ELASTIC-PLASTIC FITNESS-FOR-SERVICE ASSESSMENT OF CLASS 1 NUCLEAR PIPE ELBOW Usama Abdelsalam (1) , R. Pavia (2) , S. Jeremia (3) & Dk Vijay (3) (1) Ph.D., P.Eng., Kinectrics Inc./Contractor, Toronto, Ontario, Canada (2) Bruce Power L.P, Toronto, Ontario, Canada (3) Kinectrics Inc., Toronto, Ontario, Canada ABSTRACT Carbon Steel piping systems experience wall thinning due to the flow accelerated corrosion phenomenon. A review of the inspection reports of a typical nuclear pipe indicated that the predicted minimum wall thickness at the next scheduled outage may not meet the required minimum acceptable wall thickness that meets the ASME SEC III NB-3600 criteria for Service Levels A, B, C & D. This finding triggered a fitness for service assessment to extend the operating life of this pipe. This paper summarizes the detailed finite element analysis performed to re-establish a less conservative MAWT for the tight radius elbow addressing the catastrophic failure mode under the primary pressure loading, the plastic instability under bending moment with coincident pressure loading, and the fatigue crack initiation under both thermal transients and seismic loadings. The analysis presented in this paper qualified a uniform wall thickness profile with 88% of the pressure-based thickness and a local wall thickness profile with a minimum of 75% of the pressure-based thickness. These qualified MAWT solutions significantly extend the component life with a significant positive economic impact. NOMENCLATURE CANDU CANada Deuterium Uranium. COG CANDU Owners Group. CSA Canadian Standard Association. E Elastic Modulus EFPY Equivalent Full Power Years FAC Flow Accelerated Corrosion FFS Fitness For Service FFSG Fitness For Service Guidelines FRS Floor Response Spectrum g Gravitational Acceleration = 386.4 in/s 2 K Power Law Constant MAWT Minimum Acceptable Wall Thickness n Strain Hardening Parameter P Pressure PD Design Pressure PEN C Engineering Plastic Collapse Load Peval Evaluation Pressure SAM Seismic Anchor Movement SFm Structural Factor for Primary Force SFb Structural Factor for Primary Bending Moment SFe Structural Factor for Secondary Force or Moment Su Ultimate Strength T Temperature tmin sp Pressure-based thickness of a straight pipe tnom Nominal wall thickness. ϵ True Strain σ True Stress INTRODUCTION In a typical CANDU Nuclear Generation Station feeder pipes experience significant wall thinning at the tight radius elbows/bends regions. The Canadian Standards Association (CSA) Standard CAN/CSA-N285.4 permits the use of fitness for service assessments to justify continued operation to an arbitrary evaluation period. A review of an outlet feeder thinning assessment at 21.61 EFPY indicated that the operating time to the MAWT of 3.15 mm is 24.02 EFPY with a thinning rate of 0.158 mm/EFPY. To mitigate the risk of pre-mature replacement of an outlet feeder pipe, the CANDU Owners Group Feeder Fitness for Service Guidelines [1] & [2] Appendix E Level 3 assessment methods are used to develop a less conservative MAWT leading to valuable life extension of this feeder pipe. The major relevant failure modes considered under Level 3 assessment are: 1. Plastic collapse, 2. Plastic instability, and 3. Crack initiation. 1 Copyright © 2019 ASME Proceedings of the ASME 2019 Pressure Vessels & Piping Conference PVP2019 July 14-19, 2019, San Antonio, Texas, USA PVP2019-93303 Attendee Read-Only Copy