Core Unit IV: Nuclear Physics
B. Nuclear Fission

Key Concepts

Fission is the term used to describe the splitting of a heavy nucleus into two or more smaller nuclei.

Slow moving neutrons are more easily captured by the nucleus. A moderator is a medium which causes neutrons to travel more slowly.

Graphite, heavy water, and beryllium are all excellent moderators, capable of slowing neutrons without absorbing them.

The neutrons liberated by fission travel very quickly unless moderated.

A very large amount of energy is released when an atom undergoes fission. ( 200 MeV)

In a typical fission reaction, the energy released is distributed as follows: 170 MeV of kinetic energy of fission fragments, 5 MeV of kinetic energy of neutrons, 15 MeV of energy beta particles and gamma rays, and 10 MeV as energy of antineutrinos.

An example of a typical fission is:

Mass is not conserved in a nuclear reaction. The products formed during nuclear fission have a slightly lower mass, due to the nuclear mass defect. This nuclear mass defect can be used to determine the nuclear binding energy which held the heavier nucleus together and was released when fission occurred.

Neutrons released when an atom undergoes fission are capable of causing other nuclei to undergo

fission, if the neutrons are slowed down by a moderator.

A sustained fission reaction caused in this way is called a chain reaction.

Natural uranium ore contains about 0.7% uranium-235. To increase the likelihood of sustaining a chain reaction for uranium, the fissionable isotope of uranium must be increased in its relative proportion through enrichment.

A nuclear reactor produces a sustained chain reaction at a controlled rate. The heat energy produced by the reaction is used to drive turbines, generating electricity. (Refer to section C.)

Control rods, made of materials such as cadmium which absorb neutrons, are used to control the rate of a chain reaction in a nuclear reactor.

A critical mass of fissionable material is the minimum mass that will produce a nuclear explosion. To produce a sustainable nuclear chain reaction requires more material than the critical mass.

An atomic bomb explodes when two or more sub-critical masses of fissionable material are brought together very rapidly. Chemical explosives are used to implode the sub-critical masses together to form a mass larger than the critical mass.

An atomic bomb produces devastating destruction. Its explosive force is measured in terms of the comparable number of megatons of conventional explosives that would be needed to produce similar results.

Nuclear weapons produce radioactive contamination of the environment. For this and other reasons many countries have banned atmospheric testing of these weapons.

The first atomic bombs, developed and tested by the United States during the Manhattan Project in World War II, were dropped on the Japanese cities of Hiroshima and Nagasaki in 1945. Over 110 000 people were killed and many others suffered from the effects of the explosions for years afterwards. Japan surren- dered shortly after the atomic bombs were dropped, bringing the war to an end.

Leo Szilard, one of the developers of the atomic bomb, recommended that it be tested before an inter- national audience of observers prior to being used, offering the Japanese a chance to surrender beforehand. Whether or not the atomic bomb should have been used is an issue worthy of debate.

Today the nuclear arsenals of the superpowers contain such vast supplies of nuclear weapons that, according to one scenario, if a large proportion of them were deployed simultaneously, it would render the planet virtually uninhabitable by humans. Contemporary societal reactions to this issue are growing.

Should scientists ultimately help bring about an understanding that nuclear weapons are immoral? Do such weapons threaten the existence of all forms of life on Earth? These are questions worth pondering.

Should a scientifically literate society help to reduce the potential threat of nuclear war? Can nuclear weapons be thought of as a "deterrent" if their destructive capabilities could be so severe that it may be unreasonable to consider their use for solving international conflict?

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: fission, moderator, nuclear mass defect, chain reaction, enrichment, control rods, nuclear reactor, critical mass.

2. Describe what happens during fission reaction.

3. Recognize that slow-moving neutrons are more easily captured by the nucleus of an atom.

4. Give an example of a substance which can act as a good moderator.

5. Recognize that neutrons are released during fission.

6. Recognize that a very large amount of energy is released during a fission reaction.

7. Compare the amount of energy released during a fission reaction with the amount of energy released during the combustion of a typical fossil fuel.

8. Explain how the neutrons released during a fission reaction can help to sustain the reaction.

9. Explain why enrichment is used in preparing nuclear fuels.

10. Recognize that a mass greater than the critical mass of fissionable material is needed to produce an uncontrollable chain reaction.

11. Recognize the devastating destructive power present in nuclear weapons.

12. Explain that nuclear weapons release radioactive fallout.

13. Participate, as members of a scientifically literate society, in bringing about an end to the threat of nuclear war.

Teaching Suggestions, Activities and Demonstrations

1. Arrange some dominoes to simulate a nuclear chain reaction. Have the first domino strike two, then have those two strike four, and so on. Re-arrange the dominoes to represent a sustained, controlled nuclear reaction, such as one which takes place in a nuclear reactor.

2. The energy released in a fission can also be calculated using average binding energies. If an element where A 240 (with an average binding energy of 7.6 MeV/nucleon) splits in "half", it will end up as two atoms with A 120 (with an average binding energy of 8.5 MeV/nucleon). The energy released is the number of nucleons multiplied by the change in average binding energy/nucleon:

   240 x (8.5 - 7.6) MeV/nucleon 200 MeV