For power reactors, the reactor can behave differently at power conditions due to the presence of reactivity feedback. Power reactors are initially started from hot standby mode (the subcritical state at 0% of rated power) to power operation mode (100% of rated power) by withdrawing control rods and boron dilution from the primary source coolant. During the reactor startup and up to about 1% of rated power, the reactor kinetics is exponential as in a zero-power reactor. This is due to the fact all temperature reactivity effects are minimal.
The temperature reactivity effects play a very important role during further power increase from about 1% up to 100% of rated power. As the neutron population increases, the fuel and the moderator increase their temperature, which results in a decrease in reactivity of the reactor (almost all reactors are designed to have the temperature coefficients negative). The total amount of feedback reactivity that must be offset by control rod withdrawal or boron dilution during the power increase is known as the power defect. The power defects for PWRs, graphite-moderated reactors and sodium-cooled fast reactors are:
- about 2500pcm for PWRs,
- about 800pcm for graphite-moderated reactors
- about 500pcm for sodium-cooled fast reactors
See also: Operational factors that affect the multiplication in PWRs
This exactly but in the opposite direction acts after a reactor trip (SCRAM) from the Hot Full Power state (HFP). It is logical as power defects act against power increase, they also act against power decrease. When reactor power is decreased quickly, as in the case of reactor trip, power defect causes a positive reactivity insertion, and the initial rod insertion must be sufficient to make the reactor safe subcritical. Similarly, as in the HZP state, the integral worth of all control and emergency rods (PWRs) is, for example -9000pcm. It is equal to ρ = -9000/600 = -15β = -0.09. (β= 600pcm = 0.006). Also, in this case, a prompt drop occurs. The prompt drop is quicker than the deaccumulation of heat from fuel pellets. For this negative reactivity, the prompt drop is equal to:
n2/n1 = 0.006/(0.006+0.09)=0.063
but the subsequent power decrease is strongly influenced by changes in core (fuel and moderator) temperatures. After reaching stable temperature, the neutron flux may continue to fall (when subcritical) according to the stable period. Obviously, if the power defect for PWRs is about 2500pcm (about 4 βeff), the control rods must weigh more than 2500pcm to achieve the subcritical condition. The control rods must weigh more than 2500pcm plus the value of SDM (SHUTDOWN MARGIN) to ensure the safe subcritical condition. The total weight of control rods is design-specific, but, for example, it may reach about 6000 to 9000pcm. To ensure that the control rods safely shut down the reactor, they must be maintained above a minimum rod height (rods insertion limits) specified in the technical specifications.