Nuclear energy is fascinating on various points: enormous amount of energy, securing fuels, possibility of solving the global warming, etc. However, it's known that using nuclear energy involves dangerous and safety of nuclear power plant (NPP) is required. Study about NPP safety has been conducted by many researchers. In addition, concern for NPP safety has increased in resent years because of happening of some NPP accidents and newly establishment of NPP. In this paper, detection method of core abnormal phenomena is considered. They are: construction of this method, sensitivity analysis of transient phenomena, and examination for the validity of this method. Transient phenomena dealt with this study are: (A) small break and leak, (B) change in feedwater flow rate, (C) loss of feedwater heating. As a result, this method shows some validity to (B) and (C) but doesn't show to (A).