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					POGIL – Nuclear Fission & Fusion

Background
Fission and fusion are two processes that alter the nucleus of an atom. Nuclear fission provides the energy in
modern day nuclear power plants and fusion is the source of the sun’s energy. The use of fission in power
plants can help conserve or one day possibly eliminate the need for fossil fuels. Without the energy produced
by fusion of hydrogen in the sun, the Earth would quickly change into a cold planet that could not support life as
we know it.

Transmutation can be accomplished through many types of reactions, including fission. Nuclear transmutations
always obey two fundamental conservation laws:

    (1) Mass number is conserved
    (2) Electrical charge is conserved

Energy and mass are not conserved, but can be interconverted according to Einstein’s equation:
                                                                2
                                                     E = mc

Where E = energy, m = mass, and c = the speed of light

Nuclear Fission
The process of fission occurs when a nucleus splits into smaller pieces. Fission can be induced by a nucleus
capturing slow moving neutrons, which result in a highly unstable nucleus.




                                               http://www.arpansa.gov.au/images/basics/fission.jpg

Watch the following clip for a brief explanation (double-click on Fission.mov) :

                                                      Fission.mov

For an animation of this process, view the PhET model linked below and click on the “Fission – One Nucleus”
tab.

http://phet-web.colorado.edu/sims/nuclear-physics/nuclear-physics.jnlp

Nuclear Fusion

Fusion occurs when 2 nuclei join together to form a larger nucleus. For a fusion reaction to occur, the atoms of
hydrogen must be heated to millions of degrees and subjected to high pressure. When done properly, the
positively charged nuclei of the hydrogen isotopes, which naturally repel each other, combine.
The following are some examples of nuclear equations illustrating fusion:

2       2    3    1
1   H + 1H → 1H + 1 p
2
1
        3
    H + 1 H → 24 He + 01n
3
2
                            1
    He + 23 He → 24 He + 2( 1 H )

Key Questions
   1. Differentiate between fission and fusion. How can you identify each?

Fission involves the splitting of a massive nucleus to produce energy, whereas fusion involves the smashing
together of lighter nuclei to form a more massive nuclei and energy.

      2. How do deuterium (hydrogen-2) and tritium (hydrogen-3) differ?

Deuterium contains one less neutron than tritium.
   3. What quantities are conserved in nuclear transmutations?

Mass number and atomic charge (atomic number) are always conserved even though mass is not.


      4. The fusion equations show the production of atoms of several different elements, even though each
         reaction begins with isotopes of hydrogen. Knowing the starting elements, can one predict what
         element will forma as a result of a given reaction? Explain why or why not.

You could narrow down the possible answers because you are limited to products that do not exceed the total
mass number of the reactants. Also, you know that one of the products of fusion must be larger than either of
the starting nuclei.

    5. The fission equations show the production of many different elements, even though each reaction
        begins with Uranium-235 and one neutron. How is this possible given the conservation laws for nuclear
        reactions?
Many different combinations of daughter nuclei are possible. The conservation laws say nothing about numbers
of atoms being conserved, only mass number and charge.

Exercises
   6. An equation in the model shows the fusion of two deuterium nuclei to form a nucleus of tritium. Suggest
       another product that might form in this reaction.

                                                2       2                4
                                                1   H + 1H →             2   He

      7. Describe how to find the identity of the species X in the equation:

                                          235       1         152
                                            U +
                                           92       0   n →    60   Nd + X + 3( 01n )
To solve this problem, mass number and charge must be conserved. So, 235 + 1 = 152 + x + 3. Solving for x
will yield the mass number of the unknown. Using the same methodology, the atomic number of x can also be
determined.

      8. What is missing in the following reaction?

                                    235
                                      U + 01n → _____ 144Cs ___ + 2( 01n ) +
                                     92                55
                                                                                        90
                                                                                        37   Rb

      9. An atom of U-235 absorbs a neutron and produces an atom of Sb-51 and four neutrons. Identify the
         other nuclide formed in this reaction. Write the equation to support your answer.
    235
      U + 01n → 4( 01n ) +
     92
                                 121
                                   Sb +
                                  51
                                                  111
                                                   41   Nb



    10. Propose an additional series of products possible from Uranium-235 fission, other than those used as
        examples or exercises in this activity. Write a nuclear equation to illustration your answer.

    Answers may vary, but should include the production of 2-4 neutrons and two daughter nuclei. Answers
    must obey the conservation of mass number and charge.

    11. Identify the following equations as fission or fusion.
                             2       2   3    1
                             1   H + 1H →1H + 1 p                                       [Fission] [Fusion]
                       235             1           141          92             1
                        92 U + n →     0            56   Ba +   36   Kr +   3( 0   n)      [Fission] [Fusion]
                       235
                        92 U + 01n →               138
                                                    54   Xe +   95
                                                                38   Sr + 3( 01n )         [Fission] [Fusion]
                       3                   3
                       2   He +            2
                                                               1
                                               He → 24 He + 2( 1 H )                       [Fission] [Fusion]

Research

    12. What is the source of energy in fusion and fission reactions? Explain your answer as well as identifying
        your source(s) of reference.

The energy comes from the nucleus, called the binding energy. References can vary from the text (Ch. 34) to
various other sources.

    13. Name one problem associated with the use of nuclear energy and briefly explain why it is a problem.
        Also identify how that problem might be addressed.

-Poor social perception; educational programs could address this.
-For fission, the generation of hazardous radioactive waste is a very big issue. There are some who believe that
the waste can be safely stored, but that is debatable. It would be optimal to develop a methodology to convert
the waste into inert by products, possibly through fusion.
Many other answers are possible.

    14. During the 1950’s, when many countries performed above ground tests of nuclear weapons, there was
        much concern about the radioactive fallout when the by-products of uranium fission landed on
        populated areas. One major concern dealt with the fallout of radioactive strontium isotopes and a
        suspected link to increases in leukemia and bone cancer. What part of the body might absorb
        strontium? Explain your answer.

The bones, in particular white blood cells and lymphocytes could absorb strontium. This is evidenced by the
types of cancerous outbreaks noted with exposure to radiation from a weapons test.

    15. As a group, respond to the pre-questions below, and then watch the video clip and respond to the post-
        questions that follow:

    Background
    At the National Ignition Facility, run by Lawrence Livermore National Laboratory, scientists are creating a
    system to replicate the fusion described above by using lasers to create the high heat and pressure needed.
    At the center of the project is a gold cylinder the size of a dime. This gold cylinder, called the hohlraum,
    houses a capsule containing the hydrogen isotopes – the fuel for the fusion reaction. NIF scientists will blast
    the hohlraum with 192 laser beams simultaneously (containing a total of 1.8 million joules of energy) for a
    few billionths of a second. The cylinder will produce x-rays that compress and heat the capsule resulting in a
    nuclear fusion reaction.

          Pre-questions:
        (a) What do you already know about fission, fusion, and nuclear reactions?

        (b) List as many ways humans generate power as you can think of and label each as renewable or non-
        renewable.

        (c) What are isotopes?



    Control- Click to open the weblink:

                                   http://www.kqed.org/quest/television/view/842

    Post-questions:
    (d) What are the reactants and products for the reaction at NIF? Write a chemical equation showing this.




    (e) Is this an exothermic or endothermic reaction? How do you know?

The whole goal is to create a sustaining fusion reaction such that energy can be generated.



    (f) On what outside factors, beyond the chemical reactants, does this reaction depend?

Extreme pressure, carefully controlled temperature, and a massive amount of initial activation energy [heat] (to
initiate the fusion reaction).

    (g) How does this story demonstrate E=mc2?

As the hydrogen is fused, mass is converted to energy. The amount of energy generated is equivalent to the
mass “lost” times the square of the speed of light.

    (h) What is the controversy with the research being done at NIF? What are your thoughts on it?

    The caution is that NIF will be used to test nuclear weapons processes that are currently banned. NIF is
    seen as a possible loop-hole to allow the US government a way to continue to investigate and test fusion
    weaponry.


    Critical Thinking
    16. Imagine that you could compare the masses of different atoms by grabbing their nuclei with your bare
        hands and shaking the nuclei back and forth. If you were to plot all the elements, you would end up with
        a graph much like this:




           (a) Is this plot surprising? Explain why or why not.
NO. It makes sense that the large the nucleus, the more massive it is.
            (b) The mass of a nucleus and the mass of the nucleons that make up a nucleus are very different
                things. What do you think a graph of mass/nucleon vs. atomic number would look like? Sketch
                your prediction in the space below.

The graph would most likely be predicted to be a horizontal line.

            (c) Compare your sketch to the figure below. What does this figure tell you about the mass of
                nucleons in different elements?




                 From the graph, it is evident that the mass of a nucleon varies depending upon which elemental
                                                         nucleus it is located.
              (d) Will the process of fission or fusion release energy from the nucleus of iron?
Iron will not release energy whether it is fused or fissioned. It is in the “sink” of graph above. Either fusion or
fission will necessitate a net input of energy.


Conclusion: In concise paragraphs, respond to the following questions:
   □ How do the processes of fission and fusion fundamentally differ? Provide an example for each other
      than those shown in this POGIL activity.
   □ How do the waste products of fission and fusion differ? Why would we favor fusion as a means of
      energy production rather than fission?
   □ Explain the role of the relationship between mass and energy with fission and fusion.
   □ Why are small nuclei generally used in fusion, whereas large nuclei are split in fission processes?
      [HINT: refer to the illustration in question 16d.]

				
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posted:11/29/2011
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