Investigating the stability characteristics of natural-circulation boilingwater reactors using root loci of a reduced-order model

Linear stability analysis of a natural-circulation boiling water reactor (B WR) and the underlying thermal-hydraulic subsystem is performed using a red uced-order BWR dynamic model. The root-locus method is used to examine the stability of the system. The relation between the poles of the system and t he physical processes causing the instabilities is investigated. For a natu ral-circulation thermal-hydraulic system, the two types of instabilities (t ype-I and type-II oscillations) can clearly be attributed to the dynamics o f different types of pressure drops. However, it is not possible to associa te these instability types with certain poles of the system. The root loci of a reactor with weak void reactivity feedback and those of the thermal-hy draulic system behave similarly: The same pole pair remains the least stabl e one as the operating conditions move from the type-I instability region t o the type-II region. In the case of a reactor with strong void reactivity feedback, an exchange in the stability of two pole pairs is found: The leas t stable pole pair in the type-II region is not the same as in the type-I r egion.