Nuclear Energy

1
Nuclear Energy Elementary Particles

• Nuclear Fission and Fusion
• Introduction to Elementary Particles

• ANNOUNCEMENT Final Exam, Friday, May 15 505 -
  705 PM
• Alt exam sign up sheet at the front
• Rooms TBA in email
• Material from Chapters 15-24, 26-30 (slightly
  more on 28-30)
• Two pages of notes (8.5 x 11) allowed
• 33 multiple choice questions plus test code
• Scantron will be used - bring 2 HB pencils
  calculator

2
Nuclear Fission

• A heavy nucleus splits into two smaller nuclei
• Mass of the products is less than the initial
• Emc2 -gt KE, can be harnessed for electric power
• Fission of 235U by a slow (low energy) neutron
• 236U is an intermediate, short-lived state
• Lasts about 10-12 s
• X and Y are called fission fragments
• Many combinations of X and Y satisfy the
  requirements of conservation of energy and
  charge. Example

3
Sequence of Events in Fission

• The 235U nucleus captures a thermal (slow-moving)
  neutron
• This capture results in the formation of 236U
• Excess energy of this nucleus causes strong
  oscillations. The 236U nucleus becomes highly
  elongated, force between protons contributes
• Splits into two fragments and several neutrons

1
2
3
4
4
Energy in a Fission Process

• Binding energy for
  heavy nuclei
  7.2 MeV per
  nucleon
• Binding energy for
  final nuclei is
  8.2 MeV per
  nucleon
• An estimate of the energy released
• Assume a total of 240 nucleons
• Releases about 1 MeV per nucleon
• 8.2 MeV 7.2 MeV
• Total energy released is about 240 MeV
• Much larger than the energy in chemical processes

5
Chain Reaction Diagram

• 3 final neutrons can trigger fission in other
  nuclei
• This process is called a chain reaction

6
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 maximum value of K from uranium fission is
  2.5
• In practice, K is less than this
• A self-sustained reaction has K 1
• When K 1, the reactor is said to be critical
• The chain reaction is self-sustaining
• When K lt 1, the reactor is said to be subcritical
• The reaction dies out
• When K gt 1, the reactor is said to be
  supercritical
• A run-away chain reaction occurs

7
Basic Reactor Design

• Fuel elements consist of enriched uranium
• The moderator material helps to slow down the
  neutrons. Typical neutrons are 2 MeV, slower
  neutrons are captured more easily
• The control rods absorb neutrons, lowers K
• Explosives A nuclear explosive uses Kgt1

8
Nuclear Fusion

• Nuclear fusion 2 light nuclei combine to form a
  heavier nucleus
• The mass of the final nucleus is less than the
  masses of the original nuclei

• Final heavier nucleus has a higher binding
  energy, less mass
• Loss of mass is accompanied by a release of
  energy

9
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
• Density of nuclei must be high enough to ensure a
  high rate of collisions

• The proton-proton cycle is a series of three
  nuclear reactions believed to operate in the Sun
• Energy liberated is primarily in the form of
  gamma rays, positrons and neutrinos
• 21H is deuterium, and may be written as 21D

10
Making a Fusion Reactor

• Substantial effort going into fusion reactor
  research
• Would use water as input and not have radioactive
  byproducts
• The proton-proton cycle is not feasible for a
  fusion reactor
• High temperature density required not suitable
  for fusion reactor
• Most promising reactions use deuterium (D)
  tritium (T)
• Deuterium is available in almost unlimited
  quantities in water and is inexpensive to extract
• Tritium is radioactive and must be produced
  artificially
• The Coulomb repulsion between two charged nuclei
  must be overcome before they can fuse

2
11
Plasma Confinement

• Need High temp 108 K
• High density over a short time n number of ions
• n? ? 1014 s/cm3 for DT
• n? ? 1016 s/cm3 for DD
• Magnetic confinement device called a tokamak
• Magnetic fields confine the plasma

• Inertial laser confinement
• Fuel is put into the form of a small pellet
• It is collapsed by ultrahigh power lasers
• Inertial electrostatic confinement
• Positively charged particles are rapidly
  attracted toward an negatively charged grid
• Some of the positive particles collide and fuse

12
Madison Symmetric Torus
13
Elementary Particle Physics

• One learns more by looking at smaller distances

• Solids
• Newtons laws

• Molecules
• Kinetic theory of gases

• Atoms
• Lasers

• Nucleus (reactors)
• Subnuclear Structure (accelerators)

14
Rutherford Experiment

• Rutherford Experiment led to orbital model

• Use high energy particles to probe deeper

15
Accelerated charged particles

• Particle accelerator laboratories
• Cyclotron

Uniform B into page
16
From before on particle physics

• We have talked about several particles
• Electron, photon, proton, neutron
• Many particles have internal constituents
• Not fundamental atoms have proton and neutron
• Protons and neutrons have internal components
  also
• We have talked about various forces
• Electromagnetic and gravity
• Other forces - weak(radioactive decays) and
  strong
• We have a method of exploring further.
  Accelerators

17
Particles and fields

• But what are particles and forces?
• Is there are way to describe the two at once?
• An answer lies in considering everything as
  fields.
• Particles are quanta of a corresponding field.
• What does this mean?
• Think about photons.
• One photon means the electromagnetic field has
  (Plancks const)x(frequency) hf of energy.
• Two photons means 2hf of energy.

18
Particles as fields

• Electromagnetic field spread out over space.
• Stronger near the the source of the
  electric/magnetic charge - weaker farther away.
• Electromagnetic radiation, the photon, is the
  quanta of the field.
• Describe electron particles as fields
• Makes sense - the electron was spread out around
  the hydrogen atom.
• Wasnt in one place - had locations it was more
  or less probable to be. Stronger and weaker like
  the electromagnetic field.
• Electron is the quanta of the electron field.
• Everything can be described as fields with
  quant(particles) which can be individually
  observed and studied