Fission vs Fusion

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Objective

SWBAT compare and contrast the two stimulated nuclear processes: fission and fusion.

Big Idea

Fission and fusion are opposite nuclear processes that generate tremendous amounts of energy.

Background

Although my intent in structuring this unit was to keep fission and fusion far away from each other, it backfired.  By this point, students had little recollection of nuclear fission.  Since there were only two days left in the semester that I could spend on this unit, I had to do some direct instruction on fission and fusion together here at the end.  Unfortunately, this led to my exact fear-- students confused the two on the unit exam.  Most had it straightened out by the final exam.

I'm hoping next year to teach more context with this unit, including half-lives with the radioactive decay.  This year we left half-lives out as they are beyond the assessment boundary for PS1-8.  We were also fairly short on time.  As our curriculum settles down after two years of revising and beginning to implement the NGSS content, we are seeing more avenues to bring real life situations into the classroom.

This lesson is aligned to the NGSS via HS-PS1-8Develop 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. 

Fusion Bellringer

5 minutes

When students enter the room, I have out a piece of paper on each table.  Once the bell rings, I ask students to write down, without their notes, everything they remember about nuclear fusion from the previous day.  They can work as a table, or pass the paper around the table.

Common items were:

  • Fusion happens in the sun
  • Millions of degrees
  • More energy than we'd ever need
  • Small elements sticking together to make larger elements

While these are labeled as common, the most important piece, the last one, was rare around the room.

At this point, I asked students to get out their notebook or paper for notes, so we could review Fission and Fusion before the exam.

Reteaching Notes

30 minutes

This is a very bare-bones PowerPoint, as it is a review of what students have already encountered.  I begin by explaining to students that this is a review, and their writing should reflect either "I don't really remember this" or "That really helped me understand this" moments. 

For most students, this is interpreted as "I must copy everything just to be safe" or "I already know all of this, so I can just watch and listen."  I don't have any student samples where they picked and chose what they wanted to include in their notes based on their own individual understandings.

We begin with nuclear fusion.  I refer to the student bell ringers, and that students put that there were small elements making heavier ones.  I show them the animated gif and picture from the PowerPoint.  I go over the definition, and then go further to explain that elements up to Iron can be made via fusion in normal stars.  Everything heavier came from a star exploding as a nova or supernova.  I tell students the Neil deGrasse Tyson (maybe originally Carl Sagan?) line that since all elements are made in stars, their own bodies are simply recycled stars.  Some students think this is very cool, whereas others...not so much.

I then refer to the occurrence of fusion, and about half of my students are confused by the "other stars" after the Sun.  So we discuss how the sun is the closest star, and the recent discovery of exo-planets.  I also explain the different between thermonuclear, or hydrogen, fusion bombs developed after 1953 and the atomic fission bombs dropped on Japan in 1945.

I use the atomic vs nuclear weapons discussion to transition to nuclear fission.  I let students watch the gif and the picture first to refresh their memory about fission.  Then I refer back to the first computer simulation of the nuclear unit, the PhET Fission investigation before Thanksgiving.  We then define fission again as the splitting of a heavy nucleus into smaller nuclei.

I follow that with a discussion of nuclear power in Illinois, and show the map of where the nuclear plants are relative to our location on the edge of Chicago.  Invariably, students ask how an accident at one of the plants would affect us, and we talk about it briefly, but I use the picture of the Braidwood station to allay their fears, explaining how the containment buildings are structured to keep any accident inside.

The final slide I ask the students to focus just on the temperature and the energy.  We show how hot fusion reactions are compared to burning fossil fuels or even fission.  I also write out the energy amounts on the board in long hand as students are fully fluent in scientific notation.   We discuss how fusion is the desired energy source, since it is clean and gives so much power, and students ask if the high temperature is why it doesn't work.  Another student will usually follow up with "Is that why it makes such a great bomb?"  I explain that it is precisely why it is such a good weapon.

Unit Test Study Guide

15 minutes

With the notes complete, we begin to review for our unit exam with the remaining time. I choose to have students review independently so I can gauge their readiness.  I have found in our games and class activities that students will passively listen to other students' answers and claim that as "participating".

First I have students read and highlight the Introduction to Nuclear Chemistry reading.  I do not give students the quiz, as I want them to just read through this now to refresh themselves on the three decays and two stimulated processes (fusion, fission). 

When students finish, they take the first nuclear chemistry test review which has parts 1 and 2 of the review.  Students use the remainder of the period to work on the review.  I remind them to work without their notes first, to practice recall and identify what they already know.

Here is a student sample of part 1, which is focused on spontaneous decay and alpha, beta and gamma emission.

Part two is focused on the difference between fission and fusion.  Although these were taught at the beginning and end of the unit respectively, students still confused them on the exam.

The next day, students will get Nuclear Chemistry Study Guide pt 3 after we review parts 1 and 2.  Part 3 is more summative, requiring students to compare and contrast the different decays, fission and fusion, and complete an additional decay chain.  T

These style of charts are on the exam, so I wanted students to see the format prior the exam.  This provides students a day and a half of individual, supported, in class review for the unit exam.

Students averaged a 67% on the exam, which was the highest average of the year.  This was the first exam that wasn't purely multiple choice, and students struggled in the short answer with comparing and contrasting fission and fusion and explaining why some isotopes undergo alpha decay whereas others undergo beta decay.  These will be areas of focus when we revisit our curriculum next summer.