A comparative study of bifurcation characteristics in single and two-zone fuel models of a BWR: A case study of the advanced heavy water reactor (AHWR)

•A simplified model with refined radial fuel heat transfer modeling is studied.•Method of multiple scales for nonlinear analysis and oscillation characteristics.•Refinement in radial fuel heat transfer improves the overall system stability. A simplified nuclear-coupled thermal–hydraulic model following lumped-parameter approach is presented in which radial heat transfer modeling is refined by dividing the fuel pellet into two concentric rings (two distinct zones). Two different approaches to get the boundary between the two zones have been employed which essentially alter the heat capacities of the two zones of the fuel rod and give rise to development of two different versions of the two-zone model. Employing parameters relevant to an advanced heavy water reactor (AHWR), a comparison between the two-zone fuel model and a single-zone fuel model (Verma et al., 2016) shows that a two-zone model gives a larger stability threshold. A nonlinear analysis exploiting method of multiple time scales (MMTS) indicates that two-zone models exhibit both sub- and super-critical Hopf bifurcations. These analytical findings are verified against numerical simulations. A parametric study is performed by varying relevant system parameters to study their effects on the reactor dynamics. The study suggests that refinement in the modeling of the radial fuel heat transfer leads to an improvement in linear as well as global stability threshold of the reactor dynamical system.