A model for troubleshooting and a theoretical calculation model of leakage were established to analyze the output decline caused by drainage problem in a nuclear power plant. The leakage rate was calculated by comparing thermal power and feed water flow before and after the unit startup and shutdown under the same operating conditions. In this study

the measurement of a main steam pipeline leakage was conducted

locating the drainage problem by different ways. In addition

the failure model of the thermostatic trap was determined and showed that the pipeline layout of liquid level gauge and the incorrect installation of steam trap leaded to frequent high level alarm of steam trap. At the same time of correcting the pipeline layout

a new free floating trap was used to replace the previous trap. The application result shows the reliability of the fault analysis and better energy saving effect. The analysis

treatment and suggestions of this abnormal output can provide reference for similar nuclear power plants.