PhET Simulation: Nuclear Fission
Journal Entry # 33
After completing this activity, you will be able to explain the concept of a “chain reaction” and how it applies to nuclear fission of Uranium. You will also be able to explain the purpose of the control rods in a nuclear reactor.
Click to run the simulation.
Fission - One Nucleus:
Select the “Fission: One Nucleus” tab at the top left. Experiment with shooting the neutron gun and watching what happens. Then answer the questions below.
1) What happens when the U-235 nucleus is “hit” with a neutron? There are two steps
here- describe them both in as much detail as you can.
Select the “Chain Reaction” tab at the top. Experiment with changing the settings and shooting the neutron gun and watch what happens. Then answer the questions below.
2) Set the initial number of U-235 nuclei to 100. What happens when you fire the
3) Explain what makes this a “chain reaction”.
4) Set the initial number of U-238 nuclei to 100. Explain what happens when you fire
the gun and if this is a chain reaction or not.
5) Set the initial numbers of U-235 nuclei and U-238 nuclei to the numbers in the table
below. Record your results.
U-235 100 70 50 30 0
U-238 0 30 50 70 100
% of U-235 fissioned after 1 firing
# firings required to fission all U-235 N/A
6) What happens to the reaction as the proportion of U-238 nuclei increases?
7) If you were trying to design the most efficient fission reactor possible, what ratio of
U-235 to U-238 would you want? Explain why.
Select the “Nuclear Reactor” tab at the top. Experiment with changing the settings and firing the neutrons and watch what happens. Then answer the questions below.
8) The bar graphs on the right of the display show the “Power Output” and the “Energy
Produced”. What is the difference between these two quantities?
9) Watch very closely to the fission reactions as they happen. Specifically watch
happens to the loose neutrons after the reaction.
a) What happens if the neutrons hit another nucleus?
b) What happens if the neutrons hit a control rod?
10) Compare the chain reaction that occurs when the control rods are inserted further into
the reactor versus when they are pulled all/mostly out of the reactor.
11) If the purpose of a nuclear reactor in a power plant is to produce energy, why are
there control rods?
Why should we learn about nuclear energy?
How does this affect our lives?
Watch the videos below.
Activity by K. Gates from phet.colorado.edu/simulations.