Shakedown and Fatigue Evaluations of Overlay Cladding on Reactor Pressure Vessels

Overlay cladding is classified to non-pressure boundary. Not only the ASME Boiler & Pressure Vessels Code Section III [1] but also the JSME Design and Construction Code [2] prescribe that no structural strength shall be attributed to cladding and the presence of the cladding shall be considered with respect to both the thermal analysis and the stress analysis. This means the codes do not require stress evaluation for overlay cladding itself. If overlay cladding has a fatigue crack, the crack may grow and extend to the base metal. Thus overlay cladding may give an influence on the integrity of base metal in the pressure boundary. The thermal expansion of stainless steel cladding is different from that of base metal made of low alloy steel, and this difference causes discontinuity of stress distribution between the cladding and the base metal. It is questionable that a stress evaluation line is set on such stress distribution including discontinuity between the cladding and the base metal. An evaluation method based on elastic-plastic analysis is preferable to evaluate such portion. ASME B&PV Sec.III and Sec.VIII, Div. 2 [3] have plastic analysis provisions. Also the JSME D&C Code issued a code case on alternative design methodology by using elastic-plastic finite element analysis for Class 1 vessels [4, 5]. In this paper, shakedown, fatigue and environmental fatigue evaluations are performed for the overlay cladding of direct vessel injection nozzle of Reactor Pressure Vessel by using the JSME Code Case on the alternative design methodology.