PSI - Issue 17

Tamás Fekete / Procedia Structural Integrity 17 (2019) 464–471 Tamás Fekete / Structural Integrity Procedia 00 (2019) 000 – 000

468

5

pressurized thermal shock (PTS) calculations – see e.g. Fekete (2018) – are a typical example of these investigations for nuclear power plant reactor pressure vessels). ○ Investigations of the possible consequences of significant dynamic effects, stemming from the environment (e.g. earthquake analyses). During accident assessments, temperature and stress-strain field histories are determined – for the interval of the accident plus the interval in which the System reaches its stable state again – . In addition, fracture mechanics calculations are carried out for damage-tolerant design. For these calculations, the design standards give a rather general guidance; the use of safety calculation guidelines is strongly advised. The material parameters to be applied in the computations are described in the design standards or in the safety calculation guidelines. As an outcome of System design safety calculations, the designed service lifetime of the System is defined as the minimum of either its design allowed lifetime , or its expected service lifetime . Since the main objective of the design safety calculations is to provide a lifetime estimate that is predictive in the sense that the System can be operated with very high reliability, at least until the designed service lifetime it is imperative that the calculations have adequate predictive power. Therefore, the conditions as well as the results of the calculations should be regularly checked on the functioning System. Based: on (1) assumptions and constraints on which System design safety computations depend, as well as on (2) particular results of the calculations, an elaboration of the Maintenance, Surveillance, Inspection and Testing (MSIT) system plan will continue, until its final form is reached (see e.g. Trampus (2014)). The purpose of developing the MSIT system (see Trampus (2014)) is that the necessary observations and measurements are carried out on the functioning System. Their results are evaluated, and if deviations are detected, the necessary actions are performed as required. Based on the results of design safety computations, the selection of safety critical zones come to an end, so the MSIT system plan focuses primarily on these critical zones. 3.2. Structural Integrity for the functioning System After manufacturing and assembling the parts – which is completed by the installation of the MSIT system – , the System is ready to be put into operation. When commissioned, it starts functioning according to the intentions of the design. During operation, the operating staff collects information through the MSIT system, – hand in hand with the concerned professional and scientific community. They investigate whether or not the System, as well as its structural materials – primarily in the safety critical zones – , are in suitable conditions (are in ‘good enough health’) that may allow further operation. When processing the information provided by the MSIT system, two types of approaches can be used: • A restricted approach, when the information is used only to check the validity of design safety calculations; this approach was used for many decades e.g. in conventional power plants. • A realistic approach, when the information serves as a starting point to give more realistic ‘ technically allowed lifetime ’ assessments of the System. Strictly speaking, these computations are called Structural Integrity calculations. The significance of these calculations and the importance of the soundness of their methodologies are increasing by the day, since the feasibility of long-term operation (i.e. operation between 50 and 80 years) of functioning reactor pressure vessels in the nuclear energy sector has become a more and more relevant issue. In the meantime: ○ new operational needs (e.g. load following operation) have emerged; and ○ the environmental standards as well as the safety requirements are becoming increasingly stringent. Structural Integrity calculation methodologies are nowadays directed by standards and guides; furthermore, they are supported by newer results of applied research, technical development and the best engineering practices. Although applying the latest achievements of experimental methods and information technology all highly contributed to improving the accuracy of safety evaluations of large-scale pressure systems, some basic theoretical weaknesses of standard assessment methodologies – e.g. the poorly described load-history effects – have so far not been eliminated. Therefore, the predictive potential of these methods has not improved significantly. This justifies the need for general methodological questions of Structural Integrity to be studied.