Transcript Chapter 45

Chapter 45
Applications of Nuclear Physics
Processes of Nuclear Energy
Fission
 A nucleus of large mass number splits into two smaller nuclei.
Fusion
 Two light nuclei fuse to form a heavier nucleus.
Large amounts of energy are released in both cases.
Introduction
Interactions Involving Neutrons
Because of their charge neutrality, neutrons are not subject to Coulomb forces.
As a result, they do not interact electrically with electrons or the nucleus.
Neutrons can easily penetrate deep into an atom and collide with the nucleus.
Section 45.1
Fast Neutrons
A fast neutron has energy greater than approximately 1 MeV.
During its many collisions when traveling through matter, the neutron gives up
some of its kinetic energy.
For fast neutrons in some materials, elastic collisions dominate.
 These materials are called moderators since they moderate the originally
energetic neutrons very efficiently.
 Moderator nuclei should be of low mass so that a large amount of kinetic energy is
transferred to them in elastic collisions.
 Materials such as paraffin and water are good moderators for neutrons.
Section 45.1
Thermal Neutrons
Most neutrons bombarding a moderator will become thermal neutrons.
 They are in thermal equilibrium with the moderator material.
 Their average kinetic energy at room temperature is about 0.04 eV.
 This corresponds to a neutron root-mean-square speed of about 2 800 m/s.
 Thermal neutrons have a distribution of speeds.
Section 45.1
Neutron Capture
Once the energy of a neutron is sufficiently low, there is a high probability that it
will be captured by a nucleus.
The neutron capture equation can be written as
n  ZA X 
1
0
A1
Z
X* 
A1
Z
X γ
 The excited state lasts for a very short time.
 The product nucleus is generally radioactive and decays by beta emission.
Section 45.1
Nuclear Fission
A heavy nucleus splits into two smaller nuclei.
Fission is initiated when a heavy nucleus captures a thermal neutron.
The total mass of the daughter nuclei is less than the original mass of the parent
nucleus.
 This difference in mass is called the mass defect.
 Multiplying the mass defect by c2 gives the numerical value of the released
energy.
 This energy is in the form of kinetic energy associated with the motion of the
neutrons and the daughter nuclei after the fission event.
Section 45.2
Short History of Fission
First observed in 1938 by Otto Hahn and Fritz Strassman following basic studies
by Fermi.
 Bombarding uranium with neutrons produced barium and lanthanum.
Lise Meitner and Otto Frisch soon explained what had happened.
 After absorbing a neutron, the uranium nucleus had split into two nearly
equal fragments.
 About 200 MeV of energy was released.
Section 45.2
Fission Equation: 235U
Fission of 235U by a thermal neutron
236
n 235
U

92
92 U*  X  Y  neutrons
1
0

236U*
is an intermediate, excited state that exists for about 10-12 s before
splitting.
 X and Y are called fission fragments.
 Many combinations of X and Y satisfy the requirements of conservation of energy
and charge.
Section 45.2
Fission Example: 235U
A typical fission reaction for uranium is
141
92
1
n  235
92U  56 Ba  36 Kr  3  0 n 
1
0
Section 45.2
Distribution of Fission Products
The most probable products have mass
numbers A  95 and A  140.
There are also 2 to 3 neutrons released
per event.
Section 45.2
Energy in a Fission Process
Binding energy for heavy nuclei is about 7.2 MeV per nucleon.
Binding energy for intermediate nuclei is about 8.2 MeV per nucleon.
An estimate of the energy released
 Releases about 1 MeV per nucleon
 8.2 MeV – 7.2 MeV
 Assume a total of 235 nucleons
 Total energy released is about 235 MeV
 This is the disintegration energy, Q
This is very large compared to the amount of energy released in chemical
processes.
Section 45.2
Chain Reaction
Neutrons are emitted when
235U
undergoes fission.
 An average of 2.5 neutrons
These neutrons are then available to trigger fission in other nuclei.
This process is called a chain reaction.
 If uncontrolled, a violent explosion can occur.
 When controlled, the energy can be put to constructive use.
Section 45.3
Chain Reaction – Diagram
Section 45.3
Enrico Fermi
1901 – 1954
Italian physicist
Nobel Prize in 1938 for producing
transuranic elements by neutron
irradiation and for his discovery of
nuclear reactions brought about by
thermal neutrons
Other contributions include theory of
beta decay, free-electron theory of
metal, development of world’s first
fission reactor (1942)
Section 45.3
Nuclear Reactor
A nuclear reactor is a system designed to maintain a self-sustained chain
reaction.
The reproduction constant K is defined as the average number of neutrons
from each fission event that will cause another fission event.
 The average value of K from uranium fission is 2.5.
 In practice, K is less than this
 A self-sustained reaction has K = 1
Section 45.3
K Values
When K = 1, the reactor is said to be critical.
 The chain reaction is self-sustaining.
When K < 1, the reactor is said to be subcritical.
 The reaction dies out.
When K > 1, the reactor is said to be supercritical.
 A run-away chain reaction occurs.
Section 45.3
Moderator
The moderator slows the neutrons.
 The slower neutrons are more likely to react with
235U
than 238U.
 The probability of neutron capture by 238U is high when the neutrons have high
kinetic energies.
 Conversely, the probability of capture is low when the neutrons have low kinetic
energies.
 The slowing of the neutrons by the moderator makes them available for
reactions with 235U while decreasing their chances of being captured by 238U.
Section 45.3
Reactor Fuel
Most reactors today use uranium as fuel.
 Naturally occurring uranium is 99.3% 238U and 0.7% 235U

238U
almost never fissions
 It tends to absorb neutrons producing neptunium and plutonium.
 Fuels are generally enriched to at least a few percent 235U.
Section 45.3
Pressurized Water Reactor – Diagram
Section 45.3
Pressurized Water Reactor – Notes
This type of reactor is the most common in use in electric power plants in the US.
Fission events in the uranium in the fuel rods raise the temperature of the water
contained in the primary loop.
 The primary system is a closed system.
This water is maintained at a high pressure to keep it from boiling.
This water is also used as the moderator to slow down the neutrons.
Section 45.3
Pressurized Water Reactor – Notes, cont.
The hot water is pumped through a heat exchanger.
The heat is transferred by conduction to the water contained in a secondary
system.
This water is converted into steam.
The steam is used to drive a turbine-generator to create electric power.
Section 45.3
Pressurized Water Reactor – Notes, final
The water in the secondary system is isolated from the water in the primary
system.
 This prevents contamination of the secondary water and steam by the
radioactive nuclei in the core.
A fraction of the neutrons produced in fission leak out before inducing other
fission events.
 An optimal surface area-to-volume ratio of the fuel elements is a critical
design feature.
Section 45.3
Basic Design of a Reactor Core
Fuel elements consist of enriched
uranium.
The moderator material helps to slow
down the neutrons.
The control rods absorb neutrons.
All of these are surrounded by a
radiation shield.
Section 45.3
Section 45.3
Control Rods
To control the power level, control rods are inserted into the reactor core.
These rods are made of materials that are very efficient in absorbing neutrons.
 Cadmium is an example
By adjusting the number and position of the control rods in the reactor core, the K
value can be varied and any power level can be achieved.
 The power level must be within the design of the reactor.
Section 45.3
Reactor Safety – Containment
Radiation exposure, and its potential health risks, are controlled by three levels of
containment:
Reactor vessel
 Contains the fuel and radioactive fission products
Reactor building
 Acts as a second containment structure should the reactor vessel rupture
 Prevents radioactive material from contaminating the environment
Location
 Reactor facilities are in remote locations
Section 45.3
Reactor Safety – Radioactive Materials
Disposal of waste material
 Waste material contains long-lived, highly radioactive isotopes.
 Must be stored over long periods in ways that protect the environment
 At present, the most promising solution seems to be sealing the waste in
waterproof containers and burying them in deep geological repositories.
Transportation of fuel and wastes
 Accidents during transportation could expose the public to harmful levels of
radiation.
 Department of Energy requires crash tests and manufacturers must
demonstrate that their containers will not rupture during high speed
collisions.
Section 45.3
Nuclear Fusion
Nuclear fusion occurs when two light nuclei combine to form a heavier nucleus.
The mass of the final nucleus is less than the masses of the original nuclei.
 This loss of mass is accompanied by a release of energy.
Section 45.4
Fusion: Proton-Proton Cycle
H11H21 H  e   
The proton-proton cycle is a series of
three nuclear reactions believed to
operate in the Sun.
1
1
Energy liberated is primarily in the form
of gamma rays, positrons and
neutrinos.
Then
All of the reactions in the proton-proton
cycle are exothermic.
An overview of the cycle is that four
protons combine to form an alpha
particle, positrons, gamma rays and
neutrinos.
Section 45.4
H 21H32 He  
1
1
H 32 He  42 He  e   
1
1
or
3
2
He  32 He  42 He 11H11H
Fusion in the Sun
These reactions occur in the core of a star and are responsible for the energy
released by the stars.
High temperatures are required to drive these reactions.
 Therefore, they are known as thermonuclear fusion reactions.
Section 45.4
Advantages of a Fusion Reactor
Inexpensive fuel source
 Water is the ultimate fuel source.
 If deuterium is used as fuel, 0.12 g of it can be extracted from 1 gal of water
for about 4 cents.
Comparatively few radioactive by-products are formed.
Section 45.4
Considerations for a Fusion Reactor
The proton-proton cycle is not feasible for a fusion reactor.
 The high temperature and density required are not suitable for a fusion
reactor.
The most promising reactions involve deuterium and tritium.
H  21H  32 H  01n Q  3.27 MeV
2
1
H  21H  31H  11H Q  403
. MeV
2
1
H  31H  42 He  01n Q  1759
. MeV
2
1
Section 45.4
Considerations for a Fusion Reactor, cont.
Tritium is radioactive and must be produced artificially.
The Coulomb repulsion between two charged nuclei must be overcome before
they can fuse.
 A major problem in obtaining energy from fusion reactions.
Section 45.4
Potential Energy Function
The potential energy is positive in the
region r > R, where the Coulomb
repulsive force dominates.
It is negative where the nuclear force
dominates.
The problem is to give the nuclei
enough kinetic energy to overcome this
repulsive force.
Can be accomplished raising the
temperature of the fuel to approximately
108 K.
At this temperature, the atoms are
ionized and the system contains a
collection of electrons and nuclei,
referred to as a plasma.
Section 45.4
Critical Ignition Temperature
The temperature at which the power
generation rate in any fusion reaction
exceeds the lost rate is called the
critical ignition temperature, Tignit.
The intersections of the Pgen lines with
the Plost line give the Tignit.
Section 45.4
Requirements for Successful Thermonuclear Reactor
High temperature ~ 108 K
 Needed to give nuclei enough energy to overcome Coulomb forces
Plasma ion density, n
 The number of ions present
Plasma confinement time, 
 The time interval during which energy injected into the plasma remains in the
plasma.
Section 45.4
Lawson’s Criteria
Lawson’s criteria states that a net
power output in a fusion reactor is
possible under the following conditions.
 n ≥ 1014 s/cm3 for deuteriumtritium
 n ≥ 1016 s/cm3 for deuteriumdeuterium
 These are the minima on the
curves.
Section 45.4
Requirements, Summary
The plasma temperature must be very high.
To meet Lawson’s criterion, the product n must be large.
 For a given value of n, the probability of fusion between two particles
increases as  increases.
 For a given value of , the collision rate increases as n increases.
Confinement is still a problem.
Section 45.4
Confinement Techniques
Magnetic confinement
 Uses magnetic fields to confine the plasma
Inertial confinement
 Particles’ inertia keeps them confined very close to their initial positions.
Section 45.4
Magnetic Confinement
One magnetic confinement device is
called a tokamak.
Two magnetic fields confine the plasma
inside the donut.
 A strong magnetic field is produced
in the windings.
 A weak magnetic field is produced
by the toroidal current.
The field lines are helical, they spiral
around the plasma, and prevent it from
touching the wall of the vacuum
chamber.
Section 45.4
Fusion Reactors Using Magnetic Confinement
TFTR – Tokamak Fusion Test Reactor
 Close to values required by Lawson criterion
JET – Joint European Torus
 Reaction rates of 6 x 1017 D-T fusions per second were reached
NSTX – National Spherical Torus Experiment
 Produces a spherical plasma with a hole in the center
 Is able to confine the plasma with a high pressure
ITER – International Thermonuclear Experimental Reactor
 An international collaboration involving four major fusion programs is working
on building this reactor.
 It will address remaining technological and scientific issues concerning the
feasibility of fusion power.
 Fusion operation is expected to begin in 2018.
Section 45.4
Inertial Confinement
Uses a D-T target that has a very high particle density
Confinement time is very short.
 Therefore, because of their own inertia, the particles do not have a chance to
move from their initial positions.
Lawson’s criterion can be satisfied by combining high particle density with a short
confinement time.
Section 45.4
Laser Fusion
Laser fusion is the most common form
of inertial confinement.
A small D-T pellet is struck
simultaneously by several focused, high
intensity laser beams.
This large input energy causes the
target surface to evaporate.
The third law reaction causes an inward
compression shock wave.
This increases the temperature.
Section 45.4
Fusion Reactors Using Inertial Confinement
Omega facility
 University of Rochester (NY)
 Focuses 24 laser beams on the target
National Ignition Facility
 Lawrence Livermore National Lab (CA)
 Construction was completed in early 2009
 Will include 192 laser beams focused on D-T pellets
 The lasers were fired in March 2009 and broke the megajoule record for lasers.
 They delivered 1.1 MJ to a target
 Fusion ignition tests are planned for 2010.
Section 45.4
Fusion Reactor Design – Energy
In the D-T reaction, the alpha particle
carries 20% of the energy and the
neutron carries 80%.
 The neutrons are about 14 MeV.
The alpha particles are primarily
absorbed by the plasma, increasing the
plasma’s temperature.
The neutrons are absorbed by the
surrounding blanket of material where
their energy is extracted and used to
generate electric power.
Fusion Reactor Design, cont.
One scheme is to use molten lithium to capture the neutrons.
The lithium goes to a heat-exchange loop and eventually produces steam to drive
turbines.
Section 45.4
Fusion Reactor Design, Diagram
Section 45.4
Some Advantages of Fusion
Low cost and abundance of fuel
 Deuterium
Impossibility of runaway accidents
Decreased radiation hazards
Section 45.4
Some Anticipated Problems with Fusion
Scarcity of lithium
Limited supply of helium
 Helium is needed for cooling the superconducting magnets used to produce
the confinement fields.
Structural damage and induced radiation from the neutron bombardment
Section 45.4
Radiation Damage
Radiation absorbed by matter can cause damage.
The degree and type of damage depend on many factors.
 Type and energy of the radiation
 Properties of the matter
Section 45.5
Radiation Damage, cont.
Radiation damage in the metals used in the reactors comes from neutron
bombardment.
 They can be weakened by high fluxes of energetic neutrons producing metal
fatigue.
 The damage is in the form of atomic displacements, often resulting in major
changes in the properties of the material.
Radiation damage in biological organisms is primarily due to ionization effects in
cells.
 Ionization disrupts the normal functioning of the cell.
Section 45.5
Types of Damage in Cells
Somatic damage is radiation damage to any cells except reproductive ones.
 Can lead to cancer at high radiation levels
 Can seriously alter the characteristics of specific organisms
Genetic damage affects only reproductive cells.
 Can lead to defective offspring
Section 45.5
Damage Dependence on Penetration
Damage caused by radiation also depends on the radiation’s penetrating power.
 Alpha particles cause extensive damage, but penetrate only to a shallow
depth.
 Due to their charge, they will have a strong interaction with other charged particles.
 Neutrons do not interact with material and so penetrate deeper, causing
significant damage.
 Gamma rays can cause severe damage, but often pass through the material
without interaction.
Section 45.5
Units of Radiation Exposure
The roentgen (R) is defined as
 That amount of ionizing radiation that produces an electric charge of 3.33 x
10-10 C in 1 cm3 of air under standard conditions.
 Equivalently, that amount of radiation that increases the energy of 1 kg of air
by 8.76 x 10-3 J .
One rad (radiation absorbed dose)
 That amount of radiation that increases the energy of 1 kg of absorbing
material by 1 x 10-2 J.
Section 45.5
More Units
The RBE (relative biological effectiveness)
 The number of rads of x-radiation or gamma radiation that produces the
same biological damage as 1 rad of the radiation being used.
 Accounts for type of particle which the rad itself does not
The rem (radiation equivalent in man)
 Defined as the product of the dose in rad and the RBE factor
 Dose in rem = dose in rad x RBE
Section 45.5
RBE Factors, A Sample
Section 45.5
Radiation Levels
Natural sources – rocks and soil, cosmic rays
 Called background radiation
 About 0.13 rem/yr
Upper limit suggested by US government
 0.50 rem/yr
 Excludes background
Occupational
 5 rem/yr for whole-body radiation
 Certain body parts can withstand higher levels
 Ingestion or inhalation is most dangerous
Section 45.5
Radiation Levels, cont.
50% mortality rate
 About 50% of the people exposed to a dose of 400 to 500 rem will die.
New SI units of radiation dosages
 The gray (Gy) replaces the rad.
 The sievert (Sv) replaces the rem.
Section 45.5
SI Units, Table
Section 45.5
Radiation Detectors, Introduction
Radiation detectors exploit the interactions between particles and matter to allow
a measurement of the particles’ characteristics.
Things that can be measured include:
 Energy
 Momentum
 Charge
 Existence
Section 45.6
Early Detectors
Photographic emulsion
 The path of the particle corresponds to points at which chemical changes in
the emulsion have occurred.
Cloud chamber
 Contains a gas that has been supercooled
 Energetic particles ionize the gas along the particles’ paths.
Section 45.6
Early Detectors, Cont.
Bubble chamber
 Uses a liquid maintained near its
boiling point
 Ions produced by incoming
charged particles leave bubble
tracks.
 The picture is an artificially colored
bubble chamber photograph.
Section 45.6
Contemporary Detectors
Ion chamber
 Electron-ion pairs are generated as radiation passes through a gas and produces an
electric signal.
 The current is proportional to the number of pairs produced.
 A proportional counter is an ion chamber that detects the presence of the particle
and measures its energy.
Section 45.6
Geiger Counter
A Geiger counter is the most common
form of an ion chamber used to detect
radiation.
When a gamma ray or particle enters
the thin window, the gas is ionized.
The released electrons trigger a current
pulse.
The current is detected and triggers a
counter or speaker.
Section 45.6
Geiger Counter, cont.
The Geiger counter easily detects the presence of a particle.
The energy lost by the particle in the counter is not proportional to the current
pulse produced.
 Therefore, the Geiger counter cannot be used to measure the energy of a
particle.
Section 45.6
Other Detectors
The semiconductor-diode detector
 A reverse-bias p-n junction
 As a particle passes through the junction, a brief pulse of current is created
and measured.
The scintillation counter
 Uses a solid or liquid material whose atoms are easily excited by radiation
 The excited atoms emit photons as they return to their ground state.
 With a photomultiplier, the photons can be converted into an electrical signal.
Section 45.6
Other Detectors, cont.
Track detectors
 Various devices used to view the tracks or paths of charged particles directly
 The energy and momentum of these energetic particles are found from the
curvature of their path in a magnetic field of known magnitude and direction.
Spark chamber
 A counting device that consists of an array of conducting parallel plates and
is capable of recording a three-dimensional track record.
Drift chamber
 A newer version of the spark chamber
 Has thousands of high-voltage wires throughout the space of the detector
Section 45.6
Applications of Radiation
Tracing
 Radioactive particles can be used to trace chemicals participating in various
reactions.
 Example, 131I to test thyroid action
 Also to analyze circulatory system
 Also useful in agriculture and other applications
Materials analysis
 Neutron activation analysis uses the fact that when a material is irradiated
with neutrons, nuclei in the material absorb the neutrons and are changed to
different isotopes.
Section 45.7
Applications of Radiation, cont.
Radiation therapy
 Radiation causes the most damage to rapidly dividing cells.
 Therefore, it is useful in cancer treatments.
Food preservation
 High levels of radiation can destroy or incapacitate bacteria or mold spores.
Section 45.7