PSI - Issue 37

Fekete, Tamás et al. / Procedia Structural Integrity 37 (2022) 779–787 Fekete, T.: The Fundaments of Structural Integrity … / Structural Integrity Procedia 00 (2021) 000 – 000

781

3

LSPSs designed in the 1970 – 1980 s was 30 – 40 operating years, while for many systems designed nowadays, DSL has reached 60 operating years, especially in case of nuclear construction projects. 2.2. Structural Integrity Calculations of Large-Scale Pressure Systems SICs for a LSPS are computations that investigate the anticipated behaviour and the stability conditions of an in service system based on the information available at the time of calculations – i.e., the ‘ life history ’ (manufacturing, installation and operation load history) of the system components, and other relevant data [e.g., measured characteristics of structural materials, results of In-Service-Inspection ( ISI ) tests] – , and then estimate its Technically Allowable Lifetime ( TAL ), based on the foreseen further operation. The stability investigations of the system cover its structural stability – see Gilmore (1993) – and the stability issues of its structural materials – Ivanova (1998) – . As is the case with DSCs , the SICs examine the behaviour of the system from both damage tolerance and overload tolerance perspectives, including dynamic overload tolerance. As mentioned above, LSPSs are safety-critical systems that are individually designed, their main components individually manufactured and then mounted and commissioned on site. These systems, and their components, are manufactured under strict quality control constraints, and are supported by extensive material testing programs, involving ageing of structural materials, and ISI programs to monitor their fitness for service conditions. The results of these programmes have enabled SICs for LSPSs to be supported by real data and observational evidence, with an increased focus on the individual characteristics of each system, leading to a more realistic assessment of the stability conditions and the TAL of a system. These facts are at the heart of the success of many Service Lifetime ( SL ) extension projects for Nuclear Power Plant ( NPP ) units over the past two decades, which have resulted in units with originally 40 years of DSL being allowed to extend their permitted SL to 60 operating years, known as Extended Service Lifetime ( ESL ). Over the past decade, the electricity generation sector has moved towards a more environment-friendly approach, with a view to reduce the long-term environmental impact of the segment. For the same reason, there is also a need to further extend the SL of large-scale, high-power systems – and their LSPSs – , which means that the envisaged lifetime of these systems is ≈ 80 years of operation, nowadays called Long-Term Operation ( LTO ). Therefore, the role of SICs will be certainly increased, and demonstrating their predictive power will be of particular importance. Consequently, it is worth reviewing and evaluating the standards-based DSC and SIC methodology. 2.3. Methodology of standard-based Design Safety Calculations and Structural Integrity Calculations in a nutshell As mentioned earlier, the aim of constructing a pressure system is to encapsulate the technology, thereby minimising the potential interactions between the process and the external environment. While the methodologies for the DSCs of LSPSs have historically evolved with the needs of the power generation and the heavy-chemical industries, the two key objectives of DSCs have always been as follows: (1) to verify that a system being designed from elements dimensioned according to the Safety Standard, will meet the safety criteria; (2) to determine the expected Safe Operating Limits of the system. The behaviour of an LSPS is governed by the combined effects of the technology system operation, the external environment, and the behaviour of its structural materials, which depends essentially on the existence or the absence of crack-like imperfections within them. Current international good practice in DSCs is to simulate the expected behaviour of an LSPS under design and evaluate its stability conditions, based on: (1) the expected behaviour of the technology, (2) the developed design, in particular the geometry of the system with its relationship to the environment, and (3) the assumed behaviour of its structural materials postulating crack-like damage within them. In this context, DSCs produce system specific solutions to a coupled thermomechanics + fracture mechanics problem. The fundamental equations of the problem are presented below. The thermal part of the problem describes the technology-induced heat transport in the system wall as follows: