A parametric study of coupled neutronic-thermohydraulic stability of natural circulation boiling water reactors (BWRs) is performed. As an example, the stability characteristics of the Dutch Dodewaard reactor, which was cooled by natural circulation, are determined. The Dodewaard reactor can be considered as the prototype of next generation natural circulation BWRs. The stability issues that are identified for this prototype reactor are therefore important in the design of new natural circulation BWRs.

Without a riser section installed, only one region of thermohydraulic instability exists in the stability plane. The significant gravitational pressure drop in a riser section, installed to enhance the natural circulation flow, gives rise to the emergence of an additional region of instability. The oscillations in this zone become especially important during low-power/low-pressure (reactor startup) conditions. Significant damping of these oscillations occurs in a reactor, due to the nuclear void reactivity feedback.

A comparison between natural circulation in-phase and out-of-phase reactor stability is made, in particular important for large reactor cores but also yielding unexpected results for small reactors. The impact of downcomer inertia on the stability of the in-phase mode is investigated in detail. Typical trajectories in the dimensionless stability plane are calculated as a function of changing operating conditions, to investigate their influence on reactor dynamics.