HW20-20 - that more thermal neutrons will be absorbed by...

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10-13 19. A reactivity insertion into an initially critical reactor operating at steady state causes the power to increase from 100 W to 10 kW in 6 minutes. What was the value of the reactivity insertion in $? Solution: From Eq. (10.16), which can be written as P(t) = P(O) exp[tjT], the period is Since keff ~ 1, we obtain from Eq. (10.17) 20. Explain how a decrease in the boiling rate inside a boiling water reactor affects the reactivity of the reactor. Solution: Most cores are "undermoderated" so that if the amount of moderator is de- creased, such as when water turns to vapor, kef! decreases. By contrast, when the boiling rate decreases, there are fewer steam voids and hence more water in the core, so kef! increases. The explanation is as follows. With more wa- ter/moderator in the core, neutrons can slow to thermal energies more quickly with less chance of being absorbed while slowing down, and thus the resonance escape factor p increases. Also, more water means
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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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