Non-modal stability analysis of the supercritical water reactor lumped model

Compared to commercial reactors and other Generation-IV designs, Super-critical Water Reactors (SCWR) presents the unique feature of super-critical water acting as a coolant. Whereas this choice, in principle, will guarantee higher thermal efficiencies compared to standard light-water designs, thermal-hydraulic instabilities are a cause of concern, since near the thermodynamic pseudo-critical point water properties show significant variability: thus, the study of stability for the SCWR is a primary concern for its future licencing. In this context, this study further expands the analysis of stability of the SCWR using non-modal stability theory: compared to standard modal stability analysis, which focuses on the behaviour of the system, both linear and non-linear, following the exhaustion of the perturbation, non-modal theory studies on the short-term perturbation amplification in linearised systems, thus focusing on the early stages of instability even for asymptotically stable operating states. Being a preliminary work in this sense, this study adopts a simple lumped-parameter model for the SCWR, coupling point-kinetics equations with a mono-dimensional model to describe the core thermal-hydraulics, focusing on the influence of perturbations on various parameters to study their influence on the short-term response of the system. In particular, the presence of power oscillations in the early stages of the transient, potentially related to Density Wave Oscillation instability, is shown. • Supercritical water reactors may be subjected to two-phase instabilities such as Density Wave Oscillations. • Study of the stability of the SCWR from the non-modal point of view. • Transient growth of energy is observed even for asymptotically stable states. • The system reaches the new equilibrium state in an oscillatory manner.