Flow accelerated corrosion (FAC) has resulted infailures of balance of plant piping systems at nuclear power plants (NPPs) including the rupture of the suction line to the main feedwater pump at the Surrey Unit 2 Pressurized Water Reactor (PWR) in the US in 1986 and the rupture of a high pressure condensate line at the Mihama Unit 3 PWR in Japan in 2004.
In a typical PWR plant, FAC primarily affects balance of plant (non-nuclear) piping systems because the nuclear piping systems are typically constructed from stainless steel or are clad with stainless steel which is not susceptible to FAC. However, CANDU nuclear piping systems are constructed of carbon steel so under certain combinations of flow conditions and water chemistry, FAC degradation of CANDU nuclear piping is plausible.
In response to the concerns over the potential for FAC related degradation or pressure boundary systems and components, the US industry, through the Electric Power Research Institute (EPRI), undertook development of a program to manage the degradation mechanism. EPRI has issued a recommended practices document and developed predictive modelling software (CHECWORKS) that have been adopted at US and Canadian NPPs to implement and manage FAC related degradation. While the Canadian NPP operators are active participants in the EPRI programs, they form a minority group with NPPs that have the potential for FAC to impact nuclear piping systems, which are subject to different operating conditions and chemistry controls than balance of plant piping. CANDU NPP operators have established additional activities related to the management of FAC through the CANDU Owner’s Group, but the majority of the recommended practices employed in FAC management are based upon PWR experience.
Given that the CANDU NPPs are entering periods of extended operation and initiatives underway within the CANDU industry related to evaluations of Large Break Loss of Coolant Accident probabilities and leak before break evaluations for large diameter piping systems, an independent third-party technical review of the application of the EPRI recommended practices for FAC management to CANDU nuclear piping systems larger than 6 inches in diameter is justified.
Objective:
To obtain an independent, third-party review of the EPRI recommended practices, discussed in the suggested documents found in the scope of work section, for the management of flow accelerate corrosion to ensure that they address factors which may contribute to FAC susceptibility of CANDU nuclear piping systems larger than 6 inches in diameter. The outcome of the review will provide useful information for CNSC staff to consider when making licensing decisions concerning aging management strategies and fitness for service evaluations for nuclear pressure boundary systems that are potentially susceptible to FAC.
