energyphyx 1020usu 1360 chapter 6 nuclear power 2002 1 nuclear power nuclear power station, diablo...

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1 ENERGY PHYX 1020 USU 1360 CHAPTER 6 NUCLEAR POWER 2002 NUCLEAR POWER Nuclear Power Station, Diablo Canyon, CA

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NUCLEAR POWER

Nuclear Power Station, Diablo Canyon, CA

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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

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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

ElectronNucleus

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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

Protons

Neutrons

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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:

ZAXN

92235U143

92238U146

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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.

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Nuclear Reactions(2)

• In all cases the energy comes from a reduction in mass of the products before and after the nuclear reaction– If m is the mass change– The energy is given by Einstein’s equation

• E = mc2

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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– rays - energetic helium nuclei also called particles (4

2He2 )– rays - energetic electrons from the nucleus

• Neutrons change to protons with the emission of an electron– rays - very short wavelength electromagnetic waves– To maintain energy and momentum conservation neutrinos are

also sometimes emitted

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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:

– 23994Pu145 235

92U143 + 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

614C8→ 7

14N7 + e−,T12

= 5730yrs

Used to determine date of death of living plant/animal tissue

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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

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Nuclear Fission (2)

• 235U absorbs a neutron to become a very unstable isotope 236U which then undergoes fission to two fragments, 3 neutrons and rays

235U236U

Neutronn

n

n

Gammarays

GammaraysBarium

Krypton

92235U143+n→ 92

236U144→ 3690Kr54+56

143Ba87+3n

This is one of many pairs of fission products that are formed

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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

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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

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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

235U

238U

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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

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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)

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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.

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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

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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

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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.

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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.

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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

FuelRods

ControlRods

Data for at 1220MW reactor core, more information given in Table 6.1 (p.181)

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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

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Nuclear BWR Electrical Power Plant

• The generating part is the same as for a fossil fueled power plant

• Containment structure outside the reactor vessel

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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)

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Breeder Reactors (2)

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How much Energy from Uranium Fission?

n+235U→ 56144Ba+36

89Kr+3nAssume the fission reaction is:

We can compute the energy per fission by the mass loss:

Q=M 235U( )−M144Ba( )−M

89Kr( )−2mn[ ]c2=173MeV

Mass before Mass afterIn terms of measurable mass of uranium:

235 x 1.67x10-27kg produces 173 MeV (assume mp=mn)=1.67•10-27

Thus 1kg produces 173/(235x1.67x10-27) MeVOr 1kg produces 4.45x10-20 x(173/(235x1.67x10-27) )kWhI.e 1kg 235U produces 19.6x106 kWhThis corresponds to 1/0.03 ~ 33kg enriched uranium fuel

kg

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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

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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.

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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.

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Safety Issues (2)

Comparison of nuclear power plant risks with human events

Comparison of nuclear power plant risks with natural events

100 nuclear power plants

Total HumanCaused

100 nuclear power plants

Meteors

Total Natural

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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.

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Chernobyl

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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

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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

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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

25.6 tons

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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

104 in 1999

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Nuclear Power Plant Operations

1999 - 19.8%

Generating costs expressed in constant dollars.

Note nuclear cost rises from <coal to ~2 x coal in 1990.

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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•

12H1

13H2

11H0

12H1+1

3H1→ 24He2+n+17.6MeV

12H1+1

2H1→ 23He1+n+3.2MeV

12H1+1

2H1→ 13H2+n+4.0MeV

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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

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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

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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

Ene

rgy Electrostatic

repulsion

Nuclear forceattraction

d= 2 x 10-15 m

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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 fusionconfinement

1014

Breakeven

Ignition

109Confinement parameter (particle sec•cm-3)

Tem

pera

ture

(K

)

107

108

109Advance towardscontrolled fusion

(N)

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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

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Experimental Tokamak Reactor

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Inside the Torus

General Atomics Experimental Reactor

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International Thermonuclear Experimental Reactor (ITER)

• 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

Currently planned for 2010 operating date

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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.

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Nova Inertial Nuclear Fusion Device

Fusion Chamber

One Laser

Lawrence Livermore Laboratory

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Fusion Research in the USA

• Distribution of fusion laboratories in US with funds >$5M/yr (Magnetic and Inertial confinement)

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The Future of Nuclear Fusion• When will controlled nuclear fusion be a viable power source for

national use?– 50 years in the future - whenever the question is asked!

• Because of this funding has been drastically reduced for research into large pre-prototype machines

• The decision appears to be to go back to first principles and come up with new, innovative ideas on confining the hot, high pressure plasma– Perhaps active rather than passive control of the Tokamak plasma may be

the solution.– Universities may be the benefactors of the reduced and redirected funding.

• Whatever happens is does not look like the virtually unlimited energy from nuclear fusion will be on tap in the foreseeable future.– But breakthroughs do happen ...

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Learning Objectives (1)• Understand that nuclear energy comes from mass loss in

nuclear reactions converted to energy by E=mc2

• Know that the energy released is 100 million times greater than chemical energy

• Know the existence of three nuclear reactions radioactivity, nuclear fission and nuclear fusion

• Be aware of the model of the atom with negatively charged electrons surrounding a nucleus and the size scale.

• Know that the nucleus is positively charged due to the presence of protons

• Know that the other nuclear particle is the uncharged neutron

• Know of the existence of a strong nuclear force holding the nucleus together

• Understand that the number of protons defines the element• Understand that the number of neutrons in the nucleus of

an element can vary to give isotopes of the element• Be familiar with the nuclear composition notation Z

AXN

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Learning Objectives (2)• Understand that radioactivity is the spontaneous emission of radiation from a

nucleus.• Be aware of the types of emitted “rays”, alpha particles, beta particles and

gamma rays and that they are damaging to tissue• Be familiar with the term half life applied to radioactive elements• Understand the concept of nuclear fission• Be aware of the importance of the role of neutrons in fission• Understand why a chain reaction is possible in nuclear fission• Be familiar with the term critical mass in connection with nuclear fission• Know that the system used to harness fission energy is called a nuclear

reactor• Know the four main components of the core of a nuclear reactor and their

functions• Be aware that only 235U is readily fissile and that it is only 0.7% of the mined

uranium

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Learning Objectives (3)• Know that the process of increasing the amount of 235U is called

enrichment• Be aware that the neutrons in the reactor core will convert the 238U

into 239Pu a fissile material for bombs and reactors• Be familiar with the major parts of a Boiling Water Reactor power

plant and their functions• Know what is meant by a Breeder Reactor• Be aware of the limited supply of uranium accessible to mining• Understand that there are national and international security concerns

connected with the production of Pu in reactors• Be familiar with safety concerns about nuclear power plants• Understand where they stand in comparison with other hazards• Know the two major nuclear power plant accidents that have occurred

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Learning Objectives (4)• Understand that nuclear power plants produce radioactive waste.• Be aware of the time scale for the radioactivity to reduce to low

levels.• Be familiar with plans and considerations connected with

storing radioactive waste from nuclear reactors.• Be aware that expansion of the nuclear reactor program stopped

in 1978 and some reasons.• Understand what is meant by nuclear fusion.• Know the isotopes which are candidates for energy generation

by nuclear fusion.• Know the sources of deuterium and tritium.• Understand why it is difficult to bring nuclei close enough to

fuse.• Be aware that the fusion conditions can be met by high

temperatures and pressures.• Be aware of the Lawson criterion for nuclear fusion

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Learning Objectives (5)• Understand that the Lawson criterion for ignition has been

approached but not met.

• Know of the two methods being studied for confinement of nuclear fusion.

• Be familiar with the system called a Tokamak.

• Be familiar with what is meant by inertial confinement.

• Understand that future prospects for power extraction from controlled nuclear fusion are not very bright.