Lesson 7: Fission Reactor Video Game

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Description

Now that students have learned how neutrons interact with atoms (experiment 3) and how the fission chain reaction starts and continues (experiment 6), they'll take a look inside a nuclear fission reactor. What are the most important parts? What makes them important? Most importantly, how is the nuclear chain reaction controlled?

These questions will be answered as students design their own two-dimensional nuclear reactor, choosing the size and location of its key components in the role of nuclear engineers. Then, they will take on the role of nuclear reactor operator as they try to control the nuclear chain reaction with neutron poison.

Why it Matters

Nuclear fission reactors are important for producing medical equipment and electricity. Understanding what's inside a reactor can help you make informed decisions about how close or how far from a fission reactor you want to live and work. Do you know how close you are to a nuclear reactor today? If a company announced a new nuclear reactor in your area, how would you feel?

Background Information

Click here to access slides with the diagrams from this section.

In a previous lesson, we learned about the fission chain reaction. When one neutron goes in and two come out, the number of fissions can increase exponentially, causing an explosion or meltdown. Nuclear fission reactors are designed to control the chain reaction. In this lesson, we will learn what goes in a nuclear reactor and how the chain reaction is controlled.

Each nuclear fission reactor is built to make use of one of the outputs of the fission reaction. If the reactor uses the neutrons from fission to make isotopes, it is called a research reactor or a test reactor. Isotopes made in test reactors are common in the medical field. If the reactor turns the energy from fission into electricity, it is called a nuclear power plant. Electricity from nuclear power plants makes up about 20% of the electricity on the U.S. electrical grid.

Caption: Research reactors use the neutrons from fission to create isotopes. Nuclear Power Plants (NPPs) use the energy from fission to create electricity.

No matter their purpose, all fission reactors also fall into one of two other categories. These categories get their names from the energy of the neutrons causing fission: fast reactors and thermal reactors.

When neutrons are released from fission, they have a lot of kinetic energy, which means they are moving fast. We call these fast neutrons. A small number of fast neutrons can be absorbed by certain kinds of nuclear fuel to cause fission. A reactor that depends on those fast neutrons to cause fission is called a fast reactor. When fast neutrons cause fission, the reaction produces more nuclear fuel, so fast reactors are sometimes called fast breeder reactors. Because the new fuel is more useful for making nuclear weapons, the United States usually chooses to go with the other kind of reactors, thermal reactors.

Caption: Fast reactors depend on fast neutrons for fission and produce more nuclear fuel.

In thermal reactors, low energy neutrons called thermal neutrons initiate most of the fission reactions. These neutrons are released from fission as fast neutrons, then slowed down by materials in the reactor until they reach an energy level that makes them much more likely to cause fission. Unlike fast reactors, thermal reactors don’t generate more nuclear fuel. Rather, the radioactive spent fuel from thermal reactors must be carefully packaged and stored for a long time after it is removed from the reactor. Thermal reactors are common in the U.S.

Caption: Thermal reactors depend on thermal neutrons for fission and do not produce more fuel.

This video game models a thermal reactor. Let's go over the components of the reactor now.

Fuel: Nuclear fuel has atoms that are very large and very close to fission. If a thermal neutron gets close to a fuel atom, it is absorbed, causing the atom to fission. Out of each fission come two fission products (new atoms), two fast neutrons, and energy.

Moderator: Thermal reactors use a moderator to slow down fast neutrons, increasing the likelihood that they will be absorbed by a fuel atom. When a neutron runs into a moderator atom, it bounces off of the atom, but the atom absorbs some of its energy, thus lowering the energy of the neutron. It can take multiple bounces on moderator atoms for a fast neutron to become a thermal neutron. This is the only component of a thermal reactor that isn't included in a fast reactor.

Caption: When a neutron collides with a moderator atom, it scatters (or bounces) off, but loses a little energy, becoming slower. A fast neutron may collide with multiple moderator atoms before slowing down enough to be a thermal neutron.

Poison: Poison atoms absorb neutrons without becoming unstable. Reactors are built with removable neutron poison, often in the form of control rods. The control rods can be slowly inserted or removed from the reactor, making sure to absorb exactly the number of neutrons we don’t want to cause fission. In the video game, even after you release the neutrons, you are able to change the poison insertion, just as real nuclear reactor workers adjust the insertion of control rods.

Caption: When a neutron collides with a poison atom, it is absorbed.

Coolant: Whether or not a reactor plans to make electricity from the energy released during fission, the energy needs to be removed from the reactor. Soon after fission, the energy released is turned into heat. Just like heat from a stove, heat from fission raises the temperature of the fuel where it is released. Fuel needs to be below a certain temperature to work well, so we flow coolant through the reactor. The coolant goes in cold and comes out hot. The faster coolant is flowing, the more energy can be removed, which means the reactor can reach a higher power level. In the video game, the coolant flow rate drops if it isn’t being used. You may need to turn it back up as you reach higher power levels.

Caption: Coolant flows through the reactor, sweeping away the energy released during fission.

Matrix: The matrix holds the fuel, moderator, poison, and coolant in place.

Reflector: The reflector goes all around the reactor. We can't control which direction a neutron will go. The reflector, like the moderator, is bouncy for neutrons. If a neutron hits the reflector, it will bounce back into the reactor.

Caption: When a neutron collides with a reflector atom, it scatters back into the reactor rather than escaping.

Shielding: Most fission products (the atoms that come out of fission) are unstable. They may release an alpha, a beta, a gamma, or a neutron as they decay. Alpha and beta particles won't escape the fuel, but the gamma rays are likely to escape the reactor. For this reason, we place a lot of large atoms outside the reflector. The more atoms, and the larger the atoms, the lower the dose to reactor workers nearby.

Caption: Unstable fission products can release gamma rays as they radioactively decay. Those gamma rays are attenuated by the shielding atoms.

If you're interested in learning more about nuclear reactor physics, check out MIT OpenCourseWare for in-depth nuclear science lessons. In the meantime, we hope you enjoy the video game!

Student Objectives

  • The student will describe the purpose of nuclear reactors and their key components.

Learning Objectives

  1. Each nuclear fission reactor was built either to harvest the neutrons from fission to make isotopes or to harvest the energy from fission to make electricity.
  2. Each nuclear fission reactor aims to control the fission reaction.
  3. There are fast and thermal reactors, each named after the energy of the neutrons.
  4. Fast reactors can make more nuclear fuel.
  5. Thermal nuclear reactors control the fission reaction using materials with special types of atoms:
    1. Fuel atoms fission if they absorb a thermal neutron.
    2. Moderator atoms are a little bouncy, so they can turn high energy (fast) neutrons into low energy (thermal) neutrons.
    3. Reflector atoms are very bouncy, so they help keep neutrons inside the reactor.
    4. Poison atoms absorb neutrons, but they don't fission. Poison can be added or removed during reactor operation to control the chain reaction.
    5. Shielding atoms absorb (or attenuate) gamma rays, which come from unstable fission products.
  6. Nuclear reactors also need coolant to control temperature inside the reactor.

Materials List

Material Preparation

These instructions are for Windows. If you are using Apple or Linux, please use the internet to figure out how to get python, numpy, and pygame.

  1. Install Python
    1. Using your start menu, go to the Microsoft Store.
    2. Download the most recent version of Python 3.
  2. Install numpy and pygame
    1. Using your start menu, open Terminal. Do not use the open button in Microsoft Store.
    2. Type this command into the terminal and hit enter:
      python3 -m pip install -U pygame --user
    3. Type this command into the terminal and hit enter:
      ​python3 -m pip install -U numpy --user
    4. Note for troubleshooting: If the lines above don't work, check for a space at the beginning or end of the line.
  3. Download GitHub Repository
    1. Go to https://github.com/cscrawford/nse-22033-nuc-education.git and create an account if prompted.
    2. Click the green <> Code button.
    3. Click Download ZIP from the dropdown.
    4. When your download is complete, find the downloaded folder, right click, and select Extract All.
  4. Open game
    1. Once the file is extracted, right click fission_reactor_game>Open With>Python3.11.3 (the numbers may be different).

Demonstration Video

Laboratory Instructions

Download a Printable Worksheet PDF

  1. Questions
    1. This video game will help you learn about nuclear reactors. Before you start, talk with your classmates about what you want to know. Write down any questions you have.
  2. Exploration
    1. With the fission reactor game pulled up on your computer screen, click "Start Tutorial".
    2. Click through the tutorial, reading the information at the bottom of the screen.
    3. Drag and drop one or more of each kind of dot onto the gameboard.
      1. Fuel
      2. Moderator
      3. Coolant
      4. Poison
        ​
    4. When you have finished placing dots, click the down arrow on poison insertion until you reach 0%. This will make the poison dots fade away.
    5. Click start.
    6. Write down the answers to the following questions:
      1. How long did your first reactor run?
      2. What power level did it reach?
      3. Why did your reactor stop? Options are supercriticality, overheating, or running out of neutrons.
      4. What surprised you?
      5. What do you want to try next time?
      6. What do each of the following do in a fission reactor?
        1. Fuel
        2. Moderator
        3. Coolant
        4. Poison
        5. Reflector
        6. Shielding
  3. Experiment
    1. Now that you know how to play, click Reset Gameboard and try something new (Repeat steps 1.3-1.5).
    2. Write down the following:
      1. What was different about this design? What was similar to your previous design(s)?
      2. Why did your reactor stop?
      3. How long did it run?
      4. What power level did it reach?
    3. Repeat steps 2.1 and 2.2 a few more times.
    4. Write down the following:
      1. Which design ran the longest?
      2. Which design reached the highest power level?
      3. Which dot do you think is the most important, and why?
  4. Reading or listening
    1. If you haven't already, talk with your classmates about the game.
      1. What strategies worked for you?
      2. What did you learn?
      3. Which dot did you think was the most important?
    2. Take a few minutes to learn about nuclear reactors, either by reading or listening to your teacher.
    3. Discuss or write down your answers to the following questions:
      1. What is one purpose of a nuclear reactor?
      2. Some reactors are called fast reactors. What are the other reactors called?
      3. Which kind of reactors don't need a moderator?
      4. Which kind of reactors can breed more nuclear fuel?
      5. Make any corrections you need to make to your previous answers.
  5. Competition
    1. Now that you've learned about nuclear fission reactors, play the game again, this time competing with your classmates for the longest operation time and the highest power level.
    2. If you have any unanswered questions, explore the supplemental resources to find the answers.

Suggested Evaluations

  • Ask each student to show their design and explain a few design choices (size and position of dots, poison insertion level, etc.).

  • Collect and review lab notebooks or worksheets linked above.
  • Create a simple reactor design. Use the cursor as an imaginary neutron on the screen. Set it in any direction and stop when it hits a dot or the reflector. Ask the student what the 'neutron' should do. Note: In order to give you control over the speed and location of the neutron, there will be no red dot on the screen, rather, your cursor will represent the neutron.

Supplemental Resources