HW18-3 - to avoid a sudden energy release(Wigner effect 3...

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10-2 Chap. 10 Principles of Nuclear Reactors 2. Discuss the relative merits of water and graphite for use in a thermal reactor. Solution: Water: .much less expensive .can also be used as the coolant for a power reactor .boils at lower temperatures and thus require a pressure: vessel .O"a is relatively large (0.664 b) .larger f. and slows neutrons with fewer scatters .much more inert chemically .cannot be used as a structural element of the core .can produce radioactive 3H and 16N by activation .can dissociate and produce explosive hydrogen-oxygen gas mixtures Graphite: .reactor-grade (high purity) graphite is expensive .cannot be used as the coolant .solid to very high temperatures; thus no need for a pressure vessel .Ua is very small (34 mb) .smaller ~ and requires more scatters to slow neutrons .burns and thus hot graphite must be isolated from oxygen .can be used as structural elements of the core .does not activate and produce radionuclides .stores energy and must be periodically annealed
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Unformatted text preview: to avoid a sudden energy release (Wigner effect). 3. List five desirable properties of a moderator for a thermal reactor. Explain the importance of each property. Solution: (a) It should have a small A number so that it can thermalize neutrons with relatively few scatters. (b) It should have a small absorption cross section and a large scattering cross section over all neutron energies to avoid nonfuel absorption (to maximize f) and to promote thermalization by scattering interactions. (c) It should be relatively inexpensive and easily incorporated into the core (d) It should not change phase over the anticipated range of core temperatures to avoid the need for expensive pressure vessels. (e) It should be chemically stable to reduce its longevity and prevent possible accident conditions (f) It should not absorb neutrons to produce unwanted radioactivity. July 24, 2002...
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