Abstract
Catastrophic failure of some components in a large nuclear power plant, particularly those that play a principal role in ensuring nuclear safety, may lead to unacceptable radiological consequences.
The International Atomic Energy Agency provides guidance on safety categorisation, which relates to the component’s functional role, and safety classification, which relates to the component’s safety function. The key safety functions for a nuclear energy system are containment, cooling and control. Within design codes these categories and classifications are then simply related to the construction class. However, there is insufficient evidence to justify that design code compliance alone provides sufficient structural reliability for those components where the consequences of failure may be severe.
Structural components are typically passive and whilst their functional role appears relatively straightforward, analysis of the consequences of their failure can be complex. In addition, pressurised components such as pipework and vessels can fail to produce highly energetic missiles or jets. In such cases, whilst the failure of a component might not have a direct effect on the key safety functions but if missiles or jets are formed, they could lead to further consequential failures. If those failures affect components, systems or structures with an even higher safety categorisation or classification, then by association this enhances the importance of demonstrating structural integrity of the pressure boundary component which constitutes the initiating event. This process has been referred to as Structural Integrity Classification and involves a systematic review of the hazard fault tree considering all direct and indirect consequences arising from a postulated self-initiated failure of a susceptible pressure boundary system, component or part.
This paper outlines the typical requirements of a structural integrity classification system including a systematic consideration of direct and indirect consequence of failure.
The paper then describes the requirements for demonstrating enhanced structural reliability above and beyond that inferred by code compliance. This includes but is not limited to demonstration of quality of build, selection of appropriate materials, joining methods (e.g. welding) and heat treatments, adequacy of arrangements for non-destructive examination and testing including pre-service and in-service, consideration of environmental effects and through-life degradation mechanisms and finally additional requirements to demonstrate tolerance to manufacturing defects and defects which might develop in-service.
The content of this paper was developed during licensing activities conducted for a range of nuclear energy systems, mainly power stations, for a variety reactor types and reactor vendors. Although the advice has been developed specifically for deployment of those energy systems in the United Kingdom, it has potential for worldwide application.
