Following the incident (through-wall crack and crazing zones) of May 1998 on the principal mixing zone of the residual heat removal system at CIVAUX nuclear plant, EDF has initiated a R&D programme to understand the incident and assess the risks of damage on other nuclear plant mixing zones. The programme includes different sectors of development: assessment of temperature fluctuations in mixing zones, study of high-cycle thermal fatigue behaviour of austenitic steel and development of mechanical methodologies for damage assessment and propagation of crazing zones (thermal striping). A series of tests is being performed in support of the programme. The paper develops the description of each sector of the programme: tests performed, on-site measurements and numerical interpretations. The first results are presented.
Following the incident (through-wall crack and crazing zones) of May 1998 on the principal mixing zone of the residual heat removal system at CIVAUX nuclear plant, EDF has initiated a R&D programme to understand the incident and assess the risks of damage on other nuclear plant mixing zones. The programme includes different sectors of development: assessment of temperature fluctuations in mixing zones, study of high-cycle thermal fatigue behaviour of austenitic steel and development of mechanical methodologies for damage assessment and propagation of crazing zones (thermal striping). A series of tests is being performed in support of the programme. The paper develops the description of each sector of the programme: tests performed, on-site measurements and numerical interpretations. The first results are presented.
This paper presents a numerical study to tackle thermal striping phenomena occuring in piping systems. It is here applied to the Residual Heat Removal (RHR) bypass system. A large Eddy Simulation (L.E.S.) approach is used to model the turbulent flow in a T-junction. The thermal coupling between the Finite Volume CFD Code_Saturne and the Finite Element thermal code Syrthes, gives access to the instantaneous field inside the fluid and the solid. By using the instantaneous solid thermal fields, mechanical computations (as presented in (Stephan et al 2002)) are performed to yield the instantaneous mechanical stresses seen by the pipework T-junction and elbow.
In mixing tees, hot and cold fluids meet at a nozzle with different flow rates. The resulting temperature fluctuations create mechanical and thermal stresses on the pipes. Predicting the subsequent fatigue is the subject of on-going research. A new approach was developed in order to obtain a qualitative and quantitative estimate of the damage caused by a flow without necessarily acquiring measures, which is often difficult when studying flows in nuclear plant piping. Several time-series of fluid temperature from mock-up tests were studied. Four specific profiles were recognized. Internal stress was computed and fatigue was calculated according to an algorithm. The influences of the profiles, and of the temperature difference between hot and cold legs, were studied. Finally, an approach was proposed, whereby thermal fluctuations are modeled by an envelope spectral density which is transposed to the characteristics of the studied flow (standard deviation, mean temperature, cold and hot legs temperature difference, flow rate). The use of the envelope signal gives a good estimate of the thermal fluctuations but the damage is over-estimated. On-going calculations are attempting to modify the envelope spectral density to attain more realistic values.
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