Theoretical and experimental stochastic modeling analysis of PWR core heat transfer

A multi-nodal stochastic heat transfer model has been developed to study the fuel-to-coolant heat transfer and the effects of various heat flux perturbations on the core-exit coolant temperature in a pressurized water reactor (PWR). These perturbations can be associated with the fuel or the coolant. We have studied the effects of these on the dynamic responses, which are considered to be random processes. The analysis showed that the effect on the core-exit coolant temperature, of perturbations of the heat deposited in the fuel, was most influential in the frequency range 0-1 Hz. This analysis is made by transforming the dynamic model to the frequency domain and estimating the contribution to the power spectrum of the core-exit temperature from various perturbations. Further studies showed that the perturbations in the coolant flow rate was more dominant on the core-exit temperature than the perturbations in the inlet coolant temperature. The analysis of operational data from a commercial PWR showed high coherence between the neutron signal and the core-exit temperature signal, also in the 0-1 Hz frequency range. This observation, together with a multivariate time-series modeling of the signals, demonstrated the existence of unidirectional flow dynamics between the neutron and the temperature signals. Investigations of this type are useful in understanding the mechanism of coolant temperature ''noise'' behavior, the factors influencing inadequate core cooling (that is, deviation from normal heat transfer), and the sources of perturbations that affect the temperature signals. Also, diagnostic information such as coolant flow monitoring and sensor fault monitoring can be obtained using dynamic modeling and operational data