Fission

1
Fission

• Physics 12 Adv

2
Comprehension Check

1. Two deuterium nuclei fuse to form a tritium
   nuclei and a proton. How much energy is
   liberated?
2. A deuterium nucleus fuses with a nitrogen-14
   nucleus to produce boron-10 and lithium-6. If
   the nuclear masses are 2.014102u, 14.00307u,
   10.01294u and 6.015122u respectively would this
   release energy?

3
Comprehension Check

1. 4.03MeV
2. No the final mass is larger than the initial
   mass so it requires energy be converted into mass

4
Fission

• Nuclear fission is a process in which an unstable
  nucleus splits into two fragments of comparable
  size
• Fission was discovered in 1938 through
  experiements of Otto Hahn and Fritz Strassman in
  Germany based upon earlier work by Fermi
• They bombarded uranium with neutrons and found an
  isotope of barium and krypton

5
Fission Example

• When uranium-235 absorbs a neutron, it becomes
  uranium-236 (excited state)
• One of the possible fission decay processes is
  uranium-236 decaying into barium-141, krypton-92
  and three neutrons.
• Determine the energy released in this process if
  the masses are 235.043930u, 140.914411u and
  91.926156u respectively

6
Fission Example
7
Fission Example 2

• However, another possible fission decay process
  for uranium involves uranium-235 absorbing a
  neutron but in this case it decays into
  xenon-140, strontium-94 and two neutrons.
• Determine the energy released from this fission
  process if the mass of the products are
  139.921641u and 93.915361u respectively

8
Fission Example 2
9
Nuclear Chain Reactions

• Nuclear reactors depend on nuclear chain
  reactions in order to produce energy
• Once started, these reactions will continue until
  the fuel has been spent or the reaction is
  stopped
• A common reaction is the fission of uranium-235
  which releases 3 neutrons these 3 neutrons can
  then release 9 neutrons the 9 can release 27
  and so on

10
Fuel for Fission

• The two nuclei that are most likely to undergo
  fission are
• Plutonium-239
• Uranium-235
• Plutonium is a manmade element while uranium
  occurs in nature
• However, U-235 exits is very small percentages in
  nature (.7)

11
Moderators

• In order for a nucleus to undergo fission, it
  needs to absorb a slow neutron
• Unfortunately, the neutrons released from fission
  are fast neutrons
• Before the chain reaction can continue, these
  fast neutrons must become slow neutrons

12
Moderators

• A moderator is a substance that will slow a fast
  neutron
• A good moderator will slow the neutrons and offer
  a very low chance of absorption
• Some moderators included (collions/absorption)
• Water (18/560)
• Heavy Water (25/1)
• Beryllium (90/16)
• Graphite (114/6)

13
Control Rods

• A nuclear chain reaction can be maintained by
  either enriching uranium or by using heavy water
  as a moderator
• However, the rate of the chain reaction must be
  controllable
• Control rods (made of cadmium or boron) are used
  to absorb neutrons to slow the reaction as
  required

14
Coolants

• There is a primary coolant that transfers heat
  from the reactor to a boiler
• This coolant must have a high boiling point, not
  be corrosive, not absorb neutrons, be chemically
  stable, have god heat transfer properties and
  pump easily
• Some common coolants include water, heavy water,
  some organic compounds and even some gases

15
Coolants

• The secondary coolant never comes into contact
  with the primary coolant as they are both in
  contained systems
• The heat from the primary coolant is transferred
  to the secondary coolant which is used to drive a
  turbine

16
Nuclear Waste

• Reactor fuels (uranium and plutonium) are
  radioactive but emit alpha particles
• Fission products are highly unstable and as a
  result release large amounts of radiation
• A nuclear reactor will mange all nuclear waste on
  site normally waste is stored under water for a
  year and then is transferred to concrete
  containment units

17
CANDU

• CANadian Deuterium Uranium (CANDU) is one of the
  most respected and efficient nuclear reactors in
  the world
• Unlike many reactors, it does not require
  enriched uranium and does not produce weapons
  grade plutonium
• It uses heavy water as a moderator and primary
  coolant

18
Fuel Bundles

• The CANDU reactor uses uranium oxide pellets that
  are inserted into tubes and made into fuel
  bundles
• Each bundle contains about 22kg of UO2 and will
  remain in the reactor for about 17 months
• The reactor can be refuelled while in operation
• Each fuel bundle provides as much thermal energy
  as 400t of coal

19
Fusion Reactors

• Fusion reactors would be able to release more
  energy than a fission reactor and do not result
  in radioactive water produts
• However, at the present time we cannot sustain a
  fusion reaction for long enough to produce more
  energy than is required to start the reaction
• Work is ongoing at Tokamak reactors in order to
  solve this issue

20
Energy in one gram of uranium

• If the natural abundance of uranium-235 is 0.75
  in a sample of uranium, assuming that fission is
  100 complete, how much energy can be released
  from the fission of one gram of uranium (the
  remaining mass, uranium-239 does not undergo
  fission)

21
Problems

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