Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Stability analysis of the Supercritical Water Reactor by means of the root locus criterion E. Cervi, A. Cammi ⁎ Politecnico di Milano, Department of Energy, Nuclear Engineering Division, via La Masa 34, 20156 Milano, Italy ARTICLE INFO Keywords: Supercritical Density wave oscillations Stability analysis Root locus Thermal-hydraulics Neutronics SCWR Nuclear reactors ABSTRACT The Supercritical Water Reactor (SCWR) is a concept for an advanced nuclear reactor operating at high tem- perature (500 °C average core outlet temperature at nominal power) and at high pressure (25 MPa), which give the SCWR a thermal efficiency of about 45%. However, due to the strong variability of the water properties near the thermodynamic pseudocritical point, concerns are raised towards thermal-hydraulic instabilities. A simu- lation tool was developed in Matlab® from the perspective of linear systems, aimed at investigating the reactor stability and identifying potential regions of instability through a consolidated and relatively simple approach. A frequency-domain stability analysis of the SCWR is carried out with the root locus criterion, characterizing the system stability features over its entire operating power interval. The impact of the coolant flow rate on the stability is also studied. The results show that the system is stable over the whole investigated operational range. Finally, the dynamic behavior of the SCWR is compared to the Boiling Water Reactor (BWR), pointing out significant differences due to the different working points and design features of the two reactors. The results of this study could be a starting point for further research on the SCWR, providing the designers with important feedbacks for the optimization of the SCWR coolant circuit. 1. Introduction The Supercritical Water Reactor (SCWR) is one of the six concepts under investigation in the GEN-IV international advanced reactor de- velopment program. It is a combination between the traditional Light Water Reactor (LWR) and the supercritical Fossil Power Plant (FPP). Water at a pressure and at a temperature above its critical point (p c = 22.06 MPa, T c = 373.9 °C) is called supercritical. The technology of supercritical water used as coolant is well established in the field of Fossil Power Plants, allowing to reach a larger thermal efficiency due to the increased pressure and temperature of the fluid. Since no boiling takes place in the SCWR, the reactor can be oper- ated at high temperature without any concern about the CHF (critical heat flux), which limits the operating temperature of the operating temperature of the traditional LWRs. For this reason, the system reaches a thermal efficiency of about 45%, significantly higher than the current LWRs and comparable to those of modern Fossil Power Plants (Ortega Gómez, 2009). Moreover, compared to a Boiling Water Reactor, steam separators and recirculation pumps are no longer needed, allowing significant plant simplifications and a more compact design. One of the major concerns about the SCWR is represented by thermal-hydraulic instabilities, due to the strong variability of the water density near the thermodynamic pseudocritical point. In fact, owing to the large temperature difference between the core inlet and outlet, the water density decrease in the SCWR core is even larger than in BWRs. Hence, even if no phase transition takes place, all the instability phe- nomena occurring in two-phase flows can also be observed in the SCWR. So far, the problem of thermal-hydraulic instabilities in supercritical water reactors has been addressed by many authors. Chatoorgoon (2001) studied the stability of a supercritical fluid flow in a natural circulation CANDU-X supercritical reactor (Dimmick et al., 1998) using a non-linear numerical code. Yi et al. (2004) developed a linear stability analysis code in frequency domain to study the thermal-hydraulic sta- bility of the SCWR-H, a thermal-spectrum supercritical reactor, by means of the decay ratio. Cheng and Yang (2008) developed a point- hydraulics model to study the onset of self-sustaining flow oscillations in a supercritical cooling loop, based on the definition of suited di- mensionless numbers. Ortega Gómez et al. (2008) studied the linear stability characteristics of a uniformly heated supercritical channel by evaluating the eigenvalues of a one-dimensional model. A time-domain analysis of non-linear phenomena was also presented. Moreover, a stability analysis of the SCWR, based on the U.S. reference design, was carried out by Zhao (2005). Further investigations on core-wide https://doi.org/10.1016/j.nucengdes.2018.08.004 Received 23 February 2018; Received in revised form 21 July 2018; Accepted 7 August 2018 ⁎ Corresponding author. E-mail address: antonio.cammi@polimi.it (A. Cammi). Nuclear Engineering and Design 338 (2018) 137–157 0029-5493/ © 2018 Elsevier B.V. All rights reserved. T