Thermal-Hydraulic Stability Analysis of a Natural
Rui Hu, Jiyun Zhao, and Mujid S. Kazimi
Center for Advanced Nuclear Energy Systems
Department of Nuclear Science and Engineering,
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Phone: (617) 253-8627, Fax: (617) 258-8863, E-Mail: firstname.lastname@example.org
Natural circulation cooling is a key issue in the design of next generation boiling water
reactors (BWRs) for simplicity, inherent safety, and maintenance reduction features. It
has been proven and experienced that forced circulation BWRs can be unstable during
unanticipated large-scale fluctuations in pressure, flow rate and heat generation rate.
These instabilities can make a reactor deviate from steady-state conditions and must be
thoroughly evaluated with respect to reactor safety. Given that the natural circulation
boiling water reactor (NCBWR) depends completely on natural convection to remove
heat from the reactor core at rated conditions, it generally has a higher power-to-flow
ratio comparing to forced circulation BWRs, which makes itself more suspect to
undertake unstable flow oscillations. Therefore, its stability performance is even more
important to ascertain under all expected operating conditions.
The objective of this work is to assess the characteristic of NCBWR stability, and the
sensitivity to design and operating conditions in comparison to previous BWRs. At the
first step, only channel thermal-hydraulic stability analysis at rated condition, in which
the Type-II density wave oscillations (DWO) mechanism is dominant, is discussed in this
For 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 operation conditions. A homogeneous equilibrium model
and a non-homogeneous equilibrium model with consideration of non-uniform axial
power distribution and fuel dynamics are applied in the work.
It is found that the NCBWR can be designed to be stable with 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.