Igor Varfolomeev

Probabilistic Leak-Before-Break Assessment of a Main Coolant Line

The paper presents results of a probabilistic leak-before-break (LBB) assessment of a ferritic main coolant line of a pressurized water reactor representative for German nuclear power plants. The analysis approach is based on the elastic-plastic fracture mechanics methodology, incorporating the failure assessment diagram to calculate the critical through-thickness crack size, as well as fatigue crack growth calculations to determine the flaw length at wall penetration. An essential part of this study is the collation and statistical treatment of material data, such as strength properties, crack resistance and fatigue crack growth curves, and their incorporation in the probabilistic assessment. The analysis yields negligible break probabilities, thus demonstrating the LBB behavior of the piping. This conclusion is validated by results of a sensitivity study with additional conservative assumptions and simplifications with respect to the initial crack size and material state. For the sake of simplicity, the blocking effect of the intact austenitic cladding on the crack extension, as well as the conditional probability of crack penetration through the wall during the service life are neglected in this investigation. Through an additional treatment of these issues a more realistic assessment is achieved, resulting in even smaller failure probabilities.Copyright

Consideration of Thermally Induced Bending Load in Failure Assessment of Piping Components

The primary goal of this paper is to demonstrate and validate the applicability of the failure assessment diagram (FAD) approach for predicting failure behavior of piping subjected to thermal loading caused by thermal stratification. The analysis is performed for an austenitic surge line of a PWR circuit where thermal gradients over both the pipe length and circumference produce high bending moments. For the most critical location, circumferential through-wall cracks of variable length are assumed and assessed with respect to their initiation and possible unstable propagation. The assessment result is shown to be decisively influenced by the modeling approach, which in effect depends on the categorization of the bending stresses as either primary or secondary loading. To achieve realistic fracture mechanics assessment, the whole surge line is described by a finite-element model, whereas thermal loading is imposed by mapping the temperature distribution measured under service conditions. Subsequently, elastic-plastic stress and fracture mechanics calculations are performed for the entire surge line model containing cracks of different size in the most stressed location. The analysis clearly confirms the LBB behavior. The numerical results are then compared to those based on the failure assessment diagram (FAD) approach, whereas the section bending moment determined from pipe calculations is categorized as either primary or secondary loading. It is concluded that most realistic analytical failure prediction is achieved when considering the bending moment as secondary loading. Even though the analytical approach is found in this case to produce conservative assessment results, its accuracy is concluded to be acceptable within a wide range of crack sizes considered.Copyright

Consideration of Fatigue Crack Growth Aspects in the Design and Assessment of Railway Axles

Current design rules for railway wheelsets do not directly address issues related to fatigue crack propagation. Nevertheless, the latter topic is a part of the revised safety concept for passenger trains recently adopted in German railway applications. Numerous research activities, including international cooperative projects, have been conducted in the past decade aiming at quantifying fatigue crack growth rates in railway axles and estimating their inspection intervals based on the fracture mechanics methodology. This paper summarizes some experience and findings obtained by the authors within several studies dealing with the assessment of fatigue crack propagation in railway steels. Particular aspects highlighted in the paper include material characterization, effects of the specimen geometry and crack tip constraint on fatigue crack growth rates, stress analyses of axles and wheelsets, the derivation of stress intensity factor solutions applicable to specific conditions achieved in railway axles, considerations of the variability and scatter of geometrical parameters and material data in fatigue crack growth calculations.

Application of the strain-based FAD to failure assessment of surface cracked components

Purpose – The purpose of this paper is to validate the strain-based failure assessment diagram (SB-FAD) approach for surface cracks in components subjected to displacement controlled boundary conditions. Design/methodology/approach – Numerical analyses are performed for several crack geometries and materials representative for aerospace applications. The performance of the SB-FAD is judged by comparing numerically calculated J-integrals to respective analytical estimates, using both Options 1 and 2 approximations. Findings – In the most cases, both Options 1 and 2 SB-FAD method results in reasonably conservative J-estimates. Exceptions are for surface cracks in a pressurized vessel made of a material with low-strain hardening, for which Option 2 assessment produces non-conservative results. In contrast, Option 1 assessment is conservative for all geometries considered. In general, Option 1 results in a considerable overestimation of the crack driving force, whereas Option 2 produces rather accurate result...

A Critical Evaluation of Methods for Computing Small Failure Probabilities for Ductile Piping Components

This paper focuses on the application of the failure assessment diagram (FAD) approach to accomplish robust and reliable calculations of small failure probabilities, as required for safety relevant nuclear components. Specifically, the first- and second-order reliability methods (FORM, SORM) as well as different sampling algorithms are investigated as to their performance and numerical stability. In particular, computational problems are noticed in the assessment range corresponding to the transition from ductile fracture to plastic collapse which is a typical operating regime for ferritic piping. Numerical errors are found to result due to the occurrence of multiple design points in the FORM procedure, thus leading to an underestimation of failure probabilities. Subsequent benchmark calculations reveal a high efficiency of the improved numerical algorithms within a very broad range of failure probabilities. Another aspect considered in the paper is the application of FORM/SORM to the assessment of crack instability taking into account the scatter in crack resistance curves. Finally, the methods and computer tools developed within the framework of this research are applied for evaluating some pipe burst tests described in the literature.Copyright

Integrity Assessment of a German PWR RPV Considering Loss of Constraint

The brittle failure assessment for five pressurized water reactor pressure vessels (RPV) of German nuclear power plants (NPP) has been revisited according to an advanced state of the art. Besides of recent innovation in fracture toughness curves and reference temperatures being already in the codes, also the effect of loss of constraint had to be considered when fracture toughness values determined from deep cracks in fracture toughness specimen with high multi-axial state of stress were transferred to crack configurations in the component. Thus, the available concepts were compared for their fitness for purpose, i.e. for their ability to give a fracture toughness representative to the crack configuration or flaw postulate in the component. The results of the investigation reveal a significant lower constraint in the component resulting in increased fracture toughness and showing that the brittle failure assessment based on the high constraint fracture toughness from the standard specimens can be very conservative. For consideration of the constraint conditions in the component besides the deterministic T-stress parameter also probabilistic local approach concepts based on the Weibull model were used which have the advantage of considering both the local stress strain field and the material volume under high loading. The loss of constraint was determined for several flaw postulates in the leading situations on the RPV being the coolant inlet nozzle corner and the flange joint. A considerable loss of constraint was demonstrated for flaw postulates with broken clad in the ferritic nozzle corner. Also in the flange joint the loss of constraint is evident for small flaws. In addition, for flaw postulates under the intact cladding the loss of constraint is remarkably higher than with broken postulated cladding. In summary, with the measured material toughness and the significant loss of constraint a considerable inherent margin against brittle failure can be demonstrated for the investigated load cases.Copyright

Numerical Analyses of Residual Stress Fields in Welded Components

Residual stresses in welded components can influence the lifetime significantly. Besides experimental methods of residual stress measurements numerical methods of welding simulation are an important tool to determine the whole residual stress field in a component as a basis for lifetime prediction. As examples, the residual stresses in a core shroud of a boiling water reactor (BWR) and in a cladded plate have been investigated. In case of the core shroud postulated cracks in the residual stress field of the weld have been assessed with respect to possible crack corrosion cracking. For the cladded plate, the numerical simulation of the cladding and heat treatment process was accompanied by measurements of temperature, distortions and residual stresses. In the analysis, the temperature dependent material properties as well as the transformation behavior of the ferritic base metal were taken into account. The calculated residual stresses show tensile stresses in the cladding followed by compressive stresses in the base metal that are in agreement with measurements with X-ray diffraction technique.© 2009 ASME