As the composition of the fuel changes with burnup the reactivity changes. If the power is to be kept constant equation 1.48 shows that either the source strength has to be changed by altering the accel­erator beam current, or the reactivity has to be adjusted by moving control rods. However one of the potential advantages of an ADR
over a critical reactor is the possibility of dispensing with control rods altogether. This reduces L, making more neutrons available for use (equation 1.49), and it also reduces the complexity and therefore the cost of the plant.

If the ADR has no control rods it has to remain subcritical, with a safety margin, at all times in the refuelling cycle, and this means that at the times in the cycle when the fuel is least reactive ke is significantly less than one. This has to be compensated for by increasing the beam current. If, for example, the reactivity changes by 3% during a run, with a 1% safety margin ke varies between 0.96 and 0.99, which requires the beam current to be changed by a factor of 4 to keep the power constant.

This imposes limitations on the design and cost of the accelerator, which has to be powerful enough to provide the high current needed when ke is low but is operating well below its capacity most of the time. To avoid this disadvantage, therefore, it may be preferable to accept the alternative disadvantage of installing control rods. It may then be possible to operate the plant close to critical — possibly with ke и 0.995 — at all times, with a steady beam current and steady power.