Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005.Includes bibliographical references (p. 269-277).The Supercritical Water-Cooled Reactor (SCWR) is a concept for an advanced reactor that will operate at high pressure (25MPa) and high temperature (500°C average core exit). The high coolant temperature as it leaves the reactor core gives the SCWR the potential for high thermal efficiency (45%). However, near the supercritical thermodynamic point, coolant density is very sensitive to temperature which raises concerns about instabilities in the supercritical water-cooled nuclear reactors. To ensure a proper design of SCWR without instability problems, the U.S. reference SCWR design was investigated. The objectives of this work are: (1) to develop a methodology for stability assessment of both thermal-hydraulic and nuclear-coupled stabilities under supercritical pressure conditions, (2) to compare the stability of the proposed SCWR to that of the BWR, and (3) to develop guidance for SCWR designers to avoid instabilities with large margins. Two kinds of instabilities, namely Ledinegg-type flow excursion and Density Wave Oscillations (DWO), have been studied. The DWO analysis was conducted for three oscillation modes: Single channel thermal-hydraulic stability, Coupled-nuclear Out-of-Phase stability and Coupled-nuclear In-Phase stability.(cont.) Although the supercritical water does not experience phase change, the thermodynamic properties exhibit boiling-like drastic changes around some pseudo-saturation temperature. A three-region model consisting of a heavy fluid region, a heavy-light fluid mixture region and a light fluid region has been used to simulate the supercritical coolant flowing through the core. New non-dimensional governing parameters, namely, the Expansion Number (Nexp) and the Pseudo-Subcooling Number (Npsub) have been identified. A stability map that defines the onset of DWO instabilities has been constructed in the Nexp-Npsu,b plane based on a frequency domain method. It has been found that the U.S. reference SCWR will be stable at full power operating condition with large margin once the proper inlet orifices are chosen. Although the SCWR operates in the supercritical pressure region at steady state, operation at subcritical pressure will occur during a sliding pressure startup process. At subcritical pressure, the stability maps have been developed based on the traditional Subcooling Number and Phase Change Number (also called as Zuber Number). The sensitivity of stability boundaries to different two phase flow models has been studied.(cont.) It has been found that the Homogcnouls-Nonequilibrium model (HNEM) yields more conservative results at high subcooling numbers while the Homogenous Equilibrium (HEM) model is more conservative at low subcooling numbers. Based on the stability map, a stable sliding pressure startup procedure has been suggested for the U.S. reference SCWR design. To evaluate the stability performance of the U.S. reference SCWR design, comparisons with a typical BWR (Peach Bottom ) have been conducted. Models for BWR stability analysis (Single channel, Coupled-nuclear In-Phase and Out-of-Phase) have been constructed. It is found that, although the SCWR can be stable by proper inlet orificing, it is more sensitive to operating parameters. such as power and flow rate, than a typical BWR. To validate the models developed for both the SCWR and BWR stability analysis, the analytical results were compared with experimental data. The Peach Bottom 2 stability tests were chosen to evaluate the coupled-nuclear stability analysis model. It was found that the analytical model matched the experiment reasonably well for both the oscillation decay ratios and frequencies. Also, the analytical model predicts the same stability trends as the experiment results.(cont.) Although there arc plenty of tests available for model evaluations at subcritical pressure, the tests at supercritical pressure are very limited. The only test publicly found was for the single channel stability mode. It was found that the three-region model predicts reasonable results compared with the limited test data.by Jiyun Zhao.Ph.D.