Those who work with fast reactors have to contend with the fact that an addition of moderator softens the spectrum and effectively increases the reactivity but only when this moderator is applied throughout the core. If the moderator is applied at the center, then the reactivity is liable to be reduced, whereas a reduction of moderator would cause an increase in reactivity. See Section 1.4.1.2 for a more detailed discussion of this point.

An LMFBR is not subject to moderator addition but to subtraction (Section 2.3.4). Gas-cooled and steam-cooled reactor systems are sensitive to the flooding accident, which is a moderator addition (2).

Figure 2.19 shows the variation in reactivity following the ingress of water as a result of a failure of heat exchanger tubes in the gas-cooled fast reactor system. An amount equal to 10 kgm/sec is assumed to leak in from a simul­taneous failure of three tubes. However, in this case, the addition of re­activity is at a slow enough rate for detection to remedy the situation. Temperature changes in the fuel were about 0.2°C/sec.