Reactor Theory (Reactor Operations)
Figure 2 Reactor Power Response to Positive Reactivity Addition
Returning now to Equation (4-7) for reactor period.
If the positive reactivity added is less than the value of
, the emission of prompt fission
neutrons alone is not sufficient to overcome losses to non-fission absorption and leakage. If
delayed neutrons were not being produced, the neutron population would decrease as long as
the reactivity of the core has a value less than the effective delayed neutron fraction. The
positive reactivity insertion is followed immediately by a small immediate power increase called
the prompt jump. This power increase occurs because the rate of production of prompt neutrons
changes abruptly as the reactivity is added. Recall from an earlier module that the generation
time for prompt neutrons is on the order of 10 seconds. The effect can be seen in Figure 2.
After the prompt jump, the rate of change of power cannot increase any more rapidly than the
built-in time delay the precursor half-lives allow. Therefore, the power rise is controllable, and
the reactor can be operated safely.