Structural Integrity Assessment to Justify Increase in Upper Operating Temperature Limit of AGR Dome

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The UK advanced gas cooled reactors (AGRs) use a graphite core with carbon dioxide gas as the primary coolant. There is a diaphragm above the core which separates re-entrant gas at lower temperature and higher pressure from that leaving the channel guide tubes at reactor outlet temperature. This diaphragm is known as the hot box dome. The dome is perforated to facilitate the passage of fuel and control rods into the core. The dome is fabricated in carbon-manganese steel and incorporates a number of full penetration welds which are post-weld heat treated (PWHT). The dome’s upper surface is insulated to protect it from gas at high temperature, intended to maintain the dome at a temperature of below 380°C. Since dome failure could conceivably result in gas by-passing and, hence, failing to adequately cool the core the original safety case claims that gross failure of the dome is incredible. More recently potential failure modes of the dome have been reviewed and various dome weld failure scenarios have been analysed and assessed to demonstrate a tolerance to the consequences of complete failure of certain welds. On this basis the dome could be shown to satisfy a lesser classification of high integrity, although no claim to reclassify the region has been made. Through-life temperature monitoring is carried out to demonstrate that the peak dome temperature remains below 380°C. This has shown evidence of rising temperatures, believed to arise from a reduction in the effectiveness of the upper surface insulation, an effect that was acknowledged by the original design. Work to investigate this effect has developed the understanding of the dome thermal environment which is far more complex than previously thought. The hottest parts of the dome are far smaller and more localised than previously thought, and lie immediately above the monitored locations. In order to support a case to operate for an extended life, it is now proposed that the upper temperature limit could safely be increased to 390°C. Structural integrity analyses and assessments have been carried out to support the proposed increase to 390°C and include a demonstration of the absence of a cliff-edge effect by assessing cases with the hottest parts of the dome at temperatures of 400 and 410°C. The work seeks to demonstrate adequate margins of safety against all potential failure modes. ASME III code assessment against primary stress limits has been used to guard against failure by plastic collapse and/or creep rupture. Creep-fatigue initiation assessments have been used to demonstrate margins against the formation of defects using the EDF Energy high temperature assessment procedure, R5. This has enabled the consideration of potential defects to be confined to those that might have formed during welding or PWHT and have been missed by extensive pre-service inspections. Notwithstanding the low likelihood that any such defects exist, with high confidence it may be postulated that any that do would be located in welds and be of limited size. Defect tolerance assessments have been carried out, including the calculation of limiting defect sizes in accordance with the EDF Energy R6 procedure and the growth of postulated defects by creep and fatigue using R5. Other failure and degradations mechanisms have been considered and eliminated as a potential threat by drawing on reviews of relevant operating experience on other reactors with similar materials and environments, and material property data from long term tests. This paper describes how the multi-facetted programme of work, which proposes a modification to an existing safety case, has been devised to explicitly address all conceivable modes of failure and demonstrate a robust argument against each one.