Core and reload designs are intentionally made with negative reactivity, although I have seen some preliminary designs with positive reactivity such the reload fuel (lattices) had to be redesigned until negative reactivity was achieved.Why is the void coefficient in enriched US reactors negative?
A positive moderator void coefficient would allow for a power increase in the event that a decrease in moderator density occurred, e.g., if nuclear boiling began in a single phase system such as a PWR, or vigorous boiling or dryout began in a BWR. An increase in local heat flux/transport would increase the voiding, and thus increase the local power through positive feedback. Such an event is precluded by designing a lattice and core with negative void coefficient. So a negative void coefficient assures control of power and power distribution in a reactor core.Criterion 28—Reactivity limits. The reactivity control systems shall be designed with appropriate limits on the potential amount and rate of reactivity increase to assure that the effects of postulated reactivity accidents can neither (1) result in damage to the reactor coolant pressure boundary greater than limited local yielding nor (2) sufficiently disturb the core, its support structures or other reactor pressure vessel internals to impair significantly the capability to cool the core. These postulated reactivity accidents shall include consideration of rod ejection (unless prevented by positive means), rod dropout, steam line rupture, changes in reactor coolant temperature and pressure, and cold water addition.
Criterion 29—Protection against anticipated operational occurrences. The protection and reactivity control systems shall be designed to assure an extremely high probability of accomplishing their safety functions in the event of anticipated operational occurrences.