Effectiveness of Water Chemistry Improvement against FAC in PWR Secondary System

Authors : Wataru Sugino¹, Daisuke Akutagawa¹,
                 Taku Ohira¹, Ayumi Abe¹
                 Kimitoshi Yoneda², Fumio Inada²
 

Company : ¹ The Japan Atomic Power Company
                   ² Central Research Institute of Electric
                      Power Industry

Country : Japan

Topics :

Flow Accelerated Corrosion Steam-water circuit chemistry
Non-Destructive Examination methods

ABSTRACT :

FAC of carbon steel (CS) piping is one of main issues in secondary system of Pressurized Water Reactor (PWR) nuclear power plant. FAC brought frequent inspection and replacement of thinned piping and subsequent increase of operating and maintenance cost.

FAC rate of CS piping is affected by each factor of flow dynamics, temperature, material of piping and water chemistry. Consequently, it seems reasonable to assume that the water chemistry improvement has mitigation effect against FAC of CS piping, because the factor except the water chemistry depends on the plant design.

In PWR secondary system, All Volatile Treatment (AVT), using ammonia or amines as alkaline agents and hydrazine as an oxygen scavenger, has been adopted to mitigate the corrosion of CS by forming passive film of magnetite (Fe₃O₄) on CS surface. Furthermore, in a number of PWR, High-AVT chemistry with feedwater pH around ten is adopted in order to improve corrosion mitigation effect of CS, because the magnetite has a characteristic that solubility decreases with increasing pH of the coolant.

In this presentation, FAC mitigation effect of the High-AVT chemistry with a consideration of flow dynamics is discussed based on actual wall thinning rate in Tsuruga-2 (1160 MWe PWR, commercial operation in 1987) before and after High-AVT adoption and calculated result of mass transfer coefficient (MTC) of coolant flowing through this piping by STAR-CD code ver.3.26. This calculation revealed that the High-AVT may not be effective enough to suppress FAC rate of CS piping located in high MTC region such as intrados of elbow.

Therefore, we estimated an applicability and effectiveness of Oxygenated Water Chemistry (OWC) as a new approach to FAC suppression for PWR secondary system by experiment in laboratory and demonstration test at Tsuruga-2. As a result, it is clarified that the FAC is suppressed even under extremely low dissolved oxygen concentration such as 2ppb in the coolant under High-AVT condition because hematite (Fe₂O₃) film which has significantly lower solubility than magnetite film under AVT condition formed on the CS surface.

Based on this result, the OWC will be applied in Tsuruga-2 from Sep. 2010. Wall thickness of CS piping after the OWC application will be measured by continuous monitoring system, using high-temperature and high-resolution ultrasonic probe, to evaluate the FAC mitigation effect. In this presentation, the OWC project at Tsuruga-2 is also introduced.

Speaker Biographical Information

He is Deputy Manager of department "Radiation & chemistry Engineering Group" at Tsuruga Nuclear Power Station of The Japan Atomic Power Company in Tsuruga City, Japan. This department handles water chemistry issues in respect to the safe and economical operation of nuclear power plants in Japan.