Reactors that are moderated with heavy water differ from those that are moderated with graphite in that (1) the moderator temperature is lower than it may be in graphite reactors, (2) when the moderator temperature is raised, the moderator-to-fuel ratio decreases, and (3) circulation in the moderator introduces local, random changes in reactivity that distort the flux distribution. The large negative contributions to the moderator temperature coefficient from leakage and resonance capture overshadow any positive contribution from eta that may accompany the buildup of plutonium with exposure. The effect of the coefficient on reactor safety depends upon the manner in which the D/sub 2/O circulation couples the reactor power to the moderator temperature. (auth).