The Effect of Reactivity Feedback Mechanisms on PWR Dynamics and Stability

Reactivity feedback mechanisms play an important role in the safety and stability of nuclear reactors. Usually, the mechanisms yield negative feedback, such that they reduce the power following an increase of reactivity. Such mechanisms contribute to the inherent stability of the core. However, in several reactor designs, reactivity feedbacks may become small and even positive. In these reactors, understanding the feedbacks effect on reactor stability is of high importance.

In this work, we study the effect of two feedback mechanisms on the stability of pressurized water reactor (PWR). The mechanisms include reactivity feedback of fuel temperature and reactivity feedback of coolant/moderator temperature. The analysis is based on a simplified point dynamics model of PWR. Several scenarios are studied: both reactivity coefficients are negative; both reactivity coefficients are positive; one coefficient is negative and the other is positive. The scenarios are analyzed both in the time domain, using numerical integration, and in the frequency domain, using the formalism of transfer function.

We show that when fuel coefficient is negative, the core is stable to small reactivity perturbations. However, when fuel coefficient is positive, such perturbations may yield a diverging power and reactor instability, even when coolant coefficient is negative. There exists a flux threshold, above which such instabilities may occur. Finally, we show that both periodic and stochastic reactivity perturbations can yield reactor instability.