Thermal Fatigue Screening Criteria for Identifying Susceptible Piping Components in CANDU® Stations
9th International Conference on CANDU® Maintenance - 2011 December 04-06


Presented at:
9th International Conference on CANDU® Maintenance
2011 December 04-06
Location:
Toronto, Canada
Session Title:
Learning from OPEX Session A3

Authors:
Qi Chen (AECL Nuclear Laboratories)
Chris Schefski (AECL Nuclear Laboratories)
S. Pentecost (AECL Nuclear Laboratories)
  

Abstract

In December 1987, a fatigue failure in a non-isolable section of a safety injection line at the Farley-2 plant prompted the U.S. Nuclear Regulatory Commission (NRC) to issue Bulletin 88-08 requiring thatU.S.utilities review all non-isolable branch lines to determine if they are susceptible to thermal fatigue. The thermal fatigue incident at Farley-2 was caused by stresses in the pipe wall resulting from large-scale temperature fluctuations. Shortly after the Farley-2 event, several other incidents with through-wall cracks due to thermal fatigue had occurred in plant subsystems and piping configurations similar to the Farley-2 safety injection line. Thermal fatigue cracks have also occurred in piping configurations with different geometries, such as drain, residual heat removal, and shutdown cooling suction lines in various pressurized water reactors (PWR) and boiling water reactors (BWR).

Thermal fatigue, caused by local thermal stratification phenomena, has received significant attention in the PWR and BWR communities in the past two decades. Although CANDU® stations have experienced relatively few thermal fatigue failures; the impact of this known fatigue mechanism for CANDU designs has not been rigorously assessed. Screening and evaluation methodology, which has been developed by Electric Power Research Institute (EPRI) to identify locations susceptible to thermal cycling in PWR systems, has recently been modified under a CANDU Owners Group (COG) project for application in CANDU piping systems.

This paper describes a new software tool for evaluating locations susceptible to thermal fatigue in CANDU piping systems in an effort to avoid failures that lead to costly plant shutdowns. The software, combined with engineering judgement, will assist CANDU station staff to focus their inspections on key components, therefore reducing dose, time and cost during outages. Computational Fluid Dynamics (CFD) was used to form the basis for expanding the range of validity in the EPRI Thermal Fatigue Screening criteria to the geometries found in CANDU piping systems and this work is also discussed.

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