Textbook Question
Complete and balance the nuclear equations for the following fission or fusion reactions:
(a) 21H + 21H → 32He + _
(b) 23992U + 10n¡ → 13351Sb + 9841Nb + _ 10n
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Ch.21 - Nuclear Chemistry
Chapter 21, Problem 60c
(c) What other substances are used as a moderator in nuclear reactor designs?
Verified step by step guidance1
1. A moderator in a nuclear reactor is a substance that slows down the neutrons produced during the fission process. The slowed down neutrons are more likely to cause further fission reactions, thus sustaining the chain reaction.
2. The most commonly used moderator is water, specifically light water or heavy water. Light water is ordinary water (H2O), while heavy water is water where the hydrogen atoms are replaced by deuterium (D2O).
3. Besides water, other substances can also be used as moderators. These include graphite (carbon) and beryllium. Graphite was used in the first nuclear reactors built by man, while beryllium is used in some advanced reactor designs.
4. The choice of moderator depends on various factors such as the type of reactor, the fuel used, and the desired neutron energy spectrum. Each moderator has its own advantages and disadvantages.
5. For example, water is cheap and readily available, but it also absorbs neutrons, reducing the efficiency of the reactor. Graphite does not absorb neutrons as much, but it can catch fire in case of a reactor accident. Beryllium is a good moderator and reflector of neutrons, but it is more expensive and less common than water or graphite.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Nuclear Moderators
Nuclear moderators are materials used in nuclear reactors to slow down fast neutrons produced during fission. By reducing the speed of these neutrons, moderators increase the likelihood of further fission reactions, enhancing the reactor's efficiency. Common moderators include water, heavy water, and graphite, each with distinct properties that affect reactor design and operation.
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Types of Moderators
Different types of moderators can be employed in nuclear reactors, each influencing the reactor's performance. Light water (ordinary water) is the most widely used, while heavy water (deuterium oxide) allows for a more efficient reaction with natural uranium. Graphite, another common moderator, is used in some reactor designs for its ability to absorb fewer neutrons, thus maintaining a higher neutron economy.
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Reactor Design Considerations
The choice of moderator is crucial in reactor design as it affects the reactor's criticality, safety, and fuel efficiency. Factors such as neutron absorption cross-section, heat capacity, and chemical stability under radiation are considered when selecting a moderator. The design must balance these factors to optimize performance while ensuring safety and regulatory compliance.
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Related Practice
Textbook Question
Complete and balance the nuclear equations for the following fission reactions:
(a) 23592U + 10n → 16062Sm + 7230Zn + _ 10n
(b) 23994Pu + 10n → 14458Ce + _ + 2 10n
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Textbook Question
The spent fuel elements from a fission reactor are much more intensely radioactive than the original fuel elements. (b) Given that only two or three neutrons are released per fission event and knowing that the nucleus undergoing fission has a neutron-to-proton ratio characteristic of a heavy nucleus, what sorts of decay would you expect to be dominant among the fission products?
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