Page 63 - Dynamics and Control of Nuclear Reactors

5.5 Power ascension 55

15

10

Power

5

0
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
Time (min)
FIG. 5.2
Responses of a subcritical reactor to reactivity step increases of 10 cents per step, starting
at a subcritical reactivity of 50 cents.

reactivity step increases. Note the larger increase in the power change and the longer
time required to stabilize following a reactivity change as the reactor gets closer to
critical. In this example, the initial subcritical reactivity has a value of 50 cents,
with subsequent step reactivity insertions of 10 cents.

5.5 Power ascension
After criticality is achieved, the rise to desired power begins. An increment of reactiv-
ity is added and the power ascent begins. See Chapter 4 for illustrations of power
increases in a critical reactor. Operators monitor the reactor power and the reactor
period (the time required for the power to increase by a factor of e) to ensure that
the power is increasing at an acceptable rate. As the desired power level is approached
the operators reduce the reactivity in order to reach criticality (reactivity¼0) at the
desired power level.

Exercises

5.1. Confirm that Eq. (5.1) follows from the point kinetics equations (as modified
by adding a source term, S).
5.2. Consider the kinetics Eqs. (3.25) and (3.26) for one delayed neutron group.
Insert a source term S(t) in Eq. (3.25) equal to 1.0/s. Calculate the fractional
power for successive step reactivity insertions of 5, 10, 15, 20, and 25 cents.