The reactivity inputs due to control rod malfunctions are limited both in the rate of addition and in the magnitude of the addition due to the design of the control rods themselves. It is usual to consider control rod malfunctions that add reactivity while the reactor is at power and the system is hot, and also during the start-up procedure.

2.3.2.1 At Power

When the system is hot, thermal feedbacks are immediately available, because any power change will produce a significant fuel temperature change.

The following trip signals would be available: (a) control rod drive sen­sors; (b) period meters if they are included in the system; (c) high flux; and

(d) high coolant outlet temperature eventually. Knowing the trip signals available and the delays between the monitored parameter reaching the trip value and the rods commencing to move into the core, it is possible to define a highest safe rate of addition and a largest acceptable step addition of reactivity. In a typical LMFBR a step addition of 60 cents may be accom­modated and rates of up to $ 3 or $ 4/sec could be acceptable. Naturally it is also possible to design the control system to do better than this if needed by shortening the delays in the electronics and accelerating the rods when inserting the shut-down absorbers.

Figures 2.11 and 2.12 show the effect of adding terminated ramps of reactivity to a typical LMFBR in terms of its power rise and increase in

temperatures. Figure 2.13 demonstrates the difference between adding re­activity as a terminated ramp and as a step, and it also shows the very significant improvement which arises from an increase in the Doppler coefficient. Such an analysis defines the need for protective system response times of a given value depending upon the control rod malfunctions which are possible: if the control accident could add reactivity at a rate of $ 2/sec, then the protective system would need to cut back reactivity following a high flux signal in about 0.25 sec for example.