Nuclear Energy

1
Nuclear Energy

• Nuclear Fission - the splitting of two atoms
• Nuclear fusion the combining of 2 atoms
• Both processes release energy

2
Atomic structure

• Atoms are composed of protons , neutrons and
  electrons discovered in the 1930s
• Protons positively charged
• q1.60217648710-19 C
• m1.67262158 10-27 kg
• Neutrons no charge
• m1.6749 x 10-27 kg
• Electron negative charge
• q -1.60217648710-19 C
• m 9.1093821510-31 kg

3
Atomic structure

• Protons and neutrons make up the nucleus
• Electrons orbit the nucleus at specific distances
  known as energy levels
• Atomic number number of protonsZ
• Atomic mass number A Z N, where N is the
  number of neutrons
• Atomic mass total mass of the electrons,
  protons and neutrons
• Atomic weight the ratio of the average mass of
  an atom to 1/12 of the mass of an atom of
  carbon-12.
• Ion if an atom loses or gains an electron and
  has a net charge
• Isotope-atom has the same number of protons but
  different number of neutrons

4
Atomic Structure

• Atoms are held together by forces
• There are four forces in nature
• Gravity -force between masses
• Electrostatic forces-like charges repel, unlike
  charges attract
• Strong nuclear force - causes an attraction
  between protons and neutrons
• Weak nuclear force causes protons to transform
  into neutrons and neutrons in to protons

5
Atomic Structure

• In atoms, the electrostatic force is holding the
  electrons to the nucleus, since the electrons are
  negatively charged and the protons are positively
  charged.
• The protons in the nucleus are being pushed apart
  by the electrostatic force since they have the
  same charge, but the strong nuclear force
  overcomes this repulsion on the atomic size
  scales and holds the protons together.

6
Fission

• In the late 1930s, it was discovered that if a
  uranium nucleus was bombarded by neutrons, it
  absorbed the neutron and became an unstable
  isotope of uranium, which then spilt into 2
  separate atoms (Krypton and Barium) and emitted
  more neutrons and gamma rays

7
Is mass conserved?

• Now the mass of the fission products plus the
  excess neutron should equal the mass of the
  initial incident neutron and the uranium. But it
  doesnt.
• Where did the mass go?
• Well, remember E mc2 - mass cannot be created
  or destroyed, only converted to and from energy,
  so the missing mass must be converted into energy.

8
How much energy

• 200 MeV is released per fission event
• The fission of 1 g of uranium or plutonium per
  day liberates about 1 MW.
• This is the energy equivalent of 3 tons of coal
  or about 600 gallons of fuel oil per day
• No CO2 emissions!
• Vastly superior in terms of energy per amount of
  fuel

9
Self sustaining or chain reaction

• The fission reaction itself releases neutrons,
  these can be used to fission additional nuclei,
  so the elements are there for a sustained or
  chain reaction.
• Tremendous power capability made this an ideal
  weapon.

10
Fission Bombs

• Created in response to a fear that Nazi Germany
  would develop one first, which would tip the
  balance of power and possibly the outcome of WWII
  in their favor.
• Manhattan Project Secret US project to develop
  a nuclear weapon
• Developed 3 nuclear devices, one with 235U and
  two with 239PU.
• One was tested in New Mexico in 1945, the other
  two were dropped on Hiroshima and Nagasaki Japan
  in August 1945, ending WWII in the Pacific.

11
Critical Mass

• In order to sustain a chain reaction, one needs a
  specific amount of fissionable material, called
  the critical mass
• Critical mass is the smallest amount of fissile
  material needed for a sustained nuclear chain
  reaction.
• Creating the critical mass was one of the
  challenges that faced the Manhattan project

12
The devices

• Fat Man and Little Boy
• Little Boy device dropped on Hiroshima
• Gun-type device
• One mass of U-235, the "bullet," is fired down a
  gun barrel into another mass of U-235, rapidly
  creating the critical mass of U-235, resulting in
  an explosion.

13
The devices

• Fat Man
• Tested in New Mexico and dropped on Nagasaki
• Used Plutonium rather than Uranium
• Implosion style device
• The required implosion was achieved
• by using shaped charges with
• many explosive lenses to produce
• the perfectly spherical explosive
• wave which compressed the
• plutonium sphere.

14
Effects of a Fission explosion

• Blast Damage
• Thermal radiation
• Electromagnetic Pulse
• Ionizing radiation
• earthquake

15
Blast Damage

• 40-50 of the total energy released is in the
  blast.
• Most of the destruction due to blast effects
• Blast wind may exceed 1000 km/h.

16
Thermal Radiation

• 35-45 of the energy released is in thermal
  radiation
• Burns occur
• Eye injures
• Flash Blindness-caused by the initial bright
  flash, can last up to 40 minutes
• Retinal burns scarring due to the direct
  concentration of explosions thermal energy on the
  eye-rare the fireball needs to be in the direct
  line of sight
• Firestorms-gale force winds that blow in from all
  sides towards the center of a fire

17
Electromagnetic pulse

• The nuclear explosion produced high energy
  electromagnetic radiation-Gamma rays.
• The Gamma rays interact with (scatter) electrons
  and produce higher energy electrons.
• Long metal objects (cables, etc) act as antenna
  and generate high voltages and currents, which
  can damage or destroy electrical equipment.
• No known biological effects, though useful
  against Sentinels (The Matrix).

18
Ionizing radiation

• About 5 of the energy
• In the form of neutrons, gamma rays, alpha
  particles and electrons, moving at nearly the
  speed of light
• Neutrons transmutate (change the atomic structure
  of) the surrounding matter, often making it
  radioactive. This adds to the radioactive fallout

19
What does this have to do with Nuclear Energy

• It sets the historical context and shows the
  power released
• To point out that this is NOT what will happen if
  there is an accident in a nuclear power
  facility-they do not blow up like explosive
  devices. More on that later

20
Nuclear reactor

• In a nuclear power plant, the energy to heat the
  water to create steam to drive the turbine is
  provided by the fission of uranium, rather than
  the burning of coal.
• Fuel is 3 235U and 97 238U. 235U is an isotope
  of 238U. The chain reaction will only occur in
  the 235U, but naturally occurring uranium has
  both present in it.
• The neutrons coming from a fission reaction have
  an energy of 2Mev. They are too energetic to
  sustain a nuclear reaction in 235U.
• Need to slow them down to energies on the order
  of 10-2 so they can sustain fission in the 235U

21
Slowing the neutrons down

• A moderator is used to slow down the neutrons and
  cause them to lose energy
• The moderator could be water or graphite
• The lower energy neutrons are called thermal
  neutrons
• Some of the neutrons will be absorbed by 235U
  instead of causing a fission reaction or by 238U
  and resulting in the emission of a gamma ray in
  both cases.
• Absorption of a neutron by 238U can result in the
  creation of 239Pu which is also fissionable

22
Creating Plutonium

• So 238U captures a neutron creating 239U
• 239U undergoes a beta decay in with a half life
  of 24 minutes and becomes 239Np (Neptunium)
• 239Np then beta decays with a half life of 2.3
  days into 239Pu.
• 239Pu has a half life of 24,000 years
• 239Pu can also undergo fission by the slow
  neutrons in the core, with an even higher
  probability
• So as it builds up in the core, is contributes to
  the fission reaction

23
Breeder reactor

• A reactor designed to produce more fuel (usually
  239Pu ) than it consumes.
• 239Pu does not occur naturally, and it is more
  fissile than 235U.
• Leads to the possibility of reactors that can
  create their own fuel, and only need limited
  mounts of naturally occurring uranium to operate.
• Also leads to the danger of countries creating
  additional nuclear fuels for weapons development
• Caution-reactor must be designed to produce
  weapons grade plutonium, jut because someone has
  a nuclear reactor does not mean they create
  weapons grade plutonium

24
Reactor design

• PWR pressurized water reactor
• Core where the action is. Fuel assembly is kept
  in here (fuel is usually in the form of fuel
  rods)
• Fuel rods are surrounded by the water which acts
  as the moderator. This water is kept under high
  pressure so it never boils-it heats a seconds
  water source which turns into steam
• Control rods are slid in and out from the top to
  control the fission rate-in an emergency they can
  be dropped completely into the reactor core,
  quenching the fission
• Once the steam is generated, this works just like
  a fossil fuel power plant
• Can run without refueling for up to 15 years if
  the initial fuel is highly enriched
• Used in submarines and commercial power systems

25
Reactor design

• BWR Boiling water reactor
• Core where the action is. Fuel assembly is kept
  in here (fuel is usually in the form of fuel
  rods)
• Fuel rods are surrounded by the water which acts
  as the moderator and the source of steam
• Control rods are slid in and out from the bottom
  to control the fission rate-in an emergency they
  can be dropped completely into the reactor core,
  quenching the fission. Also, boron can be added
  to the water which also efficiently absorbs
  neutron
• Once the steam is generated, this works just like
  a fossil fuel power plant

26
Fuel Cycle

• Fuel rods typically stay in a reactor about 3
  years
• When they are removed, they are thermally and
  radioactively hot
• To thermally cool them they are put in a cooling
  pond.
• Initial idea was that they would stay in the
  cooling pond for 150 days, then be transferred to
  a facility which would reprocess the uranium an
  plutonium for future use.

27
Nuclear waste disposal

• This idea ran into problems.
• Fear that the plutonium would be easily available
  for weapons use halted reprocessing efforts in
  1977
• Note that it is very difficult to extract weapons
  grade plutonium from spent fuel rods
• Plan is now to bury the waste deep underground,
  in a place called Yucca Mountain, Nevada

28
Nuclear waste

• The spent fuel rods are radioactive
• Radioactivity is measured in curies
• A curie is 3.7x1010 decays per second
• A 1000 MW reactor would have 70 megacuries of
  radioactive waste once it was shut down
• After 10 years, this has decayed to 14 MCi
• After 100 years, it is 1.4MCi
• After 100,000 years it is 2000 Ci