Dry Casks 101: What Do Robots Have to do With Dry Cask Storage?

Darrell Dunn
Materials Engineer

CASK_101finalCutting-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.

img2 (002)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.

Cutaway Cask Mockup with Robot (002)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.

Dry Cask 101 – Radiation Shielding

Drew Barto
Senior Nuclear Engineer

CASK_101finalWe’ve talked before about how the uranium in nuclear fuel undergoes fission during reactor operations. The fission process turns uranium into a number of other elements, many of which are radioactive. These elements continue to produce large amounts of radiation even when the fuel is no longer supporting a chain reaction in the reactor. So shielding is necessary to block this radiation, and protect workers and the public.

As we discussed in an earlier blog post, the four major types of radiation differ in mass, energy, and how deeply they penetrate people and objects. Alpha radiation—particles consisting of two protons and two neutrons—are the heaviest type. Beta particles—free electrons—have a small mass and a negative charge. Neither alpha nor beta particles will move outside the fuel itself.

drycaskshieldingBut spent fuel also emits neutron radiation (particles from the nucleus that have no charge) and gamma radiation (a type of electromagnetic ray that carries a lot of energy). Both neutron and gamma radiation are highly penetrating and require shielding.

Shielding is a key function that dry storage casks perform, but the two main types of dry storage casks are configured in slightly different ways.

For welded, canister-based systems, shielding is provided by a thick (three feet or more) steel-reinforced concrete vault that surrounds an inner steel canister. The thick concrete shields both neutron and gamma radiation, and may be oriented either as an upright cylinder or a horizontal building.

In bolted cask systems, there is no inner canister. Bolted casks have thick steel shells, sometimes with several inches of lead gamma shielding inside. They have a neutron shield on the outside consisting of low-density plastic material, typically mixed with boron to absorb neutrons.

drycask101_radiationshielding_CompimagesThe NRC reviews spent fuel dry cask storage designs to ensure  they meet our limits on radiation doses beyond the site boundary, under normal and accident conditions, and that dose rates in general are kept as low as possible. Every applicant must provide a radiation shielding analysis as part of the application for a new storage system, or an amendment to an existing system. This analysis uses a computer model to simulate radiation penetration through the fuel and thick shielding materials under normal operating and accident conditions.

We review the applicant’s analysis to ensure it has identified all the important radiation-shielding parameters. We make sure they’re modeled conservatively, in a way that maximizes radiation sources and external dose rates. We may create our own computer simulation to confirm the dose rates provided in the application. That helps us to ensure the design meets off-site radiation dose rate requirements under all conditions.