| Summary: | The Supercritical Water-Cooled Reactor (SCWR) is a concept for an advanced reactor that will
operate at high pressure (25MPa) and high temperature (500ºC 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. 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-Npsub 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. It has been found that the Homogenous-
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 2) 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. Although
there are 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.
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