Challenging issues in iPWR with single phase water naturally circulating system

While nuclear energy plays an important role for addressing global energy issues, the nuclear technology does not only focus on the large capacity reactor anymore, but also it is concentrating more on the Small Modular Reactor (SMR) development. SMR is gaining interest due to its potential for various application areas of nuclear energy such as desalination, district heating, electricity generation and propulsion. SMR is also required to meet the certain elevated safety standard such as adopting fully passive safety system or total elimination of large break LOCA while demonstrating high economic performance. In order to meet both safety and economy goals, a lot of recently developed SMRs (especially iPWR) rely on passive systems, but they do not limit the adoption of passive systems for the safety related system only. In other words, for simplifying the SMR design to achieve high reliability, modularity and ease of transportation, integrated system often appears as the main concept of the design along with the extension of passive system concept to the normal operating system as well (i.e. full power natural circulation). However, there are many challenges await for such innovative design and this paper will discuss about these issues by categorizing three operational modes of the pressurized water reactor system, (1) reactor start-up sequence, (2) full power operating condition (3) shut down cooling period. A special attention is given to the Deteriorated Turbulent Heat Transfer (DTHT) regime since it was demonstrated in the previous studies that the normal turbulent flow can be significantly deteriorated in terms of heat transfer when the major driving force is the buoyancy force. It will be demonstrated in this paper that as the system operation relies more to the passive system, more caution is required to design out from this heat transfer regime to maintain certain safety margin.