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Chapter 45

Applications of Nuclear

Physics

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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 either case

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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

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Fast Neutrons A fast neutron has energy greater than 1 MeV

During its many collisions when traveling

through matter, the neutron gives up some ofits kinetic energy to a nucleus

For some materials and fast neutrons, elastic

collisions dominate These materials are called moderators since they

moderate the originally energetic neutrons very

efficiently

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Thermal Neutrons Most neutrons bombarding a moderator

will be come thermal neutrons

They are in thermal equilibrium with themoderator material

Their average kinetic energy at roomtemperature 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

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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

The excited state lasts for a very short time

The product nucleus is generally radioactive and

decays by beta emission

++++

X*XXn1A

Z

1A

Z

A

Z

1

0

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Nuclear Fission A heavy nucleus splits into two

smaller nuclei

Fission is initiated when a heavynucleus captures a thermal neutron

The total mass of the products is less

than the original mass of the heavynucleus This difference in mass is called the

mass defect

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Short History of Fission First observed in 1939 by Otto Hahn

and Fritz Strassman following basic

studies by Fermi

Lise Meitner and Otto Frisch soon

explained what had happened

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Fission Equation:235

U Fission of235U by a thermal neutron

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

neutronsYX*UUn236

92

235

92

1

0+++

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Distribution of Fission

Products The most probable

products have mass

numbersA 140andA 95

There are also an

average of 2.5

neutrons releasedper event

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Fission Event Described by

the Liquid-Drop Model A slow neutron approaches the 235U nucleus The 235U nucleus captures a thermal neutron

This capture results in the formation of236U*,and the excess energy of this nucleus causesit to deform and oscillate

The 236U* nucleus becomes highly elongated,

and the force of repulsion between theprotons tends to increase the distortion

The nucleus splits into two fragments,emitting several neutrons in the process

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Fission Described by the

Liquid-Drop Model Diagram

(a) Approach (b) Absorption (c) Oscillation (d) Fission

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Energy, cont. 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 theamount of energy released in chemicalprocesses

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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

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Chain Reaction Diagram

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Active Figure 45.3

(SLIDESHOW MODE ONLY)
http://../Active_Figures/Active%20Figures%20Media/AF_4503.htmlhttp://../Active_Figures/Active%20Figures%20Media/AF_4503.html

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Enrico Fermi 1901 1954 Nobel Prize in 1938 for

producing transuranic

elements by neutronirradiation

Other contributionsinclude theory of beta

decay, free-electrontheory of metal,development of worldsfirst fission reactor(1942)

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Nuclear Reactor A nuclear reactoris a system designed to

maintain a self-sustained chain reaction

The reproduction constantKis defined asthe average number of neutrons from eachfission event that will cause another fissionevent

The maximum value ofKfrom uranium fission is2.5 In practice, Kis less than this

A self-sustained reaction has K= 1

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KValues When K= 1, the reactor is said to be critical

The chain reaction is self-sustaining

When K < 1, the reactor is said to besubcritical The reaction dies out

When K > 1, the reactor is said to be

supercritical A run-away chain reaction occurs

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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 producingneptunium and plutonium

Fuels are generally enrichedto at least a

few percent 235U

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Moderator The moderator slows the neutrons

The slower neutrons are more likely to react with235

U than238

U 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 moderatormakes them available for reactions with 235U while

decreasing their chances of being captured by 238U

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Pressurized Water Reactor

Diagram

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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 rodsraise the temperature of the water containedin the primary loop The primary system is a closed system

This water is maintained at a high pressure tokeep it from boiling This water is also used as the moderator to

slow down the neutrons

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Notes, cont. The hot water is pumped through a heat

exchanger

The heat is transferred by conduction tothe water contained in a secondarysystem

This water is converted into steam The steam is used to drive a turbine-

generator to create electric power

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Notes, final The water in the secondary system is isolated

from the water in the primary system

This prevents contamination of the secondarywater 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

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Basic Reactor Design 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

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Control Rods To control the power level, control rods are

inserted into the reactor core

These rods are made of materials that arevery efficient in absorbing neutrons Cadmium is an example

By adjusting the number and position of the

control rods in the reactor core, the K valuecan be varied and any power level can beachieved The power level must be within the design of the

reactor

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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

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Reactor Safety Radioactive

Materials Disposal of waste material

Waste material contains long-lived, highly radioactiveisotopes

Must be stored over long periods in ways that protect theenvironment

Present solution is sealing the waste in waterproofcontainers 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 willnot rupture during high speed collisions

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Nuclear Fusion Nuclearfusion 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

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Fusion in the Sun All stars generate energy through fusion

The Sun, along with about 90% of other stars,

fuses hydrogen Some stars fuse heavier elements

Two conditions must be met before fusion

can occur in a star: The temperature must be high enough

The density of the nuclei must be high enough to

ensure a high rate of collisions

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Proton-Proton Cycle The proton-proton

cycle is a series of

three nuclearreactions believed tooperate in the Sun

Energy liberated is

primarily in the formof gamma rays,positrons andneutrinos

HHHeHeHe

or

eHeHeH

Then

HeHH

eHHH

1

1

1

1

4

2

3

2

3

2

4

2

3

2

1

1

3

2

2

1

1

1

2

1

1

1

1

1

+++

+++

++

+++

+

+

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Fusion Reactions, final Because high temperatures are

required to drive these reactions, they

are called thermonuclear fusionreactions All of the reactions in the proton-proton

cycle are exothermic An overview of the cycle is that four

protons combine to form an alphaparticle and two positrons

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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 canbe extracted from 1 gal of water for about 4

cents

Comparatively few radioactive by-products are formed

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Considerations for a Fusion

Reactor The proton-proton cycle is not feasible for a

fusion reactor

The high temperature and density requiredare not suitable for a fusion reactor

The most promising reactions involve

deutrium and tritium2 2 3 1

1 1 2 0

2 2 3 1

1 1 1 1

2 3 4 1

1 1 2 0

H H H n 3 27 MeV

H H H H 4 03 MeV

H H He n 17 59 MeV

.

.

.

Q

Q

Q

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Considerations for a Fusion

Reactor, cont. Tritium is radioactive and must be

produced artificially

The Coulomb repulsion between twocharged nuclei must be overcome

before they can fuse

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Potential Energy Function The potential energy is

positive in the region r>R, where the Coulomb

repulsive forcedominates

It is negative where thenuclear force dominates

The problem is to givethe nuclei enoughkinetic energy toovercome this repulsiveforce

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Critical Ignition Temperature The temperature at

which the power

generation rate in anyfusion reaction exceeds

the lost rate is called

the critical ignition

temperature, Tignit The intersection of the

Pgen with the Plost line is

the Tignit

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Requirements for Successful

Thermonuclear Reactor High temperature ~ 108 K

Needed to give nuclei enough energy to overcomeCoulomb forces

At these temperatures, the atoms are ionized,forming aplasma

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

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Lawsons Criteria Lawsons criteria states

that a net power output

in a fusion reactor ispossible under the

following conditions n 1014 s/cm3 for

deuterium-tritium n 1016 s/cm3 for

deuterium-deuterium

These are the minima

on the curves

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Requirements, Summary The plasma temperature must be very high

To meet Lawsons criterion, the product n

must be large For a given value ofn, the probability of fusion

between two particles increases as increases

For a given value of

, the collision rate increases as nincreases

Confinement is still a problem

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Confinement Techniques Magnetic confinement

Uses magnetic fields to confine the plasma

Inertial confinement Particles inertia keeps them confined very

close to their initial positions

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Magnetic Confinement One magnetic confinement

device is called a tokamak Two magnetic fields confine

the plasma inside the donut A strong magnetic field isproduced in the windings

A weak magnetic field isproduced by the toroidalcurrent

The field lines are helical,they spiral around theplasma, and prevent it fromtouching the wall of thevacuum chamber

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Fusion Reactors Using

Magnetic Confinement TFTR Tokamak Fusion Test Reactor

Close to values required by Lawson criterion

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 ThermonuclearExperimental 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

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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

Lawsons criterion can be satisfied bycombining high particle density with a short

confinement time

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Laser Fusion Laser fusion is the most

common form of inertialconfinement

A small D-T pellet is strucksimultaneously by severalfocused, high intensity laserbeams

This large input energy causes

the target surface to evaporate The third law reaction causes

an inward compression shockwave

This increases the temperature

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Fusion Reactors Using Inertial

Confinement Omega facility

University of Rochester (NY)

Focuses 24 laser beams on the target

Nova facility Lawrence Livermore National Lab (CA)

Focuses 10 laser beams on the target Has achieved n 5 x 1014 s/cm3

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Fusion Reactor Design

Energy In the D-T reaction,

the alpha particle

carries 20% of theenergy and the

neutron carries 80% The neutrons are

about 14 MeV

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Fusion Reactor Design,

Particles The alpha particles are primarily absorbed by

the plasma, increasing the plasmastemperature

The neutrons are absorbed by thesurrounding blanket of material where theirenergy is extracted and used to generateelectric power

One scheme is to use molten lithium tocapture the neutrons

The lithium goes to a heat-exchange loop and

eventually produces steam to drive turbines

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Fusion Reactor Design,

Diagram

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Some Advantages of Fusion Low cost and abundance of fuel

Deuterium

Impossibility of runaway accidents Decreased radiation hazards

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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 inducedradiation from the neutron

bombardment

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Radiation Damage Radiation absorbed by matter can

cause damage

The degree and type of damagedepend on many factors Type and energy of the radiation

Properties of the absorbing matter

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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 inthe properties of the material

Radiation damage in biological organisms isprimarily due to ionization effects in cells Ionization disrupts the normal functioning of the

cell

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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 onlyreproductive cells Can lead to defective offspring

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Damage Dependence on

Penetration Damage caused by radiation also depends on

the radiations penetrating power

Alpha particles cause extensive damage, butpenetrate 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

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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

Onerad (radiation absorbed dose) That amount of radiation that increases the energy

of 1 kg of absorbing material by 1 x 10-2 J

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More Units

TheRBE (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

Therem (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

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RBE Factors, A Sample

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RBE Factors, Notes

The values given for RBE factors are

only approximate

They vary with particle energy and with theform of damage

The RBE factor should be used as only

a first-approximation guide to the actualeffects of radiation

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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

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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

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SI Units, Table

R di ti D t t

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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

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Early Detectors

Photographic emulsion The path of the particle corresponds to

points at which chemical changes in theemulsion have occurred

Cloud chamber

Contains a gas that has been supercooled Energetic particles ionize the gas along the

particles paths

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Contemporary Detectors

Ion chamber Electron-ion pairs are

generated as radiation

passes through a gas andproduces an electric signal

The current is proportional

to the number of pairs

produced

A proportional counterisan ion chamber that detects

the presence of the particle

and measures its energy

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Geiger Counter

A Geiger counteris 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 electronstrigger a current pulse

The current is detected and

triggers a counter or speaker

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Geiger Counter, cont.

The Geiger counter easily detects the

presence of a particle

The energy lost by the particle in thecounter is not proportional to the current

pulse produced

Therefore, the Geiger counter cannot beused to measure the energy of a particle

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Other Detectors

The semiconductor-diode detector A reverse-biasp-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 areeasily excited by radiation

The excited atoms emit photons as they return totheir ground state

With a photomultiplier, the photons can beconverted into an electrical signal

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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

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Other Detectors, Final

Spark chamber Is a counting device that consists of an

array of conducting parallel plates and iscapable of recording a three-dimensionaltrack record

Drift chamber

A newer version of the spark chamber Has thousands of high-voltage wires

throughout the space of the detector

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Applications of Radiation

Tracing Radioactive particles can be used to trace

chemicals participating in various reactions Example, 131I to test thyroid action

Also useful in agriculture

Materials analysis

Neutron activation analysis uses the fact thatwhen a material is irradiated with neutrons, nuclei

in the material absorb the neutrons and are

changed to different isotopes

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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 orincapacitate bacteria or mold spores

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