Environmental effects on the integrity of Type 316H stainless steel of an AGR coolant.

  • Athanasios Gredis

Student thesis: Phd


The UK's Advanced Gas-cooled Reactor (AGR) nuclear plants, operated by EDF Energy, are using a CO2 based primary circuit gas coolant. The boilers of these reactors have numerous stainless steel (SS) components, which can experience premature cracking due to the severely aggressive environments. Stainless steel components operate in a carburising environment under the creep regime, and thus undergo carburisation over time, which in turn alters the mechanical properties at the surface of the components. Carburised material is more susceptible to premature creep cracking due to the embrittlement of the outer surface, therefore a necessity of new and fundamental understanding of the environmentally assisted degradation and failure mechanisms is required in order to extend the lifetime of the AGRs safely and economically. This study focuses on the use of laboratory oxidation experiments under creep to investigate the interaction of the carburising environment in combination with creep deformation. Parent, cold worked (CW) and cracked SS material, as well as pre-carburised material (used in in-situ high resolution digital image correlation (HRDIC)) were used to further investigate the deformation properties of the carburised layer. Oxidation experiments under creep with parallel and tapered gauge specimens, revealed that carburisation was present on material that was subjected to applied stress whereas oxidation was observed on samples that underwent significant plastic deformation at high temperature. Cold word showed a beneficial effect, as both oxidation and carburisation as well as the total creep elongation were reduced. Overall, surface preparation, cold work and coolant chemistry can be carefully chosen to control the level of oxidation and carburisation, ensuring the safe operation of austenitic stainless steel components in AGR plants. Experiments with cracked specimens highlighted the effects of the carburising environment on oxidation and cracking behaviour of Type 316H SS and in-situ HRDIC experiments with carburised material explored the cracking behaviour of the carburised layer, although further work needs to be performed to investigate its response to deformation. Overall, those findings can be used to increase the service life of these components by extrapolating the materials' behaviour for long exposures, but more investigative work needs to be performed at more prolonged exposure times since the 500 hour exposure time used in this study may not be totally representative of long term plant conditions.
Date of Award1 Aug 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorFabio Scenini (Supervisor) & Brian Connolly (Supervisor)


  • Creep
  • Carburisation
  • Austenitic Stainless Steel
  • Advanced Gas-cooled Reactors

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