This is the second day in our nuclear chemistry unit. The opening day of the unit, we watched "Seconds from Disaster- Fukushima" and students wrote 9 questions about what they observed in the video. Today, we answer some of those questions by looking at nuclear fission.
I have deliberately opened our nuclear chemistry unit with fission this year. Last year I began with decay and then did fission and fusion back to back. Students struggled mightily in differentiating between fission and fusion. I am hopeful that since this year I am separating the two stimulated nuclear processes by a couple of weeks, that students will keep them separate easier.
The activity worksheet to accompany the PhET simulation is downloaded and modified from the PhET site.
This lesson is aligned to the NGSS via 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. In the first time trying to do this unit last school year, we took the idea of "develop models" very literally, and had students make a poster or 3D model of fission and fusion. The project became students copying clipart off the internet onto a poster.
This summer I read Steven W. Gilbert's book Models-Based Science Teaching and he stresses that many of the models we develop in our instruction are mental models. Therefore, while we will use computer and other visual models to understand how nuclear processes happen, in alignment with Science and Engineering Practice 2, I am not going to be asking students to create physical models in class unless I feel it will be very beneficial.
The Energy and Matter Cross-Cutting Concept: In nuclear processes, atoms are not conserved, but the total number of protons plus neutrons is conserved is applicable throughout this lesson.
When students are in the room, I ask what they thought of the Fukushima video from the day before. Students are somewhat alarmed, especially as I prefaced the video with the fact that there are five active nuclear power plants in Illinois. They are curious if an accident like that could happen here, and we discuss how the earthquake and tsunami were the factors that led to the disaster.
My students were very curious about the human angle this year. They wanted to know what would happen to the workers who went in to repair the reactor, and how it affected people who live in the area. We talked a little about radiation risks for cancer and radiation poisoning, but deferred some of this until later when we talk about radiation. I talk briefly about having a friend from high school who is making a documentary about the organic farmers who live in the area, and how there is still radioactivity in the rice they are growing.
Not wanting to lose the students who are into this, I write the word "Nuclear" on the board. I ask the class which part of the atom are we focused on for this unit. They respond "The nucleus" and I point out that for the rest of the semester, we don't care what is happening in the electron cloud, where so much of our prior focus was. Everything now is about the nucleus until winter break.
I then send the students back to the computers to begin our investigation of how nuclear power, from fission, works.
This video gives an overview of the simulation.
Students will work on completing my modified Nuclear fission inquiry worksheet. The original sheet from the PhET site contained some parts, such as how to make a fission bomb, that were not relevant to my learning tasks. I also modified some of the questions to make them more user-friendly for my students.
As I stated in the screencast, students struggle to get started. Once they start playing with the simulation and shooting the neutron gun, they get into the activity and work pretty well. They struggle with the idea of a chain reaction, and I liken it to knocking over dominoes where the first one helps the next one.
I have found that when using the simulations, when students struggle, encouraging them to pause the simulation or slow it down when possible is powerful aide to helping them see what is happening.
As the period winds down, I encourage students to attempt to make an atomic bomb in the Chain Reaction tab. Although I removed this piece from the worksheet, it is a nice extender for the kids who finished early, and lets them get used to trying different combinations and experimenting.
To make the bomb, you have to have a critical mass of Uranium-235 (more than 75 seems to do it) in a containment vessel. If you don't resize the containment vessel, it limits your number of nuclei. When successful, the simulation shows a picture of a test of an early A-bomb.
When students finish, I collect their papers. In reviewing them, all students got the key takeaways of fission needing the addition of neutrons to start the process, and then that chain reactions work because the first nuclei to split provide the neutrons to split later nuclei. Students overused the term "explode" in place of "splits" in referring to the fission process, a misconception that is easy to correct the following day. I was very happy with this, because in the previous year, students got so caught up on the bomb questions that they didn't get the key concepts of how fission works.
Below are two copies of student work, the first is extremely sparse in how he describes what is going on in the simulation. However, his definition of "unstable" is not bad, except for the word "explode."
The second student used some of the same terms "exploding" but gave a little more detail in terms of what was happening in each step. Although this student completes missed the point of the control rods in a reactor, she made a nice clean definition of what an unstable nuclei will do.
This third student did the best job of clearly explaining in each step what was happening.