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Unformatted text preview: that more thermal neutrons will be absorbed by the nonfuel and hence the thermal utilization factor f de-creases. However, in undermoderated cores, the change in p is greater than that in f, so that as the boiling rate decrease, fp and kef! increase causing the power to rise. 21. Explain why it is reasonable that 135Xe should have a very large cross section for neutron absorption. Solution: The nuclide l~~Xe has 81 neutrons and hence is one neutron shy of having the "magic number" of 82 neutrons (see Section 3.2.6). The nucleus l~~Xe is exceptionally stable since all the neutrons in the nucleus form completely closed shells. For this reason, it is energetically very attractive for 135Xe to absorb a neutron to complete its neutron shell structure. In fact, 135Xe has an enormous resonance near thermal energies, giving it a microscopic cross section for thermal neutrons larger than that for any other nuclide. July 24, 2002...
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- Spring '06