Abstract: Three excessive displays were founded by ultrasonic examination on the reactor pressure vessel during pre-service inspection. In this paper, we analyzed the information of the three displays on the reactor pressure vessel.We also analyzed the possible generating time of the displays through combining manufacturing completion reports and the inspection reports of installation.Subsequently,the root cause of these displays was verified by the chemical composition analysis and comparative tests. This paper made the conclusion that these displays were caused by the limonite concrete.