| dc.description.abstract | At rated conditions, a natural circulation boiling water reactor (NCBWR) depends
completely on buoyancy to remove heat from the reactor core. This raises the issue of
potential unstable flow oscillations. The objective of this work is to assess the
characteristics of stability in a NCBWR at rated conditions, and the sensitivity to design
and operating conditions in comparison to previous BWRs.
Two kinds of instabilities, namely Ledinegg flow excursion and Density Wave
Oscillations (DWO), have been studied. The DWO analyses were conducted for three
oscillation modes: Single Channel thermal-hydraulic stability, coupled neutronics regionwide
out-of-phase stability and core-wide in-phase stability. Using frequency domain
methods, the three types of DWO stability characteristics of the NCBWR and their
sensitivity to the operating parameters and design features have been determined. The
characteristic equations are constructed from linearized equations, which are derived for
small deviations around steady operating conditions.
The Economic Simplified Boiling Water Reactor (ESBWR) is used in our analysis as a
reference NCBWR design. It is found that the ESBWR can be stable with a large margin
around the operating conditions by proper choice of the core inlet orifice scheme, and for
appropriate power to flow ratios.
In single channel stability analysis, neutronic feedback is neglected. Design features of
the ESBWR, including shorter fuel bundle and use of part-length rods in the assemblies,
tend to improve the thermal-hydraulic stability performance. However, the thermalhydraulic
stability margin is still lower than that of a typical BWR at rated conditions. In
neutronic-coupled out-of-phase as well as in-phase stability analysis, the perturbation
decay ratios for ESBWR at our assumed conditions are higher than that of a typical BWR
(Peach Bottom 2) at rated conditions, due to its lower thermal-hydraulic stability margin
and higher neutronic feedback. Nevertheless, the stability criteria are satisfied.
To evaluate the NCBWR stability performance, comparison with BWR/Peach Bottom 2
at both the rated condition and maximum natural circulation condition has been
conducted. Sensitivity studies are performed on the effects of design features and
operating parameters, including chimney length, inlet orifice coefficient, power, flow
rate, and axial power distribution, reactivity coefficients, fuel pellet-clad gap
conductance. It can be concluded that the NCBWR and BWR stabilities are similarly
sensitive to operating parameters. | en_US |
