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Dynamic buckling in a next generation metal coolant nuclear reactor

Lo Frano R., Forasassi G.

Journal of Achievements in Materials and Manufacturing Engineering

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2008

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Vol. 29, nr 2

163-166

EN

Purpose: The aim of the paper is to investigate the buckling effects due to the seismic sloshing phenomena interesting for a next generation heavy liquid metal cooled reactor as for example the eXperimental Accelerator Driven System (XADS). Design/methodology/approach: In this study the structural buckling behaviour of a reactor pressure vessel, retaining a rather large amount of liquid and many internal structures, is coupled to the fluid-structure interaction because during a postulated earthquake (e.g. Design Basis Earthquake) the primary coolant surrounding the internals may be accelerated with a resulting significant fluid-structure hydrodynamic interaction (known as "sloshing"). Finite element numerical approach is applied because neither linear nor second-order potential theory is directly applicable when steep waves are present and local bulge appear with a marked decrease in strength of structure. Findings: The numerical results are presented and discussed highlighting the importance of the fluid-structure interaction effects in terms of stress intensity and impulsive pressure on the structural dynamic capability. These results allowed to determine the components mostly affected by the loading condition, in order to upgrade the geometrical design, if any, for the considered nuclear power plant (NPP). Research limitations/implications: The presented research results may be considered preliminary; thus it may be useful for a design upgrading of the reactor vessel and for achieving a first evaluation of the real components capacity to bear dynamic loads in particular in the event of a severe earthquake. Originality/value: From the point of view of the practical implication, it is worth to stress that the safety of liquid retaining nuclear structures subjected to a seismic loading is of great importance in regard to the hydrodynamic forces caused by sloshing and impulsive liquid motion determined by the liquid filling levels oscillatory phenomenon.

2

Curved thin shell buckling behaviour

Forasassi G., Lo Frano R.

Journal of Achievements in Materials and Manufacturing Engineering

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2007

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Vol. 23, nr 2

55-58

EN

Purpose: The aim of the paper is to evaluate buckling instabilities behaviour of long curved thin shell. Both initially straight and curved tubes are investigated with numerical and experimental assessment methods, in the context of NPP applications with an illustrative example for IRIS LWR integrated Steam Generator (SG) tubes. Design/methodology/approach: In this study structural buckling response tube with combination effects of geometric imperfections as well as initially bent shape under external pressure load are investigated using a non linear finite element (MSC.MARC FEM code) formulation analysis. Moreover results are presented, extending the findings of previous research activity works, carried out at Pisa University, on thin walled metal specimen. Findings: The experiments were conducted on Inconel 690 test specimen tube. The comparison between numerical and experimental results, for the same geometry and loading conditions, shows a good agreement between the elastic-plastic finite-element predictions and the experimental data. Research limitations/implications: The presented research results may be considered preliminary in the sense that it would be important to enlarge the statistical base of the results themselves, even if they are yet certainly meaningful to highlight the real problem, considering the relatively large variability of the geometrical imperfections and bending instabilities also in high quality production tubes. Originality/value: From the point of view of the practical implication, besides the addressed problem general interest in industrial plant technology, it is worth to stress that straight and curved axis tubes are foreseen specifically in innovative nuclear reactors SG design.

3

Buckling of imperfect thin cylindrical shell under lateral pressure

Forasassi G., Lo Frano R.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 18, nr 1-2

287--290

EN

Purpose: This paper investigates buckling behaviour of imperfect thin cylindrical shell with analytical and experimental assessment methods, in the context of NPP applications as, for instance, the IRIS LWR integrated Steam Generator (SG) tubes. Design/methodology/approach: In this paper, thin shell, homogeneous and isotropic material, also tube geometric imperfections as eccentricity/ovality/welding are assumed to investigate the effects of latter on the limit pressure load in conditions for which, at present, a complete theoretical analysis was not found in literature. At Pisa University a research activity is being carried out on the buckling of thin walled metal specimen, with a test equipment (and the necessary data acquisition facility), suitable for carrying out test series on this issue, as well as numerical models implemented on the MARC FEM code, were set up. Findings: The experiments were conducted on test specimens of the same material (AISI 316) tube with and without longitudinal welding. The comparison between numerical and experimental results, for the same geometry and loading conditions, shows a good agreement between the elastic-plastic finite-element predictions and the experimental data. Research limitations/implications: The presented research results may be considered preliminary in the sense that it would be important to enlarge the statistical base of the results themselves, even if they are yet certainly meaningful to highlight the real problem, considering the relatively large variability of the geometrical imperfections also in high quality production tubes. Originality/value: From the point of view of the practical implication, besides the addressed problem general interest in industrial plant technology, it is worth to stress that straight and curved axis tubes are foreseen specifically in innovative nuclear reactors SG design