Welding System Could Help Extend Lives of Nuclear Power Plants

A system for welding highly irradiated metal alloys has been developed by researchers at Oak Ridge National Laboratory (ORNL) in partnership with the Electric Power Research Institute (EPRI). The system, designed and installed in a hot cell at ORNL’s Radiochemical Engineering Development Center, safely encloses equipment for laser and friction-stir welding.

ORNL and EPRI built an enclosed welding system in a hot cell of ORNL’s Radiochemical Engineering Development Center. C. Scott White (ORNL) performs operations with remotely controlled manipulators and cameras.The system combines capabilities for laser welding and frictional stir welding of irradiated stainless steels. Courtesy of DOE LWRS and Keith Leonard.

The ORNL/EPRI system uses advanced techniques that introduce less stress than conventional welding, thereby reducing cracking. The system will help advance welding technologies for repair of irradiated materials by helping researchers develop processing conditions and evaluate post-weld materials properties.

According to researchers, advanced welding techniques will be needed as the country’s nuclear power plants, which generate approximately 20 percent of the nation’s electricity, continue to age. Strategies for keeping these plants in good working order focus on cost-effective repairs or replacements, which could require welding. Traditional welding techniques cannot be used for reliable repair when helium generation in components can exceed 5 to 10 atomic parts per million, as is the case with nuclear reactors.

Heat and stress drive helium to coalesce in stainless steel, forming bubbles along boundaries between “grains,” or micron-scale regions of order, that weaken the material. When metal is melted and resolidifies, differences in expansion and contraction between the newly solidified material and surrounding material can build tensile stresses along the weakened, helium-bubble-containing grain boundaries, inducing cracks.

“That’s the biggest problem we face with welding irradiated materials,” researcher Keith Leonard said.

In November 2017, the first tests on irradiated material were performed at ORNL with a laser welding technique that uses a primary laser to weld and secondary beams to reduce tensile stresses near the weld zone. The tests were conducted on samples, known as coupons, of irradiated stainless steel doped with 5, 10 and 20 parts of boron per million atoms. The materials were fabricated into playing-card-sized coupons and then irradiated at the High Flux Isotope Reactor (HFIR), a DOE Office of Science User Facility at ORNL. HFIR's abundant, energetic neutrons bombarded the coupons to change the boron into helium, simulating the aging that would occur in a commercial reactor after decades of radiation exposure.

Later in the same month ORNL researchers performed the first friction stir welding of irradiated stainless steel.

Unlike conventional arc welding, which employs molten materials, friction stir welding is a solid-state mixing technique that uses a rotating tool to generate friction and heat that softens materials but does not melt them. Because friction stir welding occurs below the melting point, it avoids cracking in repair welding of irradiated and helium-bearing materials. An artificial neural network monitors friction stir welding to detect conditions that could cause weld defects.

The welding system, which includes both laser and friction-stir welding equipment, is completely sealed in stainless steel to prevent the escape of radioactive contamination and other particles generated during welding of irradiated materials. It resides in a hot cell that is 20 feet wide, 33 feet long and 24 feet high and features a double-paned window filled with several feet of water between panes to attenuate radiation from the irradiated work pieces. Courtesy of DOE LWRS and EPRI LTO.

“Both repair welding technologies developed in our program are engineered to ‘proactively’ manage the stresses during welding so they potentially offer solutions for repairing (internal) reactor components with high helium levels — impossible with today’s welding repair technology,” researcher Zhili Feng said. “As reactors continue to age (and helium continues to be generated), industry increasingly needs technologies to handle high-helium-level scenarios.”

Preliminary observations showed both techniques produced welds of good quality.

Next, the researchers will explore welding materials with higher helium content and characterize the irradiated materials after they've been welded, with techniques including microstructural analysis and mechanical property assessments. They will also re-age material in HFIR that has undergone a weld repair to see how further aging affects welds.

Researchers have submitted a patent application. Once a joint patent is issued to ORNL and EPRI, companies could license the technology to make onsite repairs and replacements.

Additional information is available at the ORNL web site.

Laser welding in operation, as shown by different cameras mounted inside the new welding hot cell jointly developed by ORNL and EPRI. This experiment laser-welded an alloy called stainless steel 308 to clad a radiation-damaged 304 stainless steel surface and thereby simulate its repair. The 304 material was produced at ORNL, then irradiated at HFIR. Courtesy of DOE’s Light Water Reactor Sustainability Program and EPRI’s Long Term Operations Program.

In this example of friction stir welding, a piece of irradiated stainless steel is processed. This joining technology, which introduces lower temperature and less stress than conventional welding methods, could be used to repair irradiated materials. Courtesy of DOE’s Light Water Reactor Sustainability Program and EPRI’s Long Term Operations Program.