The Significance of Fast Moderator Feedback Effects in a Boiling Water Reactor During Severe Pressure Transients

Thermohydraulic feedback effects in a boiling water reactor during pressure transients are analyzed. Transient calculations are carried out with the one-dimensional nonlinear plant model ALMOS. A reference case of the analysis is a turbine trip without bypass (loss of heat sink) and delayed scram action. Neutron flux and fuel temperatures are investigated with respect to such feedback parameters as the Doppler coefficient, moderator feedback, and heat generation in the coolant. The amount of heat generated in the coolant is recognized as an important parameter. The sensitivity of the system to this parameter is further analyzed qualitatively by frequency response methods.