Effects Of Safe End Geometry And Local Mechanical Properties On LBB Behavior Of A Dissimilar Metal Welded Joint In Nuclear Power Plants

Dissimilar metal welded joint (DMWJ) is widely used in primary water systems of pressurized water reactors (PWRs) in nuclear power plants (NPPs), and it is a special welding structure in the main pressure loop. And it is also the key equipment for nuclear safety. Operating experience has shown that failure had always taken place in the DMWJ, not only because of the material mismatch, geometric discontinuity but also the complex loads including bending moment, internal pressure, gravity, alternating stress, seismic load and so on. The main nuclear loops, which are made from high-quality material with higher toughness and work at higher temperature, are more likely to leak firstly and then fracture (Leak before Break, referred to as LBB). At present, the effects of material property mismatch and nozzle geometry on the key parameter and result of LBB analysis have not been completely understood and studied. Based on the third generation of nuclear power technology AP1000as the research background, three-dimensional finite models were built for the DMWJ to accurately analyze the effects of material mismatch, geometric discontinuity, crack size and bending moment on the LBB safety assessment. The main achievements are as follows:(1) The LBB curve and evaluation diagram are built for the weakest property location of DMWJ, where is the interface of A508and Alloy52Mb. The safe end to pipe-nozzle was simplified into the straight-pipe with the same thickness and diameter, which leads to the increase of J-integral at the crack tip and the decrease of critical crack length for straight-pipe. What’s more, the LBB analysis result for straight-pipe was conservative.(2) Due to the small size of HAZ compared with the whole pipe-nozzle, the effect of the local mechanical properties on the J-integral is not obvious, In addition, the LBB analysis result for the actual model in which local mechanical properties are considered is conservative.(3) Since the local fracture resistances of the HAZ region and A508/Alloy52Mb interface region are much lower than those of the base metal, the non-conservative (unsafe) LBB assessment result will be produced if the fracture resistance curve of base metal is used. With increasing the applied bending moment, the degree of un-conservatism in LBB behavior becomes large. Therefore, to obtain accurate LBB assessment results for cracks located in the A508/Alloy52Mb interface region, the local fracture resistance properties of the interface region should be used.(4) With increasing the crack length, the crack opening shape changes in a order of diamond, symmetric ellipse,un-symmetric ellipse and half-ellipse at the same bending moment. With increasing the bending moments, the crack opening displacements become large, which mainly is affected by the crack axial displacement on the side far from the nozzle geometry.