In this paper the project DEFI-PROSAFE is presented. In the frame of Nugenia+ project, a work package was dedicated to the “DEFInition of reference case studies for harmonized PRObabilistic evaluation of SAFEty margins in integrity assessment for long-term operation of reactor pressure vessel” (acronym DEFI-PROSAFE). A methodology is proposed to assess safety margins in RPV integrity accounting for uncertainties propagation, because no commonly accepted European approach exists for probabilistic assessment of RPV fast fracture risk. The DEFI-PROSAFE methodology, which is based on the comparison between deterministic and probabilistic assessment, will be detailed. The experience gained from the US Screening Criteria (NUREG-1806) and past projects (ICAS, PROSIR), and guideline (IAEA TecDoc 1627) as well as aspects specific to European deterministic integrity approach have been considered. Usually for probabilistic fracture assessment of the RPV, the parameters (describing flaws, material and neutron fluence) are sampled and the RPV cylindrical region is assessed using deterministic thermal hydraulic loading for evaluation of initiation or failure conditional probability. Within DEFI-PROSAFE methodology the uncertainties in thermal-hydraulic input parameters are taken into account and their propagation in the structural assessment is considered. Comparison of RELAP5 thermal hydraulic results and mixing calculation results (KWU-MIX) with experiment relevant to PTS assessment was performed. Furthermore the DEFI-PROSAFE methodology considers RPV discontinuity regions (like RPV nozzle) and specific PIRT analysis has been performed for selection of the TH-parameters. A new benchmark for probabilistic assessment of RPV was defined within the DEFI-PROSAFE project. The benchmark definition and case studies based on previous R&D project (ICAS, PROSIR) will be presented. The performance of the new benchmark may be submitted as new European project.
The purpose of this paper is to present engineering methods used in Germany for integrity assessment of structural components with a flaw in the ductile regime. The methods are validated by more than 1500 experiments and cover through wall crack as well as part-through wall cracks (in either axial or circumferential orientation) located in base material, heat affected zone, homogeneous or dissimilar metal welds. By fulfilling the toughness requirement, the structural integrity of a cracked component in the ductile regime can be assessed conservatively (safely) by considering simplified methods for which only strength and impact energies values are needed. The physical and mathematical background of the methods for circumferential cracks (flow stress concept FSC; plastic limit load PLL) and for axial cracks (Battelle approach BMI; Ruiz approach RUIZ), as well as the historical connection with the German basis safety concept will be reviewed. Similarity with other international methods (like R6, Section XI of ASME Code, SRP 3.6.3) will be emphasized. The range of validity resulting from experimental investigations (experimental database built from own research or from literature) is summarized and the validation procedure of the methods (FSC, PLL, BMI, RUIZ) is explained. The resulting correction factors on flow stress (envelop flow stress) to account for elastic-plastic fracture conditions and to result in conservative (safe) predictions of crack instability depending on the method (FSC, PLL, BMI, RUIZ), crack orientation (circumferential and axial) and type of material (ferritic and austenitic) is given. The ratio of the computed experimental to the theoretical stress gained by these several engineering methods (German flow stress based simplified methods and other international methods) will be presented on the basis of the experimental database (with a various combination of geometry, materials and loading combination) which covers also elbows and vessels. Recommendations on the good use of the methods for ductile integrity assessment are given including also dissimilar metal welds consideration.
Non-destructive inspection records have to be characterized before evaluation of acceptance if they are evaluated as material or fabrication flaws. When multiple flaws are detected by volumetric examination, the question of interacting flaws arises if they are close to each other: can the conventional approach based on individual flaw be applied? Alternative flaw characterization requirements may be applied in lieu of using current codes by considering recent fracture mechanics research. In order to relax the conservatism of current interaction criteria, specific work was performed to describe interaction rules for flaws located in different planes. The proximity criteria are valid for linear elastic or limited elastic plastic material behavior : this is generally the case in large components. This paper presents the technical basis including validation of the proximity criteria based on a specific component with several flaws and considering 3D numerical modeling using elastic-plastic material behavior in order to check if the plastic material behavior affects the selected proximity criteria. The component is submitted to uniform (pressure) and non-uniform loading (like thermal shock).
When multiple flaws are detected in pressure retaining components during inspection, the first step of evaluation consists of determining whether the flaws shall be combined into a single flaw or evaluated separately. This combination process is carried out in compliance with proximity rules given in the Fitness-for-Service (FFS) Codes. However, the specific criteria for the rules on combining multiple flaws into a single flaw are different among the FFS Codes. In this context, revised and improved criteria have been developed, to more accurately characterize the interaction between multiple subsurface flaws in operating pressure vessels. This improved approach removes some of the conservatism in the existing ASME Code approach, which was developed in the 1970s based on two flaws interacting with each other. This paper explains in detail the methodology used to derive improved flaw proximity rules through three-dimensional FEM and XFEM analyses. After the presentation of the calculations results and the improved criteria, the paper also highlights the multiple conservatisms of the methodology using several sensitivity analyses.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.