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Advanced Structural Integrity Assessment Tools for Safe Long Term Operation

Periodic Reporting for period 2 - ATLASplus (Advanced Structural Integrity Assessment Tools for Safe Long Term Operation)

Reporting period: 2018-12-01 to 2020-05-31

Material properties are usually determined on specimens. Based on these material properties engineering structures are designed and operated. There is a high demand for safe long term operation of such structures, especially in the nuclear energetic sector. It is of high importance to know better the real limits of materials. Premature ending of a plant lifetime due to unnecessary conservatism is unfavorable economically, but one has to be certain that unnecessary risks are not taken due to our insufficient knowledge. It is well known that measured material properties depend on specimen size, type, etc. By obtaining new data, carefully analyzing, making simulations, etc. we intend to obtain new knowledge, which can be a valuable base for safe long-term operation.
For this, large scale (1:1) experiments are being planned by our partners. We participate in supporting work for these large scale experiments by material testing and simulation. A special case is the 1:5 mock-op of the VVER-440 safe end, which is not a real 1:1 scale experiment. However, in terms of complexity of the experiment it compares to experiments carried out on 1:1 scale mock-ups. We consider that having an intermediate scale between the real component scale and the specimen scale increases confidence in our simulations. The experiment addresses such complex issues as determination of material properties for non homogeneous materials, applying local and global approach to determine better simulation of crack initiation and propagation, fatigue in a complex structure with non homogeneous material, etc.
"We participate in the material characterization program for the ferritic steel WB36, planned to be used for the large scale experiments by EDF. We performed fracture mechanics oriented experiments on SENB (Single Edge Notched Bending) specimens. For these experiments Master Curve type evaluation has been done. Due to the very low number of the specimens (6 from outside and 6 from inside of the pipe), the validity criterion of the Master Curve procedure conform ASTM E 1921 standard could not be satisfied. However, as other partners also made fracture mechanics oriented experiments, it is worth comparing our results to theirs, taken into account the fact, that our partners use different types of specimens (mainly CT (Compact Tension) or SENT (Single Edge Notched Tensile)).
We elaborated a basic concept for the set-up used in the experiment which will be carried out on the VVER-440 Mockup. Main steps are: generation of a notch by EDM (Electro Discharge Machining), obtaining a crack by fatigue, final test by bending, with registration of crack extension. We intend to carry out this experiment on the Instron 8850 biaxial (axial-torsion) universal servo-hydraulic material testing system at BZN. We made analytical fracture mechanics computation for a simplified geometry (tube with crack). We realized, that a large crack is needed (with around 180 degrees circumferential opening angle) to be able to perform the test on the Instron 8850. We created simple FE models containing roughly modeled cracks with different openings for the realistic geometry of the mock-up. We performed basic elasto-plastic computations on these models. Our FE models revealed that a solid plug is needed for extension arm, the initially considered tubular extension arm can not carry sufficient bending moment to start or propagate crack in the DMW (Dissimilar Metal Weld) of the MU.
We made a more elaborated design for the supporting structure. Based on the bending moments obtained from our previous analyses we computed the size and number of bolts to hold the MU in the support. These computations were made based on the endurance limit of the bolts. However, the bolts computed by this procedure are very large, so eventually a more sophisticated solution will be needed when the number of cycles and the loads will be computed with higher precision.
We realized, that we don't know the material properties of the DMW, buttering layer and heat affected zones in a sufficient manner. To overcome this, we intend to cut a piece from the MU itself by wire EDM to be used for material testing purposes. As the experiment is a ""single shot"", we intend to make a second mock-up to test manufacturing technologies and behavior of supporting structure and mock-up prior the final test. Also, some experiments will be carried on specimens made from simple welded pieces containing same types of materials as the real MU.
Results from MULTIMETAL (EU FP7 project) suggests that it would be beneficial to put the crack in the buttering layer as it has the lowest fracture toughness. This approach presents some problems, as the crack front shall be not perpendicular to the symmetry axis of the MU. However, the VVER MU (manufactured in the framework of the STYLE project) was not made with original Russian electrodes as the joints in MULTIMETAL. Newer computations suggest, that putting crack in the DMW, perpendicular to the MU axis may also be acceptable, due to the reduced cross-section on the conical part.
Another concern regards the testing machine, as for large bending considerable forces will develop acting perpendicular to the main hydraulic cylinder of the INSTRON. As the testing machine was not conceived to carry large lateral forces, there is a risk of permanently damaging the testing machine. To overcome this, two competing solutions are considered and a third one is getting elaborated.
For experiments on specimens, Gurson type simulations has been carried out. We realized, that better simulation oriented experiments are needed, so we are taking steps in this regard."
From the point of view of experiments carried out on specimens, we can conclude that there are small details of the experiments which are usually not considered relevant . This may be true in the case of simple evaluation of the experimental results according to standards, however, in the case of more sophisticated evaluations, using detailed analytical and FE models this approach is usually not good enough. We identify these details and try to cope with them. For local approach models there are many parameters which have to be determined to match experiments with simulations. Some of the parameters can be (at least theoretically) determined by experiments, others are internal variables, which can only be determined by parameter fitting. Problems arise, when we try to use such models for a predictive approach as discrepancies arise depending on geometry, mesh, experiment type, etc. It is very little known, and it is not always easy to determine what consists of a successful application of such a model. By careful analysis of the results, identifying of inconsistencies, comparing our methodology to the ones used by our partners', etc. it is expected to pinpoint the limits of these methods and hopefully extend their applicability.
Regarding the large scale experiment on VVER mock-up, instrumentation of such a complex experiment is a challenging task. Taking part in such a complex experiment increases our expertise and has beneficial effects for our future tasks.
Results obtained from the whole project will lead to a better knowledge of the applicability limits of our experimental and simulation work thus leading to increasing confidence in predictions made for the safe long term operation of power plants.
Cracker structure with 2 arms (by BZN)
Possible support and loading mechanism with sliding auto-clutch.
Different possible locations for crack
Possible support and loading mechanism with roller and guide pins
Cutaway by EDM