Abstract: The pipe-wall thickness in a straight section, which is located behind a restriction orifice in deaerating system in 1# unit of Tianwan Nuclear Power Plant, reduced from 6.6 mm to 2.5 mm in a refueling cycle. It is prone to cause leakages that lead to a potential safety hazard for operation. This paper analyzes the location of abrasion and the mechanism of cavitation happened in this thickness-reducing pipe behind the restriction orifice, and comprehensively discusses the design deficiencies in the orifices of the deaerating system and the advantages of replacing the single-stage orifice by the multistage orifices. After the improvement, the operation requirements of water-supply pipelines in the deaerating system can be satisfied, and the erosion rate of pipelines can be effectively reduced.