PURPOSE: To make highly reliable operation possible while alleviating the burden on the plant operator by allowing quick and accurate diagnosis of abnormality in the piping of condensation system without relying on the knowledge or experience of plant operator.

CONSTITUTION: An abnormality diagnostic system 50 for condensation system piping receives a hot well water level measurement SS1 from a hot well water gauge 24, an opening signal for a hot well water level control valve 22, a make-up water flow rate measurement SS2 from a make-up water flowmeter 34, a supply water flow rate measurement SS3 from a supply water flowmeter 35, a drain flow rate measurement SS4 from a drain flowmeter 32, open/close control data of a spillover valve 27 and the hot well water level control valve 22, water level data of low pressure water supply heater group, and open/close control data of a low pressure supply water heater water level control valve. The diagnostic system 50 monitors the data, estimates the probability of breakage in the piping of condensation system based on these signals and data and issues an alarm in case of breakage.