EXAMPLE FOR THE STRUCTURAL INTEGRITY EVALUATION OF THERMAL MIXING TEE JUNCTION DESIGNED WITH SLEEVES TO PROTECT THE PRESSURE RETAINING WALL FOR THERMAL STRIPING EFFECTS UP TO 250K

摘要

The purpose of this paper is to present the structural integrity evaluation on thermal mixing T-junctions designed with thermal sleeves. The specific plant conditions and layout in the LWR piping of the main feed water system, the residual heat removal system and the emergency core cooling system for the BWR OKG2 are characterized by large temperature differences up to 250K and more than 1000 specified cycles within 40 years lifetime. Beside the investigations of low cycle fatigue evaluations are made for the high cyclic thermal fatigue and flow induced vibration. The evaluation method use computational fluid dynamic analysis (CFD) for indicating severe loads for the several plant conditions. Pre-evaluations using the steady Reynolds-Averaged Navier-Stokes model (RANS) are made to indicate the load cases with potential of large temperature fluctuation and their location. Large Eddy Simulations (LES) are prepared at the smaller region of interest for determination of high cyclic thermal loading at the inner surface of the sleeve and the shielding effect for the pressure retaining wall. Results from the CFD analysis are furthermore used for evaluation of the phenomenon of flow induced vibration. The structural and fatigue evaluation based on the design-by-analysis method using finite element models with linear and non linear elastic-plastic material behavior to ascertain the K_e -factor and their local effects on plasticity at cyclic computation. Discussed are the results with respect to the used method for the load determination, used boundary condition, simplification and the performed structural evaluation. The results will reflect among others to developed or established guidelines on thermal striping effects.