Hybrid control rod is a passive decay heat removal device, which is a new concept with combination of heat pipe and control rod by inserting neutron absorber inside heat pipe as inner structure. Heat pipe is an excellent heat transfer device using the principle of efficient convection heat transfer by phase change of working fluid and convection through a wick structure inside closed metal tube. Since heat pipe can operate without external power source, it can achieve totally passive heat transfer. It has the strength when refueling water for core cooling by ECCS cannot be injected due to impossible depressurization inside reactor vessel. In that situation, hybrid control rod can remove the decay heat from core directly as a heat pipe inserted into reactor vessel as well as shutdown of reactor as a control rod. Therefore, it can significantly reduce the possibility of core meltdown and release of radioactive material by assuring sufficient core cooling. For evaluating the concept of the hybrid control rod and its application, design features of hybrid control rod were analyzed using a CFD code. In this paper, first a conventional heat pipe was analyzed and compared with experimental results obtained in same conditions to validate the current heat pipe analysis model. The CFD predictions were in a good agreement with experimental results in terms of wall temperature. Different from general heat pipes using vacuum condition, the hybrid control rod was designed to be pressurized inside the heat pipe considering working condition of high temperature inside reactor vessel. The hybrid control rod reflecting a reactor environment was analyzed for wall temperature and thermal characteristics of conventional heat pipe. As a result, it found that the current hybrid control rod concept can remove 18.20 kW per rod at reactor condition.
