Distributed-Parameter Linear Stability Analysis of a Double-Pass Circulating Fuel Reactor

Stability analysis forms an integral aspect of safety evaluation for different nuclear reactor designs, since reactor systems can be subject to instabilities in their operational parameters. For circulating fuel reactors, modified point kinetics equations accounting for the loss and re-entry of delayed neutron precursors into the reactor core are commonly used for linear stability analysis. Additionally, the continuous stirred-tank approximation is typically assumed, equating the outflow precursor concentration with the core average. This approximation is, however, very strong, especially for a reactor with a non-trivial flow path through the core. An example of such reactor is one with a double-pass flow configuration. In this work, an alternative formulation of the equations based on the distributed parameter approach is developed and applied to the analysis of a double-pass reactor. The differences between the distributed parameter and continuous stirred-tank approaches are discussed and the resulting equations are briefly presented. Additionally, the static loss of reactivity due to precursor flushing is calculated and compared between the two approaches. Then, the linear stability of the double-pass reactor and its response to velocity perturbation are analysed – to this end, system transfer functions are derived and employed together with the Nyquist stability criterion. No significant issues with the stability of the system are found; however, behaviour specific to the double-pass system is discerned, which would not be captured by a simplified analysis, pointing at the need for the use of appropriate models.