A computational modeling tool for welding repair of irradiated materials

Weld repairs performed on irradiated reactor components can be problematic due to the presence of entrapped helium in the reactor internal components that is generated by boron and nickel transmutation under neutron irradiation. Under unfavorable conditions of high heat input, high tensile stress and high helium content, sufficient quantities of entrapped helium can migrate to and accumulate at grain boundaries during a welding operation thus resulting in cracks in the weld. Determining crack-free welding conditions solely based on extensive weldability testing for a given welding repair scenario could be very costly, time consuming, and in some cases in-feasible. In this work, a computational model has been developed and sufficiently validated as a cost-effective alternative method for making this determination. This computational model considers the transient temperature profile in the material, the stress history in the material and the helium content. It further includes a methodology for determining the amount of helium that is released under the calculated temperature/stress conditions, a method for calculating the degree to which the helium bubbles grow, and a criterion for determining if the resulting helium bubble distribution will result in cracking as the weld cools. This computational tool is benchmarked with available experimental results on welding of irradiated materials. It is shown that such a model can be a cost-effective tool for determining suitable welding process conditions (heat input and process selection) for repair welding of irradiated materials of reactor internals.