The behavior of the phase relationship between neutron flux and core-exit temperature fluctuations in a pressurized water reactor (PWR) is studied as a function of the moderator temperature coefficient of reactivity (..cap alpha../sub c/). PWR operational data indicate that the neutron noise and core-exit temperature noise cross power spectrum phase is linear in a certain frequency range, and approaches -180 deg at low frequencies. Extensive modeling studies applied to the LOFT reactor show that this low frequency phase behavior changes when ..cap alpha../sub c/ is positive, approaching zero deg at low frequencies. The analysis further showed that in the LOFT reactor, coolant flow rate fluctuation is the primary driving source causing neutron noise and core-exit temperature fluctuations. This conclusion was also confirmed by independent studies. The neutron noise-coolant temperature phase behavior may be used as a single method of monitoring the moderator temperature coefficient of reactivity during different stages of a PWR fuel cycle
