Session: CS-16-01 Fatigue and Ratcheting Issues in Pressure Vessel and Piping Design

Paper Number: 123541

123541 - Investigation Into the Effects of Cladding in Thermal Fatigue Evaluations of Nuclear Pressure Vessels and Components 

Many nuclear components are cladding with corrosion resistant material to protect the parent material from the environmental effects of the coolant.  Often, the cladding is applied by welding one or more layers of cladding material.  directly to the inside surface of the component, typical cladding materials include austenitic stainless steel and nickel-based alloys. 

Rules for the assessment of clad components are given in Paragraph NB-3122 of Section III of the ASME Boiler and Pressure Vessel Code.  Subparagraph NB-3122.3 states that, when assessing against the secondary and peak stress limits of Section III Appendix XIII, cladding must be considered in both the thermal and stress analysis and limits prescribed in Appendix XIII apply to both materials.  However, an exemption is granted that allows the presence of the cladding to be neglected provided the cladding is integrally bonded and less than ten percent of the total thickness.

It is understood that different code users have interpreted this exemption in different ways with some neglecting the cladding in both the thermal and stress analysis and others including it in the thermal analysis, but neglecting it in the stress analysis and others potentially including it in both, but only assessing the parent material for fatigue.  The addition of environmental fatigue enhancement factors adds further confusion as code rules do not clarify whether the parent or cladding factor should be applied.  NuReg/CR-6909 suggests that in many cases it is reasonable to neglect the cladding and apply the parent Fen factor, however this does not represent reality or design intent since the purpose of the cladding is to protect the parent material from the internal environment.

This paper uses a range of simple finite element models to consider the effects of these different approaches.  Models of typical plant components with a range of cladding thicknesses (between 3% and 10% of total wall thickness) under a mix of representative long range and short range transients; austenitic and nickel based alloy cladding have been considered.  The cladding has been excluded from different stages of the calculation (thermal analysis, stress analysis, fatigue evaluation and application of environmental enhancement factor) and the results from the different assessments are compared to evaluate the current code rules.

Presenting Author: Ben Pellereau Rolls-Royce

Presenting Author Biography: Specialist in nuclear pressure component structural integrity with many years of experience carrying out strength, fatigue and fracture assessments.

Authors:

Ben Pellereau Rolls-Royce
Meyrick Hockly Rolls-Royce SMR
Wolf Reinhardt Candu Energy Ltd