Improving Structural Integrity Assessment Techniques

The Network for Evaluating Structural Components (NESC) was set up over 10 years ago to address the verification of advanced structural integrity assessment techniques for safety-critical reactor components. Through its self-funded activities and associated projects, partly funded by the European Commission's DG RTD, it has developed a significant body of benchmark R&D data, in particular for large-scale tests performed under closely monitored conditions. Key developments are described in three areas a) fracture assessment for reactor pressure vessels; b) fracture assessment of dissimilar metal welds in primary piping and c) thermal fatigue damage in class 1 and class 2 piping. The added value of combining the efforts of utlities, manufacturers and R&D organisations is stressed.JRC.F.4-Nuclear design safet

Prediction of Fracture in the Transition Regime : Application to an A533B Pressure Vessel Steel

In order to model the fracture behaviour of pressure vessel steels in the transition regime, a new model has been developped in 1991 at EDF R&D Division in the framework of local approach to fracture. This approach couples the damage mechanics model developped by Rousselier which is accounting for ductile propagation of a crack, and the Beremin's model based on Weibull's statistics which stands for cleavage. It is possible to predict, by this coupled approach, safe lower bound transition curves for a temperature range up to RTNDT+50°C. It has been shown that the predictions of the model agree well with the experimental data, both in terms of fracture toughness at cleavage instability and the amount of pre-cleavage tearing. Those predicted curves have also been compared to the ASME design curve, and substantial safety margins have been exhibited

Reference upper shelf fracture toughness properties of PWR pressure vessel materials Neutral/basic flux PWR submerged-arc welds

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Local Approach to Fracture Applied to Reactor Pressure Vessel : Synthesis of a Cooperative Programme Between EDF, CEA, Framatome and AEA

A cooperative research programme in the field of the local approach to cleavage fracture applied to reactor pressure vessels is conducted between EDF, CEA, Framatome and AEA Technology. The purpose of such a programme is to have a better knowledge of this approach for RPV fracture mechanics integrity assessments regarding the risk of brittle fracture. This programme includes both an experimental part conducted by several laboratories in order to identify m and σ parameters of Beremin model on a A508 C13 steel, and numerous numerical computations of specimens and structures. Finite element programs are compared on some reference calculations in order to validate local approach to fracture postprocessors. The capability of Beremin model to explain the shallow flaw effect in cleavage fracture is finally discussed

Validation of constraint-based methodology in structural integrity of ferritic steels for nuclear reactor pressure vessels

VOCALIST (validation of constraint-based methodology in structural integrity) was a shared cost action project co-financed by DG Research of the European Commission under the Fifth Framework of the European Atomic Energy Community (EURATOM). The motivation for VOCALIST was based on the understanding that the pattern of crack-tip stresses and strains causing plastic flow and fracture in components is different to that in test specimens. This gives rise to the so-called constraint effect. Crack-tip constraint in components is generally lower than in test specimens. Effective toughness is correspondingly higher. The fracture toughness measured on test specimens is thus likely to underestimate that exhibited by cracks in components. The purpose of VOCALIST was to develop validated models of the constraint effect and associated best practice advice, with the objective of aiding improvements in defect assessment methodology for predicting safety margins and making component lifetime management decisions. The main focus in VOCALIST was an assessment of constraint effects on the cleavage fracture toughness of ferritic steels used in the fabrication of nuclear reactor pressure vessels, because of relevance to the development of improved safety assessments for plant under postulated accident conditions. This paper provides a detailed summary of the main results and conclusions from VOCALIST and points out their contribution to advances in constraint-based methodology for structural integrity assessment. In particular, the output from VOCALIST has improved confidence in the use of KJ-Tstress and KJ-Q approaches to assessments of cleavage fracture where the effects of in-plane constraint are dominant. Cleavage fracture models based on the Weibull stress, ?W, have been shown to be reliable, although current best practice advice suggests that ?W should be computed in terms of hydrostatic stress (as distinct from maximum principal stress) for problems involving out-of-plane loading. Correspondingly, the results suggest that the hydrostatic parameter, QH, is the appropriate one with which to characterize crack-tip constraint in analysing such problems. The materials characterization test results generated as part of VOCALIST have provided added confidence in the use of sub-size specimens to determine the Master Curve reference temperature, T0, for as-received and degraded ferritic RPV materials. The usefulness of correlating the Master Curve reference temperature, T0, with the constraint parameter, Q, has been demonstrated; however, the trend curves derived require further development and validation before they can be used in fracture analyses. The output from VOCALIST has contributed in providing the validation of methodology necessary to underpin the diffusion of constraint-based fracture mechanics arguments in RPV safety cases, with potential applications including WWER as well as Western-style LWR reactor types

Measurement and Prediction of Phase Transformation Kinetics in a Nuclear Steel During Rapid Thermal Cycles

Accurate prediction of the residual stress distributions in steel welds can only be achieved if consideration is given to solid-state phase transformation behavior. In this work, we assess the ability of a model for reaction kinetics to predict the phase transformations, and corresponding evolution of volumetric strain, in a nuclear pressure vessel steel when subjected to rapid weld-like thermal cycles. The cases under consideration involved the rapid heating of SA508 steel to a temperature of either 900 °C or 1200 °C for a period of 10 seconds, and subsequent cooling of the material to room temperature at rates between 0.1 and 100 °C s$^{−1}$. Predictions for the microconstituent proportions and transformation temperatures for each thermal cycle are compared to those measured through a combination of dilatometry, optical and electron microscopy, and synchrotron X-ray diffraction. In general, there was good agreement between measured and predicted transformation start temperatures and microconstituent fractions for cooling rates relevant to welding (≥ 10 °C s$^{−1}$). Even in the cases in which discrepancies were found for start temperatures, examination of the corresponding dilatation curves showed a good match between predicted and experimental transformation strain evolution. This is a very positive result in terms of residual stress prediction in welds. At slower cooling rates, significant discrepancies arose owing to the model’s incapacity to predict Widmanstätten ferrite or retained austenite, and its failure to account for the effects of carbon redistribution during transformations involving diffusion. Although not relevant to welding, improvements to the model to rectify these issues would be beneficial in terms of its wider predictive capabilities