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Effect of the mixing flows on the propagation of thermal fatigue cracks


Congress title :19th International Conference on Structural Mechanics In Reactor Technology (SMIRT 19) 
Congress location :Toronto
Congress date :12/08/2007

Document type > *Congrès/colloque

Keywords > cracks, thermal fatigue

Research Unit > IRSN/DSR/SAMS

Authors > CHAPULIOT Stéphane, GOURDIN C., MAGNAUD J. P., MERMAZ Frédéric, MONAVON A.

Publication Date > 12/08/2007

Summary

This paper covers work carried out by the CEA to study the mechanisms leading to cracking of piping as a result of thermal loading in flow mixing zones. The main goal of the work is to analyse, by calculation, the thermal loading caused by turbulent mixing in two kinds of configurations and to understand the mechanism of initiation and propagation of cracks in such components. The understanding of these thermal phenomena is difficult. One of the main obstacles to its understanding resides in the multidomain nature of the loading and associated damage, involving three complementary scientific disciplines: thermalhydraulic field, thermomechanical field and materials science. This paper describes the approach adopted by the CEA to establish natural mechanisms (turbulence, pulsing and instability) which might be the cause of any substantial thermomechanical loading in the piping. This work was initiated and supported by IRSN. Although turbulence in mixing area may be the cause of the thermal stripping (presence of highfrequency thermal fluctuations on the inner surface of the component), it cannot alone explain the propagation of deep cracks. The main reason is the "highpass filter" effect of convection. The wall cannot be subjected to convectionrelated thermal fluctuations and frequencies less than the inverse of the turbulence transit time. A straightforward frequencybased analysis of the loading, carried out as a first stage, made it possible to establish the limits of the loading created by these highfrequency events. However, turbulence can give rise to flow instability (such as pulsing) of lower frequency. But this cannot explain everything. Using two different configurations of flow mixing zones, we have study the unstable conditions of the flows. The first configuration represents the complex 3D geometry of the Civaux Unit 1. The fluctuations of temperatures in the flow are about 180°C. In this configuration, a great number of the thermalcracks appeared quickly and propagated until crossing the thickness of the component. The second configuration represents a piping connexion on the main coolant line where the temperature difference are much more important, about 320°C, but contrary to the first configuration, no crack where observed, even for a long utilization time. Thermohydromechanics linkup analyzes were carried out in order to determine propagation velocities and the conditions of cracks initiation. The experience feedback of the results obtained makes it possible to highlight the physical phenomena which must be taken into account if one wants to be able to undertake simulations in predictive matter of flows in piping. One of the originalities of this study is to carry out the overall analysis (thermalhydraulic and thermomechanical) with a single computer code, the CAST3M code developed by the CEA.


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