Flow accelerated Corrosion : experience from examination of components from Indian nuclear power plants

Authors : Vivekanand Kain, S. Roychowdhury, 
                 P. Ahmedabadi, and D.K. Barua*

Company : Bhabbha Atomic Research Centre
                   * Nuclear Power Corporation of India

Country : India

Topics

Flow Accelerated Corrosion
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ABSTRACT :

Flow accelerated corrosion (FAC) has affected a large number of components made from carbon steel in the secondary as well as primary section of light water reactors worldwide. This is primarily a corrosion process enhanced by (electro) chemical dissolution and mass transfer, rather than a mechanical process. It has caused rupture of large, medium and small diameter pipelines as well as other components carrying either single phase water or two phase (wet steam) flow. It is reported to be the only mechanism that has significant potential for large leaks in the secondary circuit. This paper describes the observations from examination of a number of components that were affected by FAC in Indian Pressurized Heavy Water Reactors in the past five years. Failure analysis done on the 10% feed water line in KAPS, non return valve (NRV) in the auxiliary feed water system of KAPS, and on the affected pipeline components from the secondary water system of KGS has been described in this paper. The surface features observed (the signature of FAC) on the ID of the pipelines affected by FAC in the primary and the secondary circuits of nuclear reactors have been described. Signatures of FAC, on components severely affected by single phase FAC (scallops) and on components affected by dual phase FAC (tiger striping). Examples of the signature patterns formed on the surfaces in both the primary and the secondary circuits of the Indian nuclear power plants have been described in this paper. Components where FAC degradation had just initiated showed that the patterns were not well defined. The signature patterns became evident only after the degradation had occurred to a large extent. FAC like features were not seen in stainless steel components also in the secondary side. Case studies showing that thinning of carbon steel components is not always attributable to FAC in the secondary circuit of the reactors are also described and reasons of these thinning demonstrated and illustrated to show the importance of establishing the cause of each thinning case. The paper also discusses if the size of the scallops is possible to be correlated to the FAC rate.

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