Review on Thermal-Hydraulic Characteristics of Nuclear Reactors Under Ocean Conditions

The thermal-hydraulic characteristics of nuclear reactors under ocean conditions are significant for reactor safety and reliability. A large number of experiments concerning this issue have been done. However, the focus of these experiments has been mainly on dynamical systems in submarines and aircraft carriers. With the development of floating nuclear power plants (FNPPs)—which are used to provide power for remote areas or ocean platforms—more studies on FNPPs are needed. The differences between FNPPs and dynamical systems bring new challenges to research of thermal-hydraulic characteristics in a nuclear reactor under ocean conditions.

Many experimental studies on natural circulation and forced circulation under ocean conditions are based on tube and rectangular channels. The effects of ocean motions on friction, flow instability, heat transfer, and critical heat flux (CHF) have been investigated. The intensity of ocean motions and their driving head have a huge impact on pressure drop, heat transfer, and flow instability, especially when the driving head is small. Flow oscillations induced by ocean motions can overlap with thermal-induced flow instability. This resonance effect can cause potential harm to systems. CHF under ocean conditions is mainly dependent on motion types and the CHF mechanism.

There are challenges with this research. First, conclusions obtained from a simple channel cannot always extend to rod bundle systems. Second, studies of ocean conditions, which always use one- or two-dimensional movements, are relatively simple and cannot reflect complicated ocean environments. Third, there is a lack of comparison between natural frequency of two-phase flow instability and frequency of ocean waves, and this may cause unwanted problems due to resonance or coupling behaviors. Fourth, CHF experiments that are performed to examine CHF events under ocean conditions are far from real reactor conditions where CHF events occur under high-temperature and high-pressure conditions, with complicated three-dimensional geometry, with open channel environments, and sometimes even under the influence of mixing vane grids [for most small modular reactor designs]. Last, there has been no work on the length effect, nonuniform heating, and mixing effect induced by spacer grids, which differentiate FNPP thermal-hydraulic characteristics from the other applications. A series of suggestions are provided for future work.