Nuclear Labs
Name: _______________________________ Page 1 of 10
Nuclear Reactor Simulation:
Go to this website http://www.ida.liu.se/~her/npp/demo.htm and do 2 of the 3 sequences. Describe
what you had to do to save the plant and what you have learned from the experience. Type your
descriptions below and email this page to me.
PhET Simulation: Nuclear Fission
(Email the answers to these questions.)
Learning Goals:
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.
Directions:
 Go to the website: http://phet.colorado.edu/simulations .
 Click on “Work, Energy, and Power” on the left hand side
 Choose the simulation titled: Nuclear Fission and then select: “Run Now”.
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.
Chain Reaction:
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 neutron gun?
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
U-238
235
% of U fissioned
after 1 firing
# firings required to
fission all 235U
100
0
70
30
50
50
30
70
0
100
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.
Nuclear Labs
Name: _______________________________ Page 2 of 10
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.
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?
Nuclear Labs
Name: _______________________________ Page 3 of 10
Nuclear Lesson
Traci Radil (email)
(When this activity was written, the simulation was called Nuclear Physics. Since then the simulation was divided into
2 simulations: Alpha Decay and Nuclear Fission)
Purpose: Describe the process of alpha radiation.
Relate potential and total energy to what happens in the nucleus.
Investigate a chain reaction.
This activity was designed for use in a Global Science class to go along with the chapter on nuclear energy.
An example of a student handout follows.
Go to http://phet.colorado.edu/new/index.php
Click on Play with Sims
Click on Quantum Phenomena on the left side
Click on Alpha Decay. Also open Nuclear Fission
Explore each section of the simulation and then answer the following questions.
Alpha Radiation
1. Describe how the 211Po atom decays into 207Pb. Include what is happening with the energy and the
time it took to decay. You may have to move through the simulation step by step.
Fission
1. How do the potential and total energy relate to the activity of the U-235 nuclei?
2. What is the result of the fission of the 235U?
Chain Reaction
1. What number of U-235 atoms gives you the highest percent of nuclei fissioned?
2. What happens when you include 238U?
3. What happens when you include a containment vessel?
Nuclear Reactor
1. How does changing the Control Adjuster affect the total energy and temperature?
Nuclear Labs
More Nuclear Decay
Name: _______________________________ Page 4 of 10
(Handwritten)
a) PhET Simulation of Nuclear Decay
Go the PhET simulations at http://phet.colorado.edu/simulations/ and choose Quantum Phenomena, Alpha
Decay. Hit the reset button a few times.
4) Label the diagram above locating the
a) total energy of the alpha particle
b) potential energy of an alpha particle due to Coulomb repulsion
c) potential energy of an alpha particle due to the strong nuclear force
d) the classical limit of how far the electron can go from the centre of the nucleus
e) the quantum limit of how far the electron must go, to be able to escape
5) How is the movement of the alpha particle different from a classical particle?
6) A particle that appears beyond the quantum limit and then repelled, is said to have ‘tunneled’ through the
barrier. How is this different from classical tunneling?
7) Why won’t a second alpha particle escape?
8) How long will it take until the next decay?
9) Is the PhET simulation intrinsically random or just pseudo random?
B) Half-Life
10) The half-life of a nucleus is the time needed until there is a 50% chance that it will have decayed. It can
be found by taking a large sample and measuring how long until half of them have decayed. Using the dice
model
a) How many rolls is one half-life? Explain how you get this answer.
Nuclear Labs
Name: _______________________________ Page 5 of 10
b) Do an experiment with many dice. What was the half-life?
c) Why were these answers different?
d) Calculate the percentage difference of your two answers.
e) What would you need to do to get them exactly the same?
f) What is the whole-life? How long will it be until all the dice have decayed?
g) Suppose you came in late and there were 15 students in the class still undecayed. How many rounds
do you think occurred while you were out? Explain how you got your answer.
11) Go to the PhET simulations at http://phet.colorado.edu/simulations/ and choose Quantum Phenomena,
Alpha Decay.
a) Measure the decay times for 25 events.
b) What was the have-life for these 25 nuclei? Explain how you calculated this.
c) Compare your answer to someone else’s. How different is it? Why is it different?
d) To three digits, the half-life of Po-111 is 520 ns according to Fred Noel Spiess in Physics Review 94,
1292-1299 (1954). How many rolls would you need to be certain to have two significant digits?
12) Radioactive decay is used to date fossils, rocks etc. They measure the ratio of decayed nuclei to
undecayed nuclei. However, for this information to be useful, you need to know what the starting value was.
Do research to find out how this is done for two different techniques.
Nuclear Labs
Name: _______________________________ Page 6 of 10
More Radioactive Decay (Handwritten)
Find and open the PhET simulation “Alpha Decay”: http://phet.colorado.edu/simulations/sims.php?sim=Alpha_Decay
Radioactive Decay in a single atom:
Open the “Alpha Decay” sim and go to the SECOND tab (single atom).
What is an alpha particle?
The simulation shows an atom’s nucleus: In this representation, are all the nucleons (protons and neutrons)
in a center “nuclear ball,” or is the nucleus distributed in scattered sections? Explain the structure.
The force keeping the protons and neutrons together is known as the nuclear force. Just like the basis for
interplanetary forces is the fundamental force “gravity,” the basis of the nuclear force is this fundamental
force:
_____________________. It gives structure to both nuclei and nucleons.
In the simulation, a diagram of the potential energy “terrain” created by the nuclear force is shown in blue.
The alpha particles are located in the diagram also… and they have a fixed total energy (shown in red).
Based on classical mechanics, would the alpha particles be able to get out of the nucleus?
In this representation, do the alpha particles stay in the “potential well” of the nucleus, or do they move into
the barrier?
Define “tunneling”:
Define “atomic number”:
When an alpha particle tunnels out of the nucleus, is the atom still an atom of the element “polonium”
(atomic number 84), or has the atom changed to a new element? If so, what element?
The simulation labels the atoms according to the “mass number” (not atomic number). Define “mass
number”:
How many protons are in the original atom (polonium-211)? How many neutrons?
How many protons are in the atom after it decays to lead-207? How many neutrons?
What is the difference, and what does that make up?
Nuclear Labs
Name: _______________________________ Page 7 of 10
The simulation has a reset button. When you reset it (many times to check well), is the time is takes the new
nucleus to decay the same, or about the same as the old time or is it random?
Radioactive Decay in a sample of atoms:
While the atomic decay is random, for larger amounts of radioactive material, the overall decay of the
population of atoms can be characterized by a constant known as the “half-life” (the time is takes for about
half of the amount to decay).
Now go to the FIRST tab (multiple atoms) and keep adding 10 atoms until the entire bucket is empty (there
are 99 total atoms). Because the simulation starts automatically, you’ll need to “Reset all Nuclei” before you
take data. Every ½ second, pause the simulation and note the number of polonium atoms. Plot below:
Number of Polonium atoms remaining
100
75
50
25
0
1
2
3
Time elasped (seconds)
What is the mathematical shape of the graph (linear, parabolic, etc.)?
Practice:
Say material “X” decays radioactively into material “Y” with a characteristic half life of two days. Assume
you start with 100 kg of X. In the graph below, plot the amount of material of “X” that remains after 2 days,
4 days, 6 days, and 10 days:
Nuclear Labs
Name: _______________________________ Page 8 of 10
Mass of X remaining (kg)
100
75
50
25
0
5
10
Time elasped since X first measured at
100 kg (days)
Say X has a safe level of 5 kg (for the given area it is located in). How long does it take to reach a safe level?
If the original level is 50 kgs (half the original size), does it take half the time to reach a safe level? Explain.
(You may want to add a plot of this sample to your graph to help you, though it isn’t required.)
Add a plot for the decay of 100 kg of material “Z,” which has a half-life of 1 day. Label this line. Then
answer this question: how long does it take material z to reach a safe level of 5 kg? How does this compare
to the time it took material X to decay to this level?
What happens to the mass lost by X in the radioactive decay? Is mass conserved, and if so, how?
Fission:
Open the “Nuclear Fission” sim: http://phet.colorado.edu/simulations/sims.php?sim=Nuclear_Fission
Start at the tab “Fission: One Nucleus”
Define fission:
Run the simulation. Before the gun is fired, is the material stable, or does it seem likely to radioactively
decay?
Nuclear Labs
Name: _______________________________ Page 9 of 10
When you operate the gun, what type of particle does it fire?
If the gun fires and “hits” the nucleus, what happens?
Switch the tab to “chain reaction” and add some uranium-238. The atom used in the previous tab was
uranium-235. Is uranium-238 “fissionable”? How does firing the gun on a uranium-238 atom change it?
(Note you can aim the gun.)
Chain reaction:
Switch to the tab “Chain Reaction”.
Reset the sim (using the button) and add lots of fissionable uranium-235. What happens and why?
Normal levels of the uranium-235 isotope are about 0.72%, with the majority being uranium-238. Round the
level up to 1% (in the favor of uranium-238), to one atom of uranium-235 and 99 atoms of uranium 238. Use
the simulation to discover if naturally-derived uranium can start a chain reaction (and therefore be useful in
either nuclear weapons or nuclear power plants): Is naturally derived uranium able to start a chain reaction,
or must the sample be “enriched”?
Use the simulation to find a minimum ratio of uranium-235 to uranium-238 (keep your total of atoms always
at 100); what is the smallest percentage that still starts a chain reaction?
How does the above compare to “weapons-grade” enriched uranium (about 80-85%)?
Use the simulation to make a nuclear weapon. What conditions are needed? (Hint, you’ll need a containment
vessel, and a certain level of enrichment that you must determine.)
What is a “dirty-bomb” (you may have to look it up online). Is a high grade of uranium needed for a dirtybomb to explode?
Nuclear Labs
Name: _______________________________ Page 10 of 10
Nuclear power plants:
Switch to the “Nuclear Reactor” tab.
What is needed to start the nuclear reactor?
What does adjusting the control rods do?
Without the control rods in position, what happens?
Homer Simpson is asleep at the Springfield Nuclear Power Plant (where he’s
known to occasionally work.. although mostly he’s asleep at the controls). When
the power plant sounds an alarm (d’oh!), what is happening, and what must
Homer do to save the day?
http://www.yoest.org/archives/homer_simpson_nnuclear_power_plant.jpg