FINITE ELEMENT ANALYSIS OF CRACK PROPAGATION IN LARGE SIZE PIPE BENDS WITH EMBEDDED CRACKS

Abstract

As per India's three stage nuclear programme. PFBR (proto type fast breeder reactor) construction work is going on at IGCAR (Indira Gandhi centre for atomic research). Kalpakkam. The critical area from safety aspect in PFBR is the primary piping system. which in itself has the pipe bend region as the most critical area. In the present dissertation work FEM analysis was done to find out the critical position for crack initiation and to know the maximum stresses at the position for a 900 pipe bend of dimensions: O.D. 580 mm_ Bend radius 840mm and wall thickness 15.5 mm. under displacement controlled loading. The dimensions of the pipe bends are similar to the pipe bends of the main cooling piping system of 500 MW PFBR. The bend flank area is found to be the critical location. For the leak before break assessment of the pipe bends as well as to evaluate the leak rate for detection of crack, the crack mouth opening displacement (CMOD) of a through thickness axial crack on the pipe bends at critical location was simulated by simplified approach. By incorporating the single and then the double curvature effect on the CMOD for flat plate geometry. CMOD for pipe bends was simulated and the approach was validated at every step by FEM. However an error of 20 % was found for CMOD of a crack on pipe bend. Since a factor of safety 10 is taken in calculation of the leak rate in PFBR leak detection system which is equivalent to 1000 % of error. so 20% of error can be considered as an acceptable error