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Nuclear Energy: Classroom Activities

Nuclear Fission Simulation


To develop a better understanding of how a neutron can give energy to a nucleus and cause fission. To understand how a chain reaction works and how a nuclear reactor works.

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Background Information

In addition to nuclear fuel rods composed of an isotope that readily splits to release energy, a moderator is an essential component in a nuclear reactor. A key characteristic of the nuclear chain reaction is that every byproduct that is produced releases more neutrons that can react with unused fuel. The moderator absorbs neutrons and limits the rate at which the fuel is spent. Otherwise the reactor would release too much energy too quickly and quickly become unstable. Various materials are used as moderators, including, water (heavy and light), graphite, lithium or beryllium, and lead and bismuth. For the safe and efficient generation of electricity a constant and predictable flow of energy is required.

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Activity Overview

Students conduct several simulations that explore fission reactions using U-235 and U-238 as a moderator. They examine the release of particles, energy released from reactions, and the effects of containment and the use of moderators in a nuclear power plant. Students also learn about the risk of creating runaway chain reactions that have a nuclear bomb effect.

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Materials and Equipment

computer, internet, simulations.

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  1. 1. Have students go to the following website:
  2. Tell students to click on the "Run Now" button to run the "Nuclear Fission" simulation at the top of the page.
  3. The simulation offers three tabs at the top. Be sure students have the "Fission: One Nucleus" tab open. Have them complete the following:
    1. Try to figure out how you can make U-235 unstable
      Students use the neutron gun to fire a neutron at the atom. This changes U-235 to U-236 through the addition of a neutron. U-236 is unstable and quickly fissions into two daughter nuclei.
    2. How do you know it's unstable?
      "Unstable" means that the nucleus has too many neutrons to hold itself together. You can tell that the nucleus is unstable because it breaks into two daughter nuclei, and because the energy graph shows that this is in an energetically unstable state (i.e., the split atom is lower energy than the unsplit U-236). There is no magic ratio of neutrons to protons that is always stable - it depends on the particular atom.
    3. How do the potential and total energy relate to the activity of the U-235 nuclei?
      Splitting the U-235 nuclei increases total energy but the potential energy remains the same.
    4. What is the result of the fission of the U-235?
      Two daughter elements are produced.
  4. 4. Have students explore the "Chain Reaction" tab. Have them answer the following:
    1. What number of U-235 atoms gives you the highest percent of nuclei fissioned?
      The great the number of U-235 atoms the higher the percent of nuclei fissioned.
    2. What happens when you include U-238?
      U-238 absorbs one neutron to become U-239 and does not fission.
    3. What happens when you include a containment vessel?
      The reaction becomes concentrated if unrestricted and all the U-235 is fissioned. Using a containment vessel creates the risk of creating a runaway chain reaction, or atomic bomb. By adding U-238, which absorbs neutrons, the amount of fissioned U-235 can be limited to prevent the reaction running away.
  5. Have students explore the "Nuclear Reactor" tab. Have them answer the following:
    1. How does changing the Control Adjuster affect the total energy and temperature?
      They control the rate of fission of the uranium in the reactor by absorbing neutrons and daughter nuclei. When partially removed, they allow a chain reaction to occur. Thus, the presence of control rods allows the reaction to be slowed or stopped, preventing the nuclear reactor from becoming a nuclear bomb.
  6. Have students discuss why U-235 is a good isotope of uranium for creating chain reactions.
    U-235 is ideal for creating a chain reaction because it splits into two daughter nuclei. Only one daughter nuclei is necessary to induce fission in another U-235. So, since the number of fission products is more than the number required to induce fission, the chain reaction keeps going.

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