Chain Reactions

What are Chain Reactions?

• Chain reactions are self-sustaining nuclear fission because each fission causes further fission.
• Note that the splitting was triggered by a single neutron that had just the right energy to excite the nucleus.

• Also during the split, two more electrons were released.
• If each of these neutrons splits subsequent nuclei, then a chain reaction occurs, as each splitting nuclei will cause yet more reactions.

• Chain reactions will not normally occur.
• In order to maintain the chain reaction in uranium, the concentration of U-235 in the uranium fuel has to be high enough.
  • Fuel-enrichment is the process of increasing the concentration of U-235, which is the fissile isotope in uranium fuel.
  • This allows for the fuel to sustain chain reactions.
• In addition, the neutrons will disperse in random directions.
  • If the amount of fissile isotope is too small, then not enough of the neutrons will be absorbed to sustain the chain reactions.
    • Critical mass is the minimum amount of the fissile isotope needed to sustain a nuclear chain reaction.

Operation of a Nuclear Reactor

• Nuclear reactors rely on chain reactions in radioactive fuels such as uranium or thorium to generate energy.

• Two factors must be controlled:
  • Number of the free neutrons in the reactor
  • Energy of the free neutrons
• The main components of the nuclear reactor are:
  • Control rods
  • Moderators
  • Heat exchangers
  • Shielding

Energy Transfers of a Nuclear Reactor

1. Nuclear energy in the fuel
2. Kinetic energy of fission products
3. Internal energy of water and steam
4. Kinetic energy of steam
5. Kinetic energy of turbines and coils
6. Electrical energy

Fuel Enrichment

• Fuel enrichment is increasing the percentage of uranium-235 in uranium fuel in order to make it of use in a nuclear power station.

Control Rods

• For a chain reaction and power generation, the percentage of uranium-235 has to be increased to around 3% to 5%.
• The purpose of a control rod is to absorb neutrons.
• The number of neutrons absorbed in controlled automatically by varying the depth of the control rods in the fuel rods.
  • Lowering the rods further decreases the rate of fission, as more neutrons are absorbed.
  • Raising the rods increases the rate of fission, as fewer neutrons are absorbed.

Moderators

• The purpose of a moderator is to slow down neutrons.
• The moderator is a material (usually water) that surrounds the fuel rods and control rods, and is a poor absorber of neutrons.
• The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum.
• The neutrons are slowed down so that they are in thermal equilibrium with the moderator.

Heat Exchangers

• The purpose of a heat exchanger is to transfer thermal energy efficiently between the water systems of a nuclear power plant.
• The coolant is a substance (usually water) that is pumped into the reactor at a cold temperature to extract the heat released by the fission reactions.
• In the heat exchanger, the coolant transfers the heat to water that is pumped in externally to produce steam.
• This steam then goes on to power electricity generating turbines.

Shielding

• The purpose of shielding is to house the reactor and absorb hazardous radiation.
• The daughter nuclei formed during fission and the neutrons emitted are radioactive.
• The entire nuclear reactor is surrounded by materials, such as steel and concrete walsl that are generally about 2 meters thick.
  • These walls absorb the emissions from the reactions to ensure that the environment around the reactor is safe.

Waste Materials from Nuclear Reactors

• Nuclear waste is the radioactive materials associated with the production of nuclear power that are no longer useful, and which may have to be stored safely for a long period of time.
• Low-level waste (LLW) is waste that has been contaminated with radioactive materials.
  • They only emit low levels of radiation.
  • Can be disposed of safely in specific landfills with proper labeling and documentation.
    • Clothes, tools or filters that have been contaminated are all examples of low-level waste.
• High-level waste (HLW) is waste that is highly radioactive.
  • For example nuclear fuel rods.
  • It can be stored in long-term deep geological repositories.

Advantages and Disadvantages of Nuclear Power

Advantages

• Extremely high energy-density.
• No greenhouse gases emitted during routine operation.
  • Some scientists think that nuclear power may be the only realistic solution to global warming.
• No chemical pollution during operation.
• A reasonably large amount of nuclear fuels are still available.
• Despite a few serious incidents, statistically over the last 50 years, nuclear power has overall proven to be a reasonably safe energy technology.

Disadvantages

• Dangerous and very long-lasting radioactive waste products.
• Expensive
• Efficiency is not high when the whole process is taken into account.
  • For example building the reactor and a landfill for the waste.
• Threat of serious accidents.
• Possible target for terrorists.
• Linked with nuclear weapons.
• Not a renewable source.