The reactor pressure vessels make up the heart of a nuclear power plant, containing the core, core shroud and coolant. These nuclear plants were built and started operating 20 to 30 years ago. The company operating this plants needed to know how this welds can be expected to perform after a long period of service. It will be very hard to replicate the effects of 20-plus years of operation. So the operator of the plant agreed with Bay Zoltan Institute to simulate the manufacturing process to better understand the characteristics of the welded joint.
MSC Software’s Marc Nonlinear FEA Software was selected because of Bay Zoltan Institute engineers experience with similar problems that Marc solved successfully and achieved very good correlation with physical experiments. The material model for heterogeneous welding was selected by utilizing BAY-LOGI engineers experience from an earlier modeling problem, where the task was to simulate repair welding on an austenitic stainless steel pipe of another type of reactor.
Based on this experience, the heterogeneous welding of safety end nozzle was molded in 3D. The joint was built from 98 weld runs. The model consists of more than 400,000 hex elements, the task was solved in approximately 18,000 nonlinear increments. “Based on the calculated stress state of the dissimilar metal weld is a good representation of the as-welded state.” Said Zoltan Bezi, Research fellow for BAY-LOGI.
“The simulation results helped us understand more deeply the nature of the welded joint and the safety margins for long term operation,” Bezi said. “By applying case specific FE simulations, the weld and its residuals stresses were realistically assessed. The properties of the weld and heat affected zone including the temperature distribution and residual stress distribution were also determined and these values were used for the subsequent structural integrity assessment. With FE, the effects of ageing and operational loading on the stress state of welds was also taken into account. This made it possible to investigate the long term operational effects. The end result demonstrate that the welding simulation provides the necessary information about the initial stage of the joint so we could be certain about long term safe operation without having to build a duplicate structure, saving millions of dollars”