PSI - Issue 48

Tamás Fekete / Procedia Structural Integrity 48 (2023) 302–309 Fekete / Structural Integrity Procedia 00 (2023) 000 – 000

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LSPS can be expected. An operating plant may be granted an extended OL , if SIC s for that plant demonstrate that the TAL of its LSPS s is more than the envisaged Extended Operating Time ( EOT ). Several NPP s, built worldwide in the last third of the 20 th century, have already been granted an Extended OL – called first-phase extended OL – , in most cases for 20 OY . Nowadays, NPP owners are preparing for second phase extended OL , to obtain additional 20 OY OL s. These NPP s will operate in the Long-Term Operation ( LTO ) range – i.e., between 50 and 80 OY s – . The methodology of SIC projects during first-phase extended OL procedures was the same as that used for the DSC s. Now, in the preparation of second-phase extended OL , a series of questions are emerging that previously had not been relevant for safety assessment, because ageing of the LSPS ’ s structural materials over decades of operation has reshaped conditions for safe operation. It is thus worth rethinking SIC methodologies from the ground up. At Centre for Energy Research in Hungary, the development of a new methodology for future SIC projects for LSPS s has been in progress for some years. The paper introduces the Modern Thermomechanics Based Nonlinear Field Theory of Fracture Mechanics ( NLFTFM ) framework, being the cornerstone of future SIC methodologies, and Before getting to the topic, a comment is in order. LSPS s are dimensioned according to internationally accepted Design Standards and Guidelines ( DSG s) – e.g., ASME Code, RCCM Code, DIN, KTA – . The purpose of system DSC s is to prove that the LSPS is guaranteed to be in service for at least the DSL . DSC s are implemented in line with the DSG s. DSG s have been developed decades ago following the disciplinary paradigm where the whole system's behavior is represented by a staggered coupling scheme of the disciplinary parts. The approach to DSC methodology is inconsistent because heat conduction theory and fracture mechanics are derived from dissipative/irreversible paradigms , strength analyses are based on reversible world view ; the description of ageing is so empirical that the issue is unclear . The concept lacks a coherent description of system level dissipation, as shown in Fig. 1. then presents the concept through an industrial case study. The Modern Thermomechanics Based Approach for SIC s

Fig. 1. A staggered disciplinary approach to DSCs, indicating the ways each discipline addresses dissipation.

The main hypothesis for the new methodology, rooted in philosophy , is that irreversibility is inevitable in our world – see e.g., Öttinger (2017), Oldofredi, Öttinger (2021), Fekete (2019, 2022) – . The hypothesis is supported by Torromé's model – Torromé (2021) – , which states nothing less than that dynamical systems are generally irreversible ; reversible behavior occurs under specific conditions. Given this hypothesis, one might plausibly assume that the physical framework theory of the methodology to be developed for SIC s is Modern Thermodynamics – a holistic theory – . Thermodynamics laws state that different physical processes in a system (represented by different fields in the theory) do not operate independently of each other, or possibly coupled in a weakly coupled scheme, but are strongly entangled through the Second Law of Thermodynamics . All physical processes produce entropy, i.e., involve energy dissipation. Couplings govern how entropy production – or dissipated power – is (re)distributed between different processes. The sketched reasoning has led to the choice of NLFTFM as the framework theory for the engineering SIC methodologies.

Fig. 2. The Modern Thermomechanics based – holistic – approach to future SIC methodologies.