Analysis of flow stability in nuclear reactor subchannels with water at supercritical pressures

The paper presents the results of the analysis by CFD models of flow stability in fuel bundle slices with upward, horizontal and downward flow orientations. Square and triangular lattice slices are both studied, basing on previous work that demonstrated the feasibility of such analyses. A uniform heat flux is applied to the slice walls in the current study, without addressing the internal structure of the rod. The STAR-CCM+ code is adopted, comparing the results obtained by a 3D model with those that were obtained by the 1D 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 set up by an in-house 1D code using a dimensionless formalism developed in previous work. Both static and dynamic instabilities are observed, in similarity with previous analyses performed for circular channels. The work represents a further step in a research aimed to establish a methodology for the analysis of flow stability in nuclear reactor cores by CFD codes.