Announcements
•Answer key for quiz 1 has been posted at websitesee folder called “quizzes 2007”.
•Quiz grades, attendance credit will be available at
a confidential grade check on the web page later
this week.
•First mid-term exam from 2006 has been posted at
the web site – see folder called “Old Exams”.
•Grades for quiz 1 based on the number of correct
answers are as follows 5 – A; 4 – B; 3 – C; 2 – D;
1 or 0 - F
Supplementary Reading, Nuclear
Chemistry
• Zumdahl Text, Sections 21.5 – 21.6,
pages 993 – 1002
• National Ignition Facility Project
http://www.llnl.gov/nif/project/
• International Thermonuclear Experimental
Reactor (ITER)
http://www.iter.org/a/index_nav_1.htm
http://www.iter.org/a/index_nav_2.htm
235U
Chain Reaction, Fission Reactor
Chicago Pile 1, December 2, 1942,
First Sucessful Nuclear Reactor
Enrico Fermi, Nobel Prize, 1938
Chain Reaction
• http://www.physics.umd.edu/lecdem/servic
es/demos/demosp4/p4-62a.mpg
• Neutrons are emitted when 235U undergoes
fission
• 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
– The principle behind the nuclear bomb, where 1 g of
235U can release energy equal to about “??” tons of
TNT
Many Radioactive Fragments form
in Fission
Many radioactive
fragments last for
many centuries
after they are
formed
Basic Reactor Design
• Fuel elements consist of
enriched 235U (natural
uranium contains 0.7%
235U and 99.3% 238U)
• 238U almost never
undergoes fission
• The moderator material
helps to slow down the
neutrons
• The control rods absorb
neutrons
To start the reactor
the control rods are
pulled out of the core
To stop the reactor
the control rods
are pushed into
the core
Fuel rods
Electricity from Nuclear Fission
In a fission reactor, enriched Uranium, which is Uranium-238
with a high concentration of Uranium-235, undergoes a process
known as Induced Nuclear Fission. Nuclear Fission occurs
when an atom of a fissionable material is struck by a neutron
and splits into two lighter atoms, releasing a massive amount of
heat and gamma radiation. The heat is used to boil water,
producing steam used to turn a generator.
Nuclear Fission Reactor
Nuclear Fusion –A Star
Fusion in the Sun
• All stars generate energy through fusion
• The Sun, along with about 90% of other stars,
fuses hydrogen
– Some “older” 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
The Sun
• Source of solar
energy
• A large nuclear
fusion reactor
• 1.39x109 m in
diameter
• ~1.5x1011 m away
from us
Structure of the Sun
Corona: Very Hot
Chromosphere: UV Radiation
Photosphere: “skin”
Convection Zone: Rising and
Falling Gas
Radiation Zone: X-rays
Core: Nuclear
Fusion
Proton-Proton Chain
Considerations for a Fusion
Reactor
• The proton-proton chain is not feasible for a fusion
reactor
– The high temperature and density required are
not suitable for a fusion reactor
– It takes a long, long time for the proton-proton
chain reaction to occur – centuries
• The most promising reactions involve
deuterium and tritium: D-T FUSION
D-T Fusion
The Process
•
•
Nuclei of two isotopes of hydrogen, deuterium (D) and tritium (T) react to produce a helium
(He) nucleus and a neutron (n). In each reaction, 17.6 MeV of energy (2.8 pJ) is liberated:
D + T 4He (3.5 MeV) + n (14.1 MeV)
Fusion Reactors
• Energy releasing fusion reactions are
called thermonuclear fusion reactions
• A great deal of effort is being directed at
developing a sustained and controllable
thermonuclear reaction
• A thermonuclear reactor that can deliver
a net power output over a reasonable
time interval is not yet a reality
Considerations for a Fusion Reactor
• Tritium is radioactive and must be
produced artificially
• One of the main problems in obtaining
energy from nuclear fusion is the fact that
the Coulomb repulsion between two
charged nuclei must be overcome before
they can fuse
• This is a competition between long range
Coulomb forces and much stronger but
very short range strong forces that bind
protons and neutrons together
binding the nucleus
the nucleus of helium contains two protons
which are both positively charged. they
should repel each other but they are strongly
bonded to each other- why?
the strong force
• an attractive force
• has an effect over a very short range
(10-15 m, about the size of the nucleus)
• leptons don’t feel this force, but particles
in the quark family do.
strong
nuclear force
Residual Strong Force
• The strong force
between the quarks
in one proton and
the quarks in
another proton is
strong enough to
overwhelm the
repulsive
electromagnetic
force
Requirements for Successful
Thermonuclear Reactor
• High temperature ~ 108 K
– Needed to give nuclei enough energy to overcome
Coulomb forces
– At these temperatures, the atoms are ionized, forming
a plasma
• Plasma ion density, n
– The number of ions present
• Plasma confinement time, τ
– The time the interacting ions are maintained at a
temperature equal to or greater than that required for
the reaction to proceed successfully
Phases
of
Fully ionized
>106 K
Matter
Plasma Forms
>104 K
>103 K
Molecules
dissociate to atoms
Gas Phase
>102 K
Liquid Phase
Solid Phase
Controlled Fusion on Earth – Can it be Done?
Breeding Tritium in a Fusion Reactor
2
3
4
1
H
+
H
→
He
+
n
1
1
2
0 fast
1
7
3
4
1
n
+
Li
→
H
+
He
+
n
0 fast 3
1
2
0 slow
1
6
3
4
0 nslow + 3 Li →1 H + 2 He
Confinement of Fusion
1. Gravity – A Star
2. Magnetic Tokamak
3. Inertial –
Laser
Principles of Inertial Confinement
Inertial Confinement Fusion Reactor
National Ignition Facility (NIF)
Hohlraum Target with D-T Pellet
Human Scale
T ~ 100 million K
has been achievedbut only for ~ 10-9 s
Operation Principles of a Tokamak
T ~ 40 million K has been achieved
International Thermonuclear Experimental
Reactor (ITER)
Human
Scale
The Future of Controlled Nuclear Fusion
The Problem with Energy Conservation
Ei
Heat, q
∆Efuel
∆Eq
∆Ew
Work, w
∆Ef
-∆Efuel = ∑∆Eq + ∑∆Ew
Piggy Bank Glass is Conserved!