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Use Computational Model to Design and Optimize Welding Conditions to Suppress Helium Cracking during Welding

Today, welding is widely used for repair, maintenance and upgrade of nuclear reactor components. As a critical technology to extend the service life of nuclear power plants beyond 60 years, weld technology must be further developed to meet new challenges associated with the aging of the plants, such as control and mitigation of the detrimental effects of weld residual stresses and repair of highly irradiated materials. To meet this goal, fundamental understanding

of the โ€œweldingโ€ effect is necessary for development of new and improved welding technologies.

Welding repair of irradiated nuclear reactor materials (such as austenitic stainless steels used for the reactor internals) is very challenging because the existence of helium in the steel, even at very low levels (i.e. parts per million), would cause cracking during repair welding. Helium is a product of the boron and nickel transmutation process under intense neutron irradiation. As the service life of nuclear reactors in the US prolongs, the amount of helium in the structural materials in certain highly irradiated areas will continue to increase, potentially to a level that the current welding repair technologies cannot be used reliably.

Under the influence of high temperatures and high tensile stresses during welding, rapid formation and growth of helium bubbles can occur at grain boundaries, resulting in intergranular cracking in the heat-affected zone (HAZ). Over the past decades, a basic understanding has been established for the detrimental effects of weld stresses on the helium induced cracking. However, practical methods for weld repair of irradiated materials are still evolving.

The overall objective of this task was to develop advanced welding technologies that can be used to repair highly irradiated reactor internals without helium induced cracking. Toward this goal, we have developed a computational model that can be used to gain a fundamental understanding of the effect of welding stress and temperature on the formation helium induced cracking during welding. The computational model was then used to design and optimize a novel welding approach to suppress helium induced cracking reported herein.