During any power decrease the temperature, pressure, or void fraction change and the reactivity of the core changes accordingly. It is difficult to
change any operating parameter and not affect every other property of the core. Since it is difficult to separate
all these effects (moderator, fuel, void etc.) the power coefficient
is defined. The power coefficient combines the Doppler, moderator temperature, and void coefficients
. It is expressed as a change in reactivity per change in percent power, Δρ/Δ% power
. The value of the power coefficient is always negative in core life but is more negative at the end of the cycle primarily due to the decrease in the moderator temperature coefficient.
Let assume that the reactor is critical at 100% of rated power and that the plant operator wants to decrease power to 75% of rated power. The reactor operator must first bring the reactor subcritical by insertion of a negative reactivity (e.g. by control rod insertion or boric acid addition). As the thermal power decreases, moderator temperature and fuel temperature decrease as well, causing a positive reactivity effect (from the power coefficient) and the reactor returns to the critical condition. In order to keep the power to be decreasing, negative reactivity must be continuously inserted (via control rods or chemical shim). After each reactivity insertion, the reactor power stabilize itself proportionately to the reactivity inserted.
- the power coefficient: Δρ/Δ% = -20pcm/% of rated power
- differential worth of control rods: Δρ/Δstep = 10pcm/step
- worth of boric acid: -11pcm/ppm
- desired trend of power decrease: 1% per minute
100% → ↓ 20 steps or ↑ 18 ppm of boric acid within 10 minutes → 90%→ next ↓ 20 steps or ↑ 18 ppm within 10 minutes → 80% → final ↓ 10 steps or ↑ 9 ppm within 5 minutes→ 75%