Thermal ageing of cast duplex stainless steels of PWR: material assessment on removed cast elbows

Some parts of reactor coolant circuit of nuclear PWR power plants - elbows on primary circuit - are made of cast duplex stainless steels. It is now identified that the mechanical characteristics of these steels can decrease upon thermal ageing after long term operation at reactor coolant circuit temperature conditions, close to 300 deg. C. The proneness to thermal ageing of these components is related to the chemical composition and the ferrite content, especially the Chromium equivalent content (Creq = %Cr + %Si + %Mo). In the context of the justification of maintaining these cast duplex components in operation, a large program of material assessment was carried out on cast elbows removed during Steam Generators Replacements (SGR). The material assessment was based on metallurgical, mechanical and chemical characterization of in-service aged components. The obtained results contribute to validate the methodology based on the on-site monitoring of the components identified as sensitive to thermal ageing. The material assessment also contribute to validate the prediction formulas established on a laboratory ageing program. The examinations performed on one SG inlet elbow removed during a SGR operation after over 100 000 hours of operation are presented in this paper. It is also explained how the results of the material assessment contribute to implement the maintenance strategy. Moreover, the duplex stainless steels specification of the cast elbows was changed into molybdenum - free grades, less sensitive to thermal ageing, which were installed from 1985 onward. Afterwards a new design of primary circuit has been developed mainly with austenitic stainless steel forged components, non sensitive to thermal ageing at the PWR operating temperature, and installed on the latest units of the N4 series plants. (authors)