Laser welding is under development at EPRI as a potentially more effective and reliable repair option than conventional welding techniques.

As nuclear reactors age, more neutrons are generated and transmutation of elements occurs in the reactor internals and reactor pressure vessel. This increased "neutron flux" gradually increases the helium level in reactor internal components and reduces the material's weldability via helium-induced cracking. The extent of cracking is heavily dependent on the heat input used for welding.
One area susceptible to such cracking is the riser brace attachment in boiling water reactors. Conventional arc welding techniques typically are not able to repair such components because their high heat input (millions of watts) would result in too much heating of the base metal. Conversely, laser welding – which operates at energy levels of only about 800 watts – provides the precise heat input control needed to avoid helium-induced cracking and excessive weld metal dilution. In addition, laser technology is particularly promising for repairing reactor internals because it can be delivered to remote locations via optical fiber.

EPRI has been evaluating laser welding for several years, and for a variety of applications. Initial research focused on weld overlay of cracked components. Utility interest ultimately led to the first known application of laser technology in a nuclear setting: Eskom's Koeberg plant in South Africa used laser welding in 2010 to mitigate through-wall cracking in an outdoor stainless steel storage tank.

In early 2011, EPRI's Welding and Repair Technology Center acquired and installed a 2kW fiber laser system. This compact, high-powered device delivers a laser beam through an optical fiber, providing focused energy control. Ongoing EPRI research activities related to laser welding include:

• Characterization of heat input parameters from the laser
• Integration of laser operation with positioning hardware to ensure accurate application of welds
• Assessment of laser system as a field-deployable welding technique
• Evaluation of key process parameters such as weld residual stress and low heat input buffer layer.

An essential element of the research program will be to demonstrate the ability of laser welding to successfully repair irradiated material samples. While EPRI can do much of the research to assess the process viability of the laser welding approach, full commercial viability can only be obtained through demonstration on irradiated materials in a test reactor. Toward this end, EPRI is collaborating with the U.S. Department of Energy (DOE) to evaluate a hybrid laser welding process for repairing highly irradiated materials under EPRI's Long-Term Operations Program and the DOE's Light Water Reactor Sustainability Program. In addition, EPRI and DOE are constructing a hot cell at the Oak Ridge National Laboratory that will perform welding experiments on irradiated materials used to construct reactor internals and vessels.

For more information about EPRI's laser welding research, contact Eric Willis at 650.855.2023, ewillis@epri.com or Greg Frederick at 704.595.2571, gfrederi@epri.com.