Stability and Control of Supercritical Water Reactor System: A Study into Concepts and Applications

Abstract

The study addresses the stability and control of nuclear systems with emphasis on the Supercritical Water Reactor (SCWR) as proposed in the Generation IV International Forum. The literature on the stability and control of the SCWR is presented. A Computational Fluid Dynamics code, STAR-CCM+, is used to study the flow stability problems in circular channels, fuel bundle slices with and without heating structures. Some of the effects of numerical discretisation, turbulence model effects, flow direction with respect to gravity and fluid properties are studied by comparing the stability thresholds identified by transient calculations with maps set up by 1D codes developed and used in previous work and results that were obtained by the 1D RELAP5 code. Flow stability in fuel bundle slices with upward, horizontal and downward flow orientations are addressed. Square and triangular lattice slices are both studied based on the work performed on the circular channel. A uniform heat flux is applied to the slice walls without addressing the internal structure of the rod. The results obtained from STAR-CCM+ by a 3D model are compared with those that were obtained by the use of RELAP5 code. The steady state characteristics of the two models are considered and the thresholds of instability identified by transient calculations are compared with maps from the 1D codes developed using a dimensionless formalism as was performed for the circular channel. Both static and dynamic instabilities are observed, in the circular channel and the fuel bundle slices, clearly showing the contiguity of these two kinds of phenomena as a function of inlet fluid subcooling. A coupled neutronic-thermal hydraulic instabilities in a subchannel slice with square lattice assembly is studied. A more realistic system is considered dealing with a slice of a fuel assembly subchannel containing the regions of pellet, gap and cladding and also including the effect of inlet and outlet throttling. A point kinetics neutronic model including six delayed neutron groups with a global Doppler and fluid density feedbacks was adopted. The response of the model to perturbations applied starting from a steady-state condition at the rated power is compared with that of a similar model developed for a 1D system code. Upward, horizontal and downward flow orientations are addressed making use of uniform and bottom peaked power profiles and changing relevant parameters as the gap equivalent conductance and the density reactivity coefficient. Though the adopted model can still be considered simple in comparison with actual details of fuel assemblies, the obtained results demonstrate the potential of the adopted methodology for more accurate analyses to be made with larger computational resources.