Lesson 18: Nuclear Energy

Download here: Ontario Curriculum Expectations

Nuclear Fission

Nuclear fission is the process of firing a neutron at a heavy atomic nucleus, like plutonium or uranium, causing it to break into two lighter radioactive nuclei. A great deal of energy is released and several neutrons are emitted. These neutrons can then collide with other heavy atomic nuclei creating more radioactive nuclei and releasing more energy and more neutrons. If the conditions are right, this can start a chain reaction. Watch this animation on nuclear fission.

Nuclear Fusion

When small nuclei collide under extreme pressure and temperature, they can combine, or fuse, into a single nucleus. Each time this happens, some of the overall mass is lost. This process is called nuclear fusion and the lost mass is converted into energy, according to Einstein’s equation, and released. Though we can’t yet produce energy by nuclear fusion, the sun can. The sun gets its energy from nuclear fusion using hydrogen as its primary fuel source. Hydrogen nuclei are fused together to eventually form helium and a great deal of energy.

Watch the video to see where the sun gets its energy:

Radioactive Decay

The amount of radioactivity in a radioactive sample decays over time. This is because a radioactive particle is very unstable, generally due to its size or energy. By emitting an alpha particle, a beta particle, or gamma radiation, the radioactive particle is able to get rid of some of its mass and energy and become more stable. All three are considered dangerous and warrant further consideration.

Alpha Particles:

• These are the same as helium nuclei – helium atoms without electrons.
• They are relatively big and heavy, about 7 300 times heavier than a beta particle.
• Due to their size, they are considered slow moving (~2.0 x 10⁷ m/s).
• Their size prevents them from penetrating very far, about 5 cm in air. They can be stopped by paper or skin.
• They have a charge of +2 due to the two missing electrons.
• Due to their size, they are strongly ionizing.

If something is ionizing, it means that it is able to remove, or knock off, electrons from other atoms. When atoms lose electrons, they become charged. Charged particles are called ions, hence the term ionizing.

Beta Particles:

• These are electrons that have formed in the nucleus of the atom.
• They are quite small and as fast (~2.9 x 10⁸ m/s) as electrons are.
• They are able to penetrate 10 m in the air but can be stopped by thin aluminium.
• Like electrons, they have a charge of -1.
• They are not as ionizing as alpha particles but can penetrate into materials much farther.

Gamma Radiation:

• A nucleus can emit excess energy in the form of a gamma ray.
• They have no mass or charge.
• They travel at the speed of light (3.0 x 10⁸ m/s).
• They can penetrate a long way, about 2 km in air, but are weakly ionizing as they tend to pass through a material rather than collide with its atoms. Thick concrete or lead will stop them.

Half-Life

The amount of radioactivity in a radioactive sample will decay over time. Each time an alpha particle, beta particle, or gamma ray is given off, one more radioactive nucleus has disappeared. This means that the older a sample becomes, the less radiation it will emit. It is not possible to predict when any particular radioactive particle will undergo radioactive decay. Some will last only seconds while others will not decay for millions of years. However, for a large sample, the average rate of decay will be proportional to the number of undecayed atoms present. This means that a 50 kg sample of radioactive material should decay at twice the rate as a 25 kg sample. As the number of radioactive particles decreases, so does the rate of decay. This actual rate will be different for every radioactive substance.

Practice Questions

1. Use the graph above to calculate the time required for the activity, in decays/second, to change from:
   1. 200 decays/s to 100 decays/s
   2. 160 decays/s to 80 decays/s
   3. 100 decays/s to 50 decays/s
   4. 40 decays/s to 20 decays/s
      

Answer

The answer to all of the questions should be approximately 20 years.

The average of your answers to the above questions is the half-life of the radioactive substance.

Half-life is the time required for half of the radioactive atoms in a sample to decay.

The longer the half-life, the longer the material remains active. The length of the half-life will depend on the substance itself as different radioactive materials decay at different rates.

Enrichment

View the half-lives of many radioactive elements.

Practice Questions

1. The form of technetium-99 used as a tracer in medicine is a gamma emitter that decays with a half-life of six hours. A fresh sample has an activity of 1200 counts/s.A) Calculate the expected activity after six hours.B) Calculate the expected activity 24 hours after the sample was made.C) Explain why measurements of the activity after six hours and after 24 hours may differ from the correct answers to A) and B).

Answer

A) Six hours is the half-life, so the activity should be half what it was originally…600 counts/s.

B) Each successive six hour period decreases the activity by one half (50%).

Start – 1200 counts/s

After 6 hours-600 counts/s

After 12 hours -300 counts/s

After 18 hours -150 counts/s

After 24 hours -75 counts/s

C) We can’t predict exactly when an atom will undergo decay. The half-life (though statistically valid and quite accurate) is not guaranteed to be 100% perfect.

Carbon-14 is used to help date ancient artefacts by using carbon dating. Most carbon has an atomic mass of 12u with six protons and six neutrons. However, a small percentage of carbon has eight neutrons and mass of 14u. This is called Carbon-14 and it is radioactive, therefore it undergoes radioactive decay. The level of decay stays fairly constant in the atmosphere so that the same proportion of C-14 is found in living things. When an organism dies, the carbon becomes “locked” in its body and is not readily exchanged with the outside world, through respiration for example. Over time, the C-14 undergoes radioactive decay and the percentage of C-14 in the organism decreases. By comparing the actual amount of C-14 in a sample to the expected average amount of C-14 in a living sample, scientists can determine how old an ancient artefact is.

Nuclear Power Plants

Electricity generation at a power plant that burns fossil fuels occurs as follows:

• Fossil fuels such as coal, petroleum, and natural gas are burned in order to generate heat.
• The heat is used to boil water and turn it into steam.
• Steam is forced past a steam turbine that rotates to generate the electricity.
• Cool water is used to condense the steam back into water.
• The water is pumped back into the boiler to be turned into steam again.

Electricity generation at a nuclear power plant is different only in how the heat is generated:

• Neutrons are fired at the nuclear fuel, uranium, in order to initiate a fission reaction.
• This releases heat energy and fast moving neutrons into the heavy-water moderator.
  • Heavy-water moderator: Some of these neutrons will initiate more fission reactions. However, fission is more successful when the neutrons are moving more slowly. To slow down, the neutrons must collide with a nucleus that will absorb some of their energy but also not capture the neutron. Deuterium, or heavy water, has this property. Deuterium is an isotope of hydrogen that contains an extra neutron.
  • The heat energy is used to heat the heavy-water moderator.
• Heat is then transferred to the ordinary water to cause it to boil.
• Electricity generation continues in the same way as in the fossil fuel powered plant.

The Pickering nuclear generating station, Pickering, Ontario. Image courtesy of Wikimedia Commons.

Although not as efficient as fossil fuel power plants (the efficiency is 30% as opposed to 33%), nuclear power plants produce far less pollution. Fossil fuel plants release oxides of sulphur, carbon, and nitrogen that contribute to smog and air pollution.