Example for the case of negative reactivity |
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This example leads us to a simplification that is not correct for reproduction of an increase in power immediately following a change in reactivity. Physically speaking, the reason is the following: the number of delayed neutrons that contribute to the chain reaction at any given time, is directly proportionate to the corresponding thermal flux just instants before, but the number of precursors that are being produced via fission is proportionate to the flux at that given instant.As we increase the keff > 1, the fission rate will increase and prompt neutrons will be generated immediately, so that for a short time, the reactor will perform as if all of the neutrons were prompt neutrons, and the flux will grow rapidly. This situation is known as prompt jump and it may be observed in the preceding figures. If we look at the last graph, we see that to remove a reactor rapidly from service, a great amount of negative reactivity is necessary in order to cause a major decline in initial power, before the delayed neutrons moderate the power reduction. When Dk = -30 mk, the power falls to a tenth of its initial value in 4 sec, but it takes 40 seconds to drop to 1/100 of its initial value.
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