Thermal-hydraulic stability analysis of natural circulation BWRs

Given that a natural circulation boiling water reactor (NCBWR) depends completely on buoyancy to remove heat from the reactor core at rated conditions, this raises the issue of potential unstable flow oscillations. The objective of this work is to assess the characteristics of NCBWR stability, and the sensitivity to design and operating conditions in comparison to previous BWRs. As a first step, the hot channel thermal-hydraulic stability at rated conditions, in which the Type-II density wave oscillations (DWO) mechanism is dominant, is discussed in this paper. When a small fraction of the parallel channels oscillates, while the bulk flow remains at steady state, neutronic feedback due to this small fraction oscillation can be neglected. Using frequency domain methods, the single channel stability characteristics of the NCBWR and its sensitivity to the operating parameters have been determined. The characteristic equation is constructed from linearized equations, which are derived for small deviations around steady operating conditions. A homogeneous equilibrium model and a non-homogeneous equilibrium model with non-uniform axial power distribution and fuel dynamics are considered in the work. It is found that the NCBWR can be designed to be stable with a large margin around the operating conditions by proper choice of the orifice scheme, and for appropriate power to flow ratios. To evaluate its stability performance, comparisons with a typical BWR (Peach Bottom 2) have been also conducted. Sensitivity studies are performed to investigate the effects of design features and operating parameters, including chimney length, inlet orifice coefficient, power, flow rate, and axial power distribution.