Light Water Reactors (LWRs) are the most widely used type of nuclear reactor in the world. However, as alternative designs continue to emerge and gain traction, they may represent the future of clean power generation.
One of those designs, the Molten Salt Reactor (MSR), uses a liquid mixture of salts as both a coolant and fuel carrier. MSRs were first proposed in the 1950s and 1960s and, despite promising results, were eventually shelved due to multiple design and operational challenges. In recent years, however, their potential benefits have generated renewed interest. Compared with present-day nuclear reactors, the MSR is projected to be cheaper, safer and produce much less nuclear waste. With that in mind, major nuclear innovation companies and research institutions are now actively working on advancing the technology.
Yong Yang, Ph.D., associate professor of nuclear engineering, has received a grant from the U.S. Nuclear Regulatory Commission to help optimize molten salt reactor design. Utilizing advanced manufacturing techniques, his research team aims to fill the knowledge gap in evaluating the materials used in MSRs and how they are fabricated.
“In the United States, our current nuclear power plant standards were created specifically for LWRs, which differ from MSRs in significant ways,” Dr. Yang said. “Materials selection is a crucial consideration when designing and manufacturing MSRs, and all materials issues, including joining and welding, must be thoroughly evaluated before licensing this innovative reactor design.”
The commercial production of MSRs presents several technological challenges, including developing materials that can withstand the reactor’s extreme temperatures and corrosive environment. While new and innovative alloys can aid in that aspect, integrating the wide variety of components in a nuclear reactor often requires joining those dissimilar metals. For example, in the heat exchanger of an MSR, which transfers heat from molten salt to a steam generator, different alloys are necessary to adapt to the diverse environments, often necessitating the use of dissimilar metal joints.
Engineers can use conventional fusion welding processes like diffusion bonding, brazing, friction stir welding, laser welding and electron beam welding for these joining. Nevertheless, these techniques may lead to considerable residual stress or abrupt changes in the composition and microstructure in the joint, making it vulnerable to poor creep performance and premature failures.
Dr. Yang pointed out that additive manufacturing (AM) has a distinguished advantage in fabricating parts with graded compositions due to its layer-by-layer process. Rather than directly joining two dissimilar metals, an AM joint can facilitate a smooth transition between materials and reduce the likelihood of flaws by minimizing the carbon diffusion and depletion of alloying elements from one base metal into the other. Utilizing new manufacturing techniques such as AM, which can reduce production time and cost while improving reactor components’ quality and safety, deploying more complex reactor designs like MSRs can be expedited.
“Previous studies show that the AM process for a given pair of alloys needs to be specifically tailored, and each gradient system requires independent development to address alloy-specific concerns,” Dr. Yang said. “We aim to show how compositionally graded transition joints between two dissimilar alloys fabricated using additive manufacturing offer more efficient and cost-effective combined alloys with improved creep and creep-fatigue resistance.”