Darrell Dunn
Materials Engineer
Cutting-edge robot technology is making it easier to inspect inside spent fuel dry cask storage systems.
You may remember from past blog posts that most spent fuel dry cask storage systems, or casks, consist of stainless steel canisters that are welded shut to safely contain the radioactive contents. The canisters are in turn placed inside thick storage overpacks to shield plant workers and the public from radiation. As these casks remain in use for longer time frames, the ability to inspect canister surfaces and welds will become an important aspect of the NRC’s confidence in their safety.
To be clear: techniques for inspecting canister surfaces and welds have been used for decades. These techniques are collectively known as nondestructive examination (NDE) and include a variety of methods, such as visual, ultrasonic, eddy current and guided wave examinations.
Where do robots come in? They are a delivery system. Robots are being developed to apply these NDE techniques inside casks. Not just any robot will do. These robots need to fit into small spaces and withstand the heat and radiation inside the cask. The state-of-the-art is evolving quickly.
To date, the Electric Power Research Institute and cask manufacturers have successfully demonstrated robotic inspection techniques to NRC staff three times: at the Palo Verde plant in Arizona (Sept. 2-3, 2015), at the McGuire plant in North Carolina (May 16-19, 2016), and just last month, at Maine Yankee (July 12-13, 2016).
At Palo Verde, the robot was used to deliver eddy current testing instrumentation inside a cask. Eddy current testing detects variations in electromagnetically induced currents in metals. Because it is sensitive to surface defects, eddy current testing is a preferred method for detecting cracks. The inspection robot was used to examine part of the mockup canister fabrication weld. An EPRI report provides a detailed description of the Palo Verde test. Future reports are expected on the McGuire and Maine Yankee demonstrations. These demonstrations are helping to refine the robots’ designs.
The Maine Yankee demo was conducted in July 2016 on a cask loaded in 2002. The demo involved a robot maneuvering a camera with a fiber optic probe, which meets the industry code for visual examinations, inside the cask. The probe was able to access the entire height of the canister, allowing the camera to capture images of the fabrication and closure welds. The welds showed no signs of degradation. The canister was intact and in good condition.
The robot was also able to obtain samples from surfaces of the cask and canister. These samples are being analyzed for atmospheric deposits that could cause corrosion.
Ultimately, if degradation is identified, cask users would select their preferred mitigation and repair option. They would have to meet the NRC’s safety requirements before implementing it.
Cask inspections are important to ensure continued safe storage of spent nuclear fuel and robots will continue to be a helpful tool in this important activity.
U.S. NRC Blog 
How old is the life of these containers?
What kind of corrosion can be handled?
Thanks, Mr. Darrel Dunn, few of my questions were answered here! Thanks for sharing the article and answering the questions of this gentleman!
How will the bottom of the canisters be inspected?
We do not expect a need to inspect the bottom of the canisters unless there is an indication of an issue. Any such indication would be obvious in the visual inspection. Most canisters have the weld at the corner of the bottom plate, where it would be visible to inspection, rather than on the bottom face.
What are the limitations of the inspection technology?
The focus today is on identifying current limitations and developing specialized tools to address them, as in the case of robot development. The nuclear industry has a long history of developing technological advancements when issues are identified that require them. That work continues. Additional development to enhance inspection capability is ongoing and will likely continue for as long as we have dry storage.
What are the specific current NRC approved mitigation options for loaded cracked canisters?
As we state in the blog post, the cask user would need to propose a repair or mitigation option that meets NRC safety requirements. Rather than trying to anticipate hypotheticals, we will review the specifics of a degraded condition and the proposed mitigation measure to see whether it would be effective and meet our requirements.
How many canisters will be inspected at each facility and how frequently?
For now, the number of canisters is dictated by each cask design’s aging management program, which the NRC must review and approve. The American Society of Mechanical Engineers is developing a “code case”—a consensus view among technical experts—that could establish a standardized number and frequency. The example aging management plan in our recently-released standard review plan for cask renewals (NUREG-1927 rev. 1) uses a frequency of every five years. That number was based on crack growth rates in very aggressive environments where the highest estimated crack growth rates could result in through-wall penetration in a period of 16 years after corrosion begins, assuming that the conditions for cracking were continuously maintained. We have since determined that failures in aggressive environments would take approximately 30 years because the conditions needed for cracking cannot be continuously maintained even in aggressive environments. As more data become available, we may need to make site-specific adjustments to frequency.
Darrell Dunn
Donna, What is your need to know here? I would have expected you to contact the NRC office for that level of information. I suspect your affiliated with groups that oppose anything connected with Nuclear Waste.
How will the bottom of the canisters be inspected? What are the limitations of the inspection technology? What are the specific current NRC approved mitigation options for loaded cracked canisters?
How many canisters will be inspected at each facility and how frequently?