Modeling of Helium Bubble induced Cracking during Welding Repair of

There are over 100 nuclear power plants operating in the U.S. that range in age from 15 to 40 years old.  May plants have obtained license extension for operation to 60 years, and operation beyond that is currently being assessed.  As plants age, it is anticipated that some internal components may require repair by welding.  Welding repair of irradiated nuclear reactor materials (such as austenitic stainless steels) is especially challenging because of the existence of large amounts of helium in the steel matrix after intense neutron exposure.  Under the influence of high temperatures and high tensile stresses during welding, rapid formation of large helium bubbles can occur at grain boundaries, resulting in intergranular cracking in the heat-affected zone (HAZ).  In this study, a refined helium bubble nucleation and growth kinetic model is developed based on the literature empirical model in the form of closed-form formulas.  Especially, the material kinetic parameters needed by the empirical model are refined using a mesoscale master equation model for helium-vacancy cluster evolution in austenite matrix.  Experimental results of high-temperature tensile tests of irradiated steel are used to validate the refined model, where the dependence of bubble size on the angle between grain boundary orientation and primary tensile stress direction are discussed.  Finally, a three-dimensional thermal-stress model incorporating the refined helium model is used to calculate the transient temperature and stress conditions and the resulting helium bubble growth kinetics under those transients.  The predicted bubble size distribution is compared with the experimental data available in the open literature.  The effect of welding parameters on the cracking tendency is evaluated using the numerical model.  The quantitative knowledge of helium bubble growth serves as much-needed prerequisite for understanding and mitigation of helium induced intergranular cracking during welding repair of irradiated nuclear reactor materials.