The chemical element is determined by the number of protons in the nucleus • Deviations in neutron count for a fixed number of protons are isotopes of the element • Protons held together by the strong nuclear force which overwhelms the electrostatic repulsion force

  5. The Nucleus (2) • The mass of protons and neutrons are almost equal: • Mp = 1.673 x 10-27kg = 1.0073 amu (atomic mass unit) • Mn = 1.675 x 10-27kg = 1.0087 amu • 1 amu is defined as 1.66 x 10-27kg • Me = 9.109 x 10-31kg (~1/1836 of mass of nuclear particles) • The magnitude of the electric charge is the same for the electron and proton, but opposite polarity • Notation used for nuclear structure • where • Z is the atomic number (number of protons) • N is the number of neutrons • A is the mass number (=Z + N) • X is the chemical symbol • E.g. two isotopes of uranium are written:

  6. Nuclear Reactions(1) • There are three nuclear reactions we will discuss: • Radioactivity • Spontaneous change to the nucleus by the emission of energetic particle(s) • Characterized by a half-life • The time for one half of the nuclei to change • Nuclear fission • The splitting of a nucleus into two parts accompanied by the emission of energy in the form of kinetic energy of emitted particles • Nuclear fusion • The joining of two nuclei to form a third resulting in a release of energy in the form of kinetic energy of the nucleus formed by fusion.

  7. Nuclear Reactions(2) • In all cases the energy comes from a reduction in mass of the products before and after the nuclear reaction • If Dm is the mass change • The energy is given by Einstein’s equation • E = Dmc2

  8. Radioactivity(1) • Radioactivity is the term used to describe the spontaneous changes that can occur when nuclei are unstable. • Many stable nuclei have unstable isotopes • We will see the importance of these when we discuss environmental issues surrounding the use of nuclear energy. • The term comes from the radiation emitted by these nuclei which were termed rays in the early studies of radioactivity • a rays - energetic helium nuclei also called a particles (42He2 ) • b rays - energetic electrons from the nucleus • Neutrons change to protons with the emission of an electron • g rays - very short wavelength electromagnetic waves • To maintain energy and momentum conservation neutrinos are also sometimes emitted

  9. Radioactivity(2) • The first three of these radiations are damaging to animal and plant tissue and are fatal in large doses. • An example of radioactive decay: • 23994Pu14523592U143 + 42He2, T1/2 = 24,000 years • T1/2 , the half life for the reaction, the time for 1/2 of the Pu nuclei to decay • Another example - Carbon 14 Used to determine date of death of living plant/animal tissue

  10. Nuclear Fission (1) • In the late 1930s German scientists discovered that when uranium nuclei were bombarded with neutrons the uranium nuclei fissioned into two fragments of about equal mass • The fission fragments had a total energy of ~160MeV • In addition two or more neutrons were also released • This turned out to be VERY important for the harnessing of the released energy • As a result of intense research during WWII it was found that only the isotope of uranium 235U underwent fission. • Also it was found that a new element formed from 238U by neutron bombardment was also a fissile element • Three bombs were produced at Los Alamos in the famous WWII Manhattan project • One was tested in the New Mexico desert • Two were used in warfare against Japan • Hiroshima and Nagasaki • A legacy of this action has been public opposition to nuclear energy

  11. Krypton Gamma rays n Neutron n n 236U 235U Gamma rays Barium Nuclear Fission (2) • 235U absorbs a neutron to become a very unstable isotope 236U which then undergoes fission to two fragments, 3 neutrons and g rays This is one of many pairs of fission products that are formed

  12. Chain Reaction (1) • To generate the fission energy rapidly and so generate large amounts of power there must be many fissions per second • This is where the importance of more neutrons being produced than the on to start the fission is crucial • Each of the released neutrons can produce another fission and so the number of fissions builds up rapidly • Until an explosion in a bomb • Or controlled by the degree of neutron absorption in a reactor • This multiplication of fission reactions is called the chain reaction, illustrated on the next slide

  13. Chain Reaction (2) • Note the exponential growth of the number of fissions • However, to facilitate this there is an important step between the emission of the neutrons after fission and their involvement in the next fission

  14. 235U 238U Chain Reaction (3) • The probability for a neutron to cause fission in a 235U nucleus (labeled Fission cross section in figure) is greatly enhanced if its energy is reduced to a low value (~0.025eV) • However it is generated at about 2MeV

  15. Critical Mass • Another factor in determining whether a chain reaction will grow or fizzle out is whether enough neutrons remain within the block of uranium. • If the block is too small too many neutrons will escape through the edge and not be available for producing additional fissions. • The minimum size for this not to occur is called the critical mass. • For a bomb with almost pure 235U it is not very large • For a reactor with only a small fraction of the uranium being 235U it is much larger • Accidents have happened and still happen when too much uranium finds itself in the same place

  16. Nuclear Reactors • Nuclear reactor is the name given to the system used to control nuclear fission and remove the energy released in fission as heat energy in the form of pressurized high temperature steam. • The steam is then used in the same manner as steam from a fossil fuel boiler to drive a turbine, turn a generator and produce electrical energy • The nuclear reactor core has four major components: • Fissile fuel to release energy from mass • A coolant to remove the heat from the fuel • A moderator to reduce the energy of the neutrons to increase the probability of their producing a fission reaction • Control rods to control the number of neutrons and thereby control the number of fissions /second (i.e. power output of the reactor)

  17. Reactor Fissile fuel(1) • Mined uranium ore is a mixture of the two isotopes 235U and 238U of which only 0.7% is the fissile isotope 235U. • In order that it can be used as a fuel in a nuclear reactor the 235U must be enriched to be 3% of the uranium. • This is difficult because the isotopes are chemically identical • Separation must use the physical difference of the masses of the isotopes • Differential diffusion speeds is a method that is used. • The enriched uranium in the form of its oxide are formed into pellets and fill a long thin tube of an alloy of zirconium. • These are called fuel rods.

  18. Reactor Fissile fuel(2) • The energy released in the fuel is converted to heat energy and conducted through the casing to a fluid which removes the heat to the heat engine (turbine) • The fuel rods remain in the reactor for ~3 years • Initially the energy comes from the fission of 235U • Later significant amounts of 238U have been converted to the man made fissile isotope 239Pu by neutron bombardment which contributes to energy release. • The formation of 239Pu in nuclear reactors is an ominous problem for the exploitation of nuclear energy which we will discuss later

  19. Reactor Coolant • The most common type of reactor to produce heat energy to generate electricity is the Boiling Water Reactor (BWR) • The coolant for the core of the reactor is regular water which is turned into steam by the heat energy resulting from the energy release by the uranium fission • It is essential that the coolant keeps flowing through the core to prevent the core temperature rising to a level where meltdown of the core occurs. • A variant is the Pressurized Water Reactor (PWR) which uses more highly enriched uranium and can operate for ~15 years with refuelling. • The water coolant remains liquid at very high pressure and temperature and then generates steam in a separate heat exchanger. • France and Russia use PWR for water cooled reactors in their nuclear power plants • Also marine nuclear power plants are normally PWR

  20. Reactor Moderator • Recall that neutrons are much more likely to cause fission if they have very low energies - fractions of an eV. • The purpose of the moderator is to reduce the neutron energy as a result of collisions between moderator nuclei and the neutrons. • The greatest energy loss occurs if the neutron collides with a nucleus having the same mass (think of billiard balls colliding) • The only candidate with equal mass is hydrogen, but it is not possible to have very dense material which is just hydrogen inside the reactor • A compromise is to use solid material with a low nuclear mass: • Graphite is a common substance used because of the high melting point of carbon and its low nuclear mass of 12 amu (neutron & proton are ~1 amu) • The hydrogen in the coolant water will also contribute to the moderation of the neutron energy and is used as the moderator in water cooled reactors.

  21. Reactor Control Rods • The method of controlling a nuclear reactor is to limit the number of neutrons available to produce further fissions of the uranium. • This can be done by introducing material which absorbs neutrons into the core of the reactor and engineering it so that the amount in the core can be varied. • The means of doing this is by control rods of a boron compound which can be inserted to variable depth in the core of the reactor. • Full insertion of the rods will shut down the chain reaction • Shut down is not instantaneous because of neutrons emitted from radioactive fission fragments in the fuel rods and the structural material of the core. • Boron can be added to the coolant in case of the need for an emergency shut down with malfunctioning control rods.

  22. Control Rods Fuel Rods Schematic Reactor Core • Fuel rods in a matrix • 46,376 rods in 193 in diameter • Gaps between fuel rods allow water to be pumped through the core. • Control rods move in between fuel rods • 177 control rods • In this design water serves as both coolant and moderator Data for at 1220MW reactor core, more information given in Table 6.1 (p.181)

  23. Schematic Boiling Water Reactor (BWR) Core • Note: • Forced circulation of coolant • Separation of steam and water • Container walls made of 6 in thick steel • Return water from condensers

  24. Nuclear BWR Electrical Power Plant • The generating part is the same as for a fossil fueled power plant • Containment structure outside the reactor vessel

  25. Breeder Reactors (1) • We noted earlier that 238U is converted to 239Pu as a result of neutron bombardment. • 239Pu is a fissile element, so we see that if we expose the naturally occurring 238U to a neutron flux we can make 239Pu - a nuclear fuel. • This is the basis of the breeder reactor which produces more nuclear fuel than it uses - hence “Breeder” • This type of reactor use 239Pu as its primary fuel which unlike 235U has a higher probability of fission for fast neutrons. • This means do not include moderator-like materials • Leads to use of a heavier nucleus coolant than water. • The coolant of choice is liquid sodium • This is why this type of reactor is called Liquid Metal Fast Breeder Reactor (LMFBR)

  26. Breeder Reactors (2)

  27. Mass before Mass after 235 x 1.67x10-27kg produces 173 MeV (assume mp=mn)=1.67•10-27 Thus 1kg produces 173/(235x1.67x10-27) MeV Or 1kg produces 4.45x10-20 x(173/(235x1.67x10-27) )kWh I.e 1kg 235U produces 19.6x106 kWh This corresponds to 1/0.03 ~ 33kg enriched uranium fuel kg How much Energy from Uranium Fission? Assume the fission reaction is: We can compute the energy per fission by the mass loss: In terms of measurable mass of uranium:

  28. Uranium Inventory • It is estimated that there is ~3x106 tonnes of uranium in the USA. (1 tonne = 1,000 kg (2,200 lb) metric ton) • About 200 tonnes of mined uranium produce 1GW.yr of electrical energy (1.1 tonnes of 235U) • The output of the 109 reactors operating in the USA is 99GW • Thus life of uranium is: • 3x106 / (200 x 99) = 152 years • If nuclear power replaced fossil fuel electrical power the time would be reduced by a factor of 5 i.e ~30 years • With breeder reactors this time would increase by a factor of nearly 200 to ~6000 years. • Because all of mined uranium could be used for fission

  29. National Security Issues • Based on previous calculations we might ask why the US has shut down its breeder reactor development program • National security • Safety • National security • Breeder reactors produce large amounts of plutonium • Small amounts of plutonium are needed to make nuclear weapons • Thus keeping track of the inventory of plutonium to the accuracy to be sure none had got into the wrong hands would be a nightmare. • International security • The government is concerned about to development of conventional nuclear reactors in other countries. • All nuclear reactors produce plutonium which remains in the spent fuel • Thus the commissioning of a nuclear power plant is also commissioning a plutonium factory.

  30. Safety Issues (1) • Another major impediment to the development of nuclear energy is the perception that nuclear power plants are more dangerous than other power plants. • This is probably a legacy of the use of nuclear weapons in WWII and subsequent testing of these weapons by the USA, USSR and other countries. • Graphic visual evidence of the blast effects of uncontrolled nuclear energy release. • Graphic descriptions of the effects of radioactivity on human beings. • Lack of public knowledge of the obscure phenomenon of radiaoactivity. • Propaganda about the results of nuclear war. • Gradual release of information on the effects of fallout on people from above ground testing of nuclear weapons. • The public were led to believe that nuclear power stations were a hairsbreadth from being nuclear bombs. • In fact the situation is very different due to safeguards built into the design of nuclear power stations.

  31. Total Natural Total Human Caused 100 nuclear power plants 100 nuclear power plants Meteors Safety Issues (2) Comparison of nuclear power plant risks with human events Comparison of nuclear power plant risks with natural events

  32. Nuclear power plant accidents • 1979 • Three Mile Island plant in Pennsylvania • Loss of coolant • Severe core damage • Minimal release of radioactive material • Containment worked • 1986 • Chernobyl plant in Ukraine • Explosion and fire destroyed reactor • Containment breached • Large release of radioactive material • High level of local fallout • Lower levels carried by wind to northern Europe • Eventually radioactive material carried around the world • In each case the accident was caused by human error in contravening correct operating procedures.

  33. Chernobyl

  34. Radioactive Waste • Although nuclear power plants do not emit dangerous materials into the atmosphere, there is a waste problem. • The fuel rods are mostly spent after 2-3 years and are removed from the reactor core and replaced by new fuel rods. • The fuel rods are very radioactive at removal. • Unstable isotopes of fission fragments • 239Pu produced from 238U in the fuel • Unstable isotopes of the structural material in the fuel rod casing. • The problem is where to put them. • First they are placed in water tanks • Cools rods heated by radioactive decay • Absorbs much of emitted radiation • Planned for ~150 days in water tanks, but many have accumulated in tanks for many years. • Nowhere else to take them

  35. Radioactive Waste (2) • The diminution of radioactivity of nuclear reactor waste with time is generally slow. • For individual isotopes it is measured by the half-life (time for one half of the nuclei to decay) • The aggregate decay of radioactivity for nuclear waste is illustrated by the example of the waste from a 1000MW reactor . • Total radioactivity after 1 year 70MCi • After 10 years 14MCi • After 100 years 1.4MCi • After 100,000 years 0.002MCi • (1Ci (Curie) = 3.7 x 1010 decays/sec, Lab sources are measured in microcuries) • The long time scales mean that storage until safe to handle must be considered on a geological time scale

  36. 25.6 tons Radioactive Waste (3) • The proposed long term storage facility under consideration is at Yucca Mountain, NV. • Containers have been approved to maintain the integrity of the unit for 1000’s years and absorb most of the emitted radiation • Radiation emission in transit is an important consideration

  37. 104 in 1999 Nuclear Power Economics • “Too cheap to meter” was the early promise of nuclear power. • In practice the power plants were very costly to build and were hard to complete on time thereby adding to capital costs. • Safety features added to the costs as did retrofits of more safety features mandated after Three Mile Island. • This has resulted in no new power plants ordered after 1978. Note decline in operable plants - due to older plants being retired

  38. 1999 - 19.8% Nuclear Power Plant Operations Generating costs expressed in constant dollars. Note nuclear cost rises from <coal to ~2 x coal in 1990.

  39. Nuclear Fusion (1) • Considerations of the binding energy of nuclei predict that if light nuclei can be made to fuse together to form a heavier nucleus, energy will be released due to a mass loss in the process. • Likely candidates for fusion reactions are the isotopes of hydrogen ( ) • Deuterium or D which is proton + neutron • Tritium or T which is proton + two neutrons • Fusion reactions involving these isotopes of hydrogen are: • D-T • D-D • D-D

  40. Diagram of Nuclear Fusion • Depicts the nuclei involved in a D-T fusion reaction • Energy multiplication is up from the thermal energy of the nuclei before they fused • Collisions convert the energy of the released particles into thermal energy

  41. Nuclear Fusion (2) • The reactions on the previous slide have been carefully studied since the D-T reaction is the energy source for hydrogen bombs and both that and the D-D reactions have been considered for controlled nuclear fusion as a power source. • D is found in water at the level of ~ 1:5000 which amounts to a huge amount of D in the oceans. • T is man-made from neutron bombardment of Lithium • It is radioactive with a 12 year halflife and is not found naturally • The basic issue with producing fusion is how to bring the nuclei close enough together given the strong electrostatic repulsion experienced by the like-charged nuclei. • They must be brought close enough so that the attractive strong nuclear force can overwhelm the repulsive electrostatic force

  42. Electrostatic repulsion Energy d= 2 x 10-15 m Nuclear force attraction Nuclear Fusion (3) • Note that classically for the electrostatic repulsion to be overcome by the strong nuclear force the relative energy must exceed 0.7MeV. • If large numbers of the particles are to exceed this energy it can be achieved by raising the nuclei to a very high temperature. • 10’s of millions of degrees • In the hydrogen bomb this is achieved by the energy released by a fission bomb. • In the sun by very high pressure in the interior • On earth two techniques have been tried. • Magnetic confinement • Inertial confinement

  43. Breakeven 109 Advance towards controlled fusion Ignition 108 Temperature (K) 107 1014 109 Confinement parameter (particle sec•cm-3) Nuclear Fusion (4) • A phenomenon known as quantum tunneling helps by not requiring the relative energy of the particle being at or above the peak in the previous diagram. • This translates into lower temperature requirements for fusion Lawson Criterion Density •conf.time >1014 sec.cm-3 The “Holy Grail” of fusion confinement (Nt)

  44. Magnetic Confinement of Fusion • The system which has emerged as most likely to achieve magnetic confinement of fusion is the TOKAMAK • From Russian words meaning toroidal magnetic chamber • After many years of work sustained ignition has not been achieved • Funding for Tokamak research in the USA has been severely cut • Below is a diagram of the principles of a Tokamak - not a power plant

  45. Experimental Tokamak Reactor

  46. Inside the Torus General Atomics Experimental Reactor

  47. International Thermonuclear Experimental Reactor (ITER) Currently planned for 2010 operating date • Because of the very high cost of fusion research an international team has been developed: • International Thermonuclear Experimental Reactor (ITER) • Plan to develop a large pre-prototype reactor

  48. Inertial Confinement of Fusion • An alternate way of reaching the Lawson criterion for pressure and confinement time at elevated temperatures is to compress and heat the D-T fuel mix with laser energy. • This relies on the reaction force on the skin of a pellet as it ablates due to heating by a laser beam. • This is actively being researched with very powerful laser installations. Note Laser beams symmetrically arranged so pellet stays in place D-T mix is compressed by inertial reaction as surfaces ablates, then heats by absorption of laser energy Still no sustained fusion by this method.

  49. One Laser Fusion Chamber Nova Inertial Nuclear Fusion Device Lawrence Livermore Laboratory

  50. Fusion Research in the USA • Distribution of fusion laboratories in US with funds >$5M/yr (Magnetic and Inertial confinement)

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