EVOLUTION OF STARS

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Nuclear Reactions	Nuclear Reactions - Fusion
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Nuclear Reactions Questions

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Q1.    Describe the two types of nuclear reactions. For which nuclei does each type release energy?  Answer

Q2.    What is radioactivity? Why is it a problem when considering commercial nuclear reactors?  Answer

Q3.    What is the difference between fission and fusion reactions? Why is one used to produce energy on Earth but the other to produce energy in stars?  Answer

Nuclear Reactions - Fusion Questions

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Q1.    Describe in simple terms the nuclear reactions which produce energy in the sun. In what way do these reactions change the chemical composition of the sun?  Answer

Q2.    Why are high temperatures required for fusion reactions to occur? Compare and contrast the proton-proton chain of reactions and the carbon (or CNO) cycle. You do not need to give the individual steps in the reactions.  Answer

Q3.    Describe in general terms the proton-proton chain of reactions. You do not need to list the individual reactions of the chain.  Answer

Q4.    What is meant by the term "catalyst" applied to carbon in the carbon cycle? Is carbon produced by these reactions.  Answer

Q5.    Why do fusion reactions never occur in our normal environment? What extraordinary condition must occur before fusion reactions can occur?  Answer

Q6.    What is the net result of the nuclear reactions which occur in the center of the sun?  Answer

Nuclear Reactions - Fission Questions

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Q1.    What condition must be met for a nuclear explosion to occur? Why can the fuel in a nuclear reactor never undergo a nuclear explosion, even in the worst possible accident?  Answer

Q2.    What is a chain reaction? How is the rate of reactions in a chain reaction controlled?  Answer

Q3.    In general terms, how is electricity produced in a nuclear power plant? How does this process differ from that used in a coal-fired power plant?  Answer

Q4.    For fission reactions: How does a chain reaction work? What conditions must be met for a chain reaction to be explosive?  Answer

Q5.    Why is preparing the fuel for a commercial nuclear reactor so expensive? Why can an explosive nuclear reaction never occur in a commercial nuclear reactor?  Answer

Nuclear Reactions Answers

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A1.    Fission reactions occur when a single large nucleus splits into two pieces. Fusion reactions occur when two small nuclei join together into a single larger nucleus. Fission releases energy only if the original nucleus is heavier than iron. Fusion releases energy only if the product is lighter than iron.

A2.    A radioactive nucleus spontaneously disintegrates by emitting a small particle (e.g., an electron, proton, or gamma ray). Such high speed particles are damaging to biological tissue. The spent fuel of a nuclear reactor, and any material that comes in direct contact with it, is highly radioactive for many thousands of years. That means it must be carefully disposed of and kept from the environment for a very long time.

A3.    Fission splits a large nucleus in two, fusion joins two or more smaller nuclei into a single larger nucleus. Fusion produces energy in stars because light nuclei are the most common on stars. The high temperatures required to overcome the electric repulsion between nuclei is a natural by-product of the gravitational collapse that formed the star. Fission is used to produce energy on Earth because the reactions are easier to control and sustain. We cannot easily maintain the temperatures required for fusion.

Nuclear Reactions - Fusion Answers

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A1.     Nuclear reactions in the sun convert hydrogen into helium. Specifically, four protons fuse in a series of steps to produce helium. In the process, a great deal of energy is released.

A2.    Since every nucleus contains protons with positive electric charge, every nucleus repels every other nucleus. The rapid motion of high temperature is required to overcome the electric repulsive force between nuclei. The proton-proton chain and the carbon cycle both convert 4 hydrogen nuclei into a helium nucleus. The proton-proton chain does so in a series of steps using only hydrogen and products produced in the reactions. The carbon cycle adds protons to a carbon nucleus, and at the end returns the carbon nucleus and a helium nucleus.

A3.    The proton-proton chain converts four protons directly into helium. Although several steps are required, no other nuclei are required. At each step, considerable energy is released by the reactions.

A4.    A carbon nucleus is used during the carbon cycle to facilitate the fusion of hydrogen into helium. At the end of the cycle, a carbon nucleus is returned that is identical to the one used in the reactions. Thus, no carbon is produced by the reactions, but there must be some carbon present initially for the reactions to proceed.

A5.    Fusion requires that two nuclei approach and touch each other. The natural electric repulsion between nuclei prevents this from happening under normal conditions of temperature and pressure. The nuclear repulsion can be overcome by the very high speeds which occur at temperatures in excess of a million degrees.

A6.    Nuclear reactions in the sun convert four hydrogen nuclei into a single helium nucleus, and in the process release a great deal of energy.

Nuclear Reactions - Fission Answers

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A1.    A nuclear explosion can only occur if there is very high purity uranium-235 present. In a conventional nuclear power plant, the uranium-235 fuel is mixed with uranium-238, whcih is not fissionable. Since these nuclei are the same chemical element, they cannot become separated by any kind of natural process or accident.

A2.    A chain reaction occurs when a neutron released by one fission stimulates another uranium nucleus to fission. If an average of more than one neutron per fission successfully stimulates additional fissions, the rate of reactions increases. If the average number of successful neutron stimulated reactions is less than one, the rate of reactions decreases. The success rate can be altered either by removing some of the fuel so that there are fewer uranium nuclei present to fission or by introducing other, neutron-absorbing materials into the fuel to keep some of the neutrons from finding uranium nuclei to stimulate.

A3.    Heat released by the nuclear reactions produces high pressure steam, which turns a turbine to generate electricity. The only difference in a coal-fired power plant is that burning coal produces the heat to make the steam instead of nuclear reactions.

A4.    When a nucleus splits in two during fission, one or more neutrons will also be ejected. These nuclei can strike other fissionable nuclei, causing them to fission. This perpetuates the chain reaction. The chain reaction becomes explosive if more than one neutron per fission stimulates additional fissions, causing the rate of reactions to rapidly increase. An explosive chain reaction can only occur if the fissionable fuel is very pure, so that the neutrons efficiently find other fissionable nuclei to fission. A certain, minimum critical mass of fuel must also be present.

A5.    The only naturally occurring nucleus that is fissionable is uranium-235. However, most uranium is U-238. Since they are the same chemical element, they are very difficult, and hence very expensive, to separate. The fuel for a nuclear reactor is only purified to around 30% U-235, not enough for an explosive chain reaction. Since no natural process, for example during an accident, can separate the two types of uranium, the reaction can never accidentally become explosive.