Exploring Nuclear Fission
with Phet & How Stuff Works
After using the PhET simulation & How Stuff Works readings, you will be able to:
· Describe how a neutron can give energy to a nucleus and cause it to fission.
· Explain the byproducts of a fission event.
· Explain how a chain reaction works, and describe the requirements for a sustained chain reaction large enough to make a bomb.
· Explain how a nuclear bomb works.
Go to PhET Simulations. Find and open the sim called Nuclear Fission
http://phet.colorado.edu/simulations/sims.php?sim=Nuclear_Fission
Click on ‘Run Now’ and select the tab that says ‘Fission: One Nucleus’
Fission: One Nucleus:
Experiment with shooting the neutron gun and watch what happens.
1) What happens when the U-235 nucleus is “hit” with a neutron? There are a number of things that happen here, describe all of them in as much detail as you can. Use the pause button when needed.
2a) In your own words, what does “unstable” mean when used to describe Uranium.
b) What causes it to be unstable.
3) Imagine that you have many U-235 atoms and you fire a neutron at one of them. What do you think will happen? Explain your prediction using words and drawings.
Chain Reaction:
Select the “Chain Reaction” tab at the top. Experiment with changing the settings and shooting the neutron gun and watch what happens.
4) Set the number of U-238 to 0 and experiment with changing the number of U-235. What happens when you fire the neutron gun at the different U-235 settings?
5) What is the minimum number of U-235 atoms required for a chain reaction.
6) Explain what makes this a “chain reaction”.
7) Click ‘Reset’ and then set the initial number of U-235 to 0 and change the number of U-238. Explain what happens when you fire the gun. Try multiple times. Explain what happens when you fire the neutron gun.
8a) What is U-238?
b) List at least 2 ways it is similar to U-235.
c) List at least 3 ways it is different from U-235.
9) 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 / 0U-238 / 0 / 30 / 50 / 70 / 100
% of 235U fissioned after 1 firing
# firings required to fission all 235U / N/A
What happens to the reaction as the proportion of U-238 nuclei increases? Explain why this happens.
10) If you were trying to design the most efficient nuclear fission bomb possible, what ratio of U-235 to U-238 would you want? Explain why.
11) Select the containment vessel box. Experiment with different numbers or U-235 & U-238. Explain any differences you see in the reaction.
12) If you wanted to explain nuclear chain reactions to someone, what would you tell them? Briefly, explain your ideas using appropriate vocabulary &/or drawings. Make certain that your answer explains why the reaction occurs AND what affects the speed of the reaction.
13) Why is U-235 a good isotope of Uranium for creating chain reactions?
How do Nuclear Bombs Work?
Go to http://science.howstuffworks.com/nuclear-bomb4.htm & Read the information on this page.
14) Define subcritical mass.
15) In what situations in the PhET simultions result in a subcritical mass?
16) Explain the difference between critical and supercritical mass.
17) What situations in the PhET simulations resulted in either critical or supercritical masses?
18) What is the job of the neutron generator in a nuclear bomb?
19) Which part of the PhET simulation represented the neutron generator?
20) What is the job of the tamper in a nuclear bomb & why is this necessary?
21) Which part of the PhET simulation “sort of” acted like a tamper?
Click on “keep reading”. You should see Fission Bomb Triggers.
On the Gun Triggered Fission Bomb image click “remove shell” then “detonate”
22) Using detail and vocabulary from this activity explain or draw what happened. (Drawings must include labels that include the vocabulary.)
23) How are subcritical & supercritical masses used in a nuclear bomb?
Activity modified from that by Drew Isola at Allegan High School
Optional: Return to the PhET simulation.
Nuclear Reactor:
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.
1) Watch very closely to the fission reactions as they happen. Specifically watch what 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?
2) 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.
3) If the purpose of a nuclear reactor in a power plant is to produce energy, why are there control rods?
4) 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?