PSI - Issue 16

Peter Trampus / Procedia Structural Integrity 16 (2019) 161–168 Peter Trampus / Structural Integrity Procedia 00 (2019) 000 – 000

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Structural integrity assessment of the pressurized components is the evaluation of their resistance against strength and fracture. Since the energy requirement for ductile failure is far greater than that required for failure in the brittle mode, the basic tool of the structural integrity assessment is the fracture mechanics. The assessment method can be deterministic or probabilistic; its scheme is shown in Fig. 2. It is visible from the scheme that the awareness of loading and environmental conditions, material properties and size and position of the existing flaws is necessary for assessing the structural integrity. All of these characteristics are subject to changes during plant operation due to ageing effects, and consequently a continuous decrease in safety margin has to be taken into account. This is primarily important in the light of long-term operation. In Fig. 2, K I is the stress intensity factor (fracture mechanics parameter) and K IC is its critical value named critical stress intensity factor or fracture toughness (material feature); P f is the probability of occurrence of failure.

Fig.2. Structural integrity assessment scheme.

The ISI requirements (scope, inspection volume, schedule, acceptance standards) are usually summarized in codes or standards developed usually by engineering associations, and are based on consensus of all stakeholders such as component manufacturers, operators, inspection organizations and regulators. Each code must be in harmony with the requirements of the given country’s nuclear safety regulation, and their use may be mandatory according to the law or regulation. In many NPP operating countries the Section XI of the ASME Boiler and Pressure Vessel Code (hereafter ASME Inspection Code) is applied for guiding the ISI, ASME (2001). The ASME Inspection Code has been prepared and published by The American Society of Mechanical Engineers. The Section XI (and other code sections) have had a substantial and verifiable influence on the nuclear codes of other countries regardless of whether they have an independent nuclear program or imported the nuclear technology. Besides the USA, only Canada, France, Germany, Japan, Russia (formerly: Soviet Union) and the Czech Republic have developed their own ISI code.

4. Experience challenging ISI/NDE

In the past decade the following operating experience examples can illustrate the outstanding importance of ISI/NDE, IAEA (2018). Those which are associated with corrosion, fatigue and irradiation or any combination of them such as irradiation assisted stress corrosion cracking (IASCC) or irradiation assisted fatigue (IAF) require more and more attention as the operating years accumulate.