In high-temperature reactors (HTRs), ingress of water introduces positive reactivity. Normally, this is controlled by the reactor itself, but in hypothetical situations, there could be a need for an active support by the control system. Calculational research identifies three reasons for the reactivity change caused by the water: (a) a negative contribution by the absorption of the hydrogen, (b) a positive contribution by the softening of the neutron energy spectrum, and (c) a reduction of the neutron leakage losses due to a shift in the neutron flux local distribution. By increasing the carbon/heavy metal ratio, the reactivity effect can be reduced to almost zero or even to negative values. In the modular pebble-bed HTR, this effect can be accomplished in a simple manner. By adding 25% of graphite spheres to the regular batches of feed fuel elements, the neutron spectrum effect is reduced, and the fractional absorption of hydrogen is increased; thus, the maximum excess reactivity is limited to 0.3%. The effect on economy and safety is negligible.