Fission and Chain Reactions

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Objective

SWBAT Compare and contrast nuclear processes (fission, fusion and radioactive decay) in terms of particle and energy emissions and changes in mass and identity of daughter isotopes.

Big Idea

Students are introduced to the process of fission by using one of PhET's funniest simulations.

Introduction

In this lesson students continue to explore NGSS Performance Expectation HS-PS1-8:  Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion and radioactive decay. This lesson has my students model the process of fission, explain the byproducts of fission, explain how a chain reaction works, and describe the requirements for a chain reaction. 

In this lesson students will engage in SP2: Developing and using models.  When it comes to nuclear chemistry, simulations are a very effective way for students to visualize what otherwise would not be possible in a chemistry classroom.  The PhET Fission simulation does an excellent job of showing the process of fission, what is produced from fission and how a chain reaction occurs.  By modeling fission, students gain a clear picture of the difference between fission and nuclear decay that was learned in the previous 2 lessons.

As a result of modeling fission using a PhET simulation, students should see the Cross Cutting Concept, Energy and Matter, by describing how a neutron can give energy to a nucleus and cause it to fission.  

Homework Check-in and Quiz

23 minutes

Homework Review 7

After checking in the homework, I have the first page of the decay series answer Key showing on the document cam. This was completed in class on the previous day, but allows them to check for correctness and ask any questions after the homework check-in. 

For the next 5 minutes, after checking in the homework, I go over the decay series and graph that was to be completed for homework. By this point in the unit students have had enough practice working on alpha and beta decay problems that most do not have questions.  The only error that might occur is a careless mistake where the wrong atomic number or mass number was written down in a decay equation.  

Many students struggle with graphing, so I allow them to take a picture of the decay series and graph in order to check for accuracy in a more efficient manner outside of class.  I do this because graphing and interpreting data is one of the Science and Engineering practices that should be worked on continuously through high school and is an integral part of my class.

Quiz 15

After the short homework Q&A, I hand out the quiz on nuclear decay.  The quiz only takes about 10 minutes since its multiple choice.  I tell them that they need a periodic table, and once the quiz is completed to put it in the folder that divides them from the person next to them.  After the quiz is completed we grade it in class.  Typically students don’t mind having another student grade their paper, so this is a strategy that I use to save time on grading and provide immediate feedback.  

80% of students scored a B or better on the quiz (student work); however, some still struggled with the extra credit decay series. I believe their difficulty is coming from taking short cuts and not writing the full decay equation.  For the students that are still struggling I provide them with another practice decay series with one-on-one help to move them past their difficulties.

Explore

20 minutes

As they were taking the quiz I handed out the Phet Nuclear Fission assignment.  This simulation provides them with their first exposure to the process of fission, how a chain reaction occurs and the basics behind fission in nuclear reactors.  Using this simulation they see that a neutron is needed to start the process of fission and in the process U-235 turns into an unstable U-236.  The unstable U-236 eventually breaks apart into two daughter products and three neutrons. 

Part I:  Fission, One Nucleus 

This is a good introduction to how fission works, although it does not express to students the amount of energy that is produced from the fission of U-235.  I recommend they should only spend a few minutes on the first part of the investigation, but make as many observations as possible about what they see.  Some possible explanations and observations:

  • As the neutron hit the U-235 it changes to U-236.  At this point it reaches a higher energy state, begins to shake and become unstable and then breaks apart.
  • In the process of breaking apart, two daughter products are produced and three neutrons are released.
  • The daughter products and neutrons fly apart with energy.

 Part II:  Chain Reaction

This part of the lesson shows students a chain reaction can occur through the process of firing one neutron at U-235.  They see that as one U-235 isotope splits three neutron are release with the potential to hit other U-235 isotopes causing the fission reaction to continue.  This process will help students develop an understanding that energy is released during fission.  As they are performing this part of the simulation they should develop an understanding that:

  • As a neutron is fired from the gun and potentially hits a U-235 isotope, depending in where it was aimed.
  • A chain reaction occur when the U-236 splits into 2 daughter products and releases 3 neutrons in the process.
  • As neutrons are propelled from U-236 hitting other U-235 isotopes, this causes the event to continue in a “chain reaction”.
  • When a neutron hits a U-238 it becomes U-239 but does not become unstable
  • A chain reaction is not produced because Neutrons are not released; therefore not hitting other isotopes.
  • Nothing happens when multiple neutrons are fired because U-238 does not become unstable and release neutrons

Part III:  Nuclear Reactor

The final part of the investigation has them explore how a chain reaction is involved in the process of creating energy in a nuclear reactor.  This part of the simulation applies a real world application to the Crosscutting Concept, energy and matter, by showing them that energy is released during a chain reaction and can be controlled by the absorption of neutrons.  The main concepts that should be obtained from this section are:

  • Neutrons are released
  • When neutrons are absorbed by control bars and the chain reaction ends.
  • Neutrons are not absorbed and more U-235 splits releasing more neutrons.
    • Every time an isotope of U-235 is split energy is released, this energy need to be controlled otherwise a meltdown will occur.

Most students get through the simulation, answering all the questions (answer key); however, due to the nature of the simulation being like a game some students need to be directed to focus on the task at hand.  At the end of the simulation students are asked to summarize the key facts of the investigation in their journals.

Evaluate

10 minutes

As a conclusion to the assignment I have them complete a 3-2-1; a technique I've adapted from Page Keeley’s, 75 Science Formative Assessments.  This strategy is a structured way for students to scaffold reflections to key learning points. This also provides me with the opportunity to see what students perceive as key learning from the assignment, and what they may be still struggling with so that further lessons can be adapted accordingly.

This strategy is very easy to implement and requires very little time.  However, it does require medium degree of thinking on the student’s part by having them reflect on their own learning.  As they are finishing their assignment with about 10 minutes left in class, I put on the board directions to write the following in their journals:

  • Three key ideas I will remember
  • Two things I am not sure about (in reference to fission)
  • One thing that I will work on for tomorrow

Once they are finished with the 3-2-1 I collect their journals at my desk so I can look them over while the next class is doing the simulation.  This provides me with immediate feedback which I will use to begin the next class periods conversation about fission.

Some think-pair-share conversation starters for the next class might be:

  • What happened to U-235 when hit by a neutron?
  • What was the difference between u-235 and U-238?
  • Explain how control rods where involved in a chain reaction.