Dry Cask Storage – The Basics

Michele Sampson
Chief, Spent Fuel Licensing Branch
Division of Spent Fuel Management

Fuel pellets, rods, and casks_r9You may have read our recent Science 101 posts in which we explained the basics of nuclear fuel and what happens when it is taken out of the reactor. We mentioned storing it in a pool, something every reactor in this country does immediately after removing the fuel. Today we want to talk about the option of storing spent nuclear fuel in dry casks.

Pools can only hold so much spent fuel. As they began filling up, utilities started looking for other ways to manage their fuel. A handful of companies developed dry storage systems. The idea is that after the fuel spends some time cooling in the pool, it can be loaded into a cask that is sealed to keep the radioactive material inside and protected.

At its most basic, a dry storage system is a cylinder that is lowered into the pool and filled with spent fuel. When full, the cylinder is raised and dried before it is sealed and placed outdoors. There are many varieties of spent fuel storage casks. All storage casks need to manage the spent fuel’s heat and contain its radioactivity, and to prevent nuclear fission (the chain reaction that allows a reactor to produce heat). The casks must resist earthquakes, tornadoes, floods, temperature extremes and other scenarios.

Casks come in different sizes. They are tall enough to hold spent fuel, which can be 14 feet long, and they can weigh up to 150 tons—as much as 50 midsize cars. In fact, plants may need a special crane that can handle heavy loads to be able to lift a loaded cask full of water out of their pool for drying. After the casks are dried and filled with helium, robotic equipment welds them closed to keep doses to workers as low as possible. Then the canisters are tested to ensure they are sealed.

And once the dry, welded canister is placed inside thick shielding, the plants use a special transporter to move the cask outdoors to where it will be stored. At that point, the radioactivity from the cask must be less than 25 millirem per year at the site boundary. That means the highest dose to someone standing at the fence for a full year would be about what you would get going around the world in an airplane. The actual dose at the site boundary is typically much lower. As of December 2014, just over 2,000 casks have been loaded and are safely storing nearly 84,000 spent fuel assemblies.

Cask designers must show their cask systems meet our regulatory requirements. The NRC staff reviews their applications in detail. We only issue an approval to systems that we know can perform safely.

Most dry storage systems in use today have the spent fuel placed into an inner metal canister that is welded shut, then placed into a large metal or metal-and-concrete cask. The canisters are designed so they can be removed and put into transportation casks for eventual shipment offsite. Some casks store the fuel horizontally, the others vertically.

drystoragegraphic)The NRC inspects the design, manufacturing and use of dry casks. These inspections ensure licensees and vendors are following safety and security requirements and meeting the terms of their licenses and quality assurance programs. NRC inspectors also observe practice runs before utilities begin moving their spent fuel into dry casks.

There are strict security requirements in place to protect the stored fuel. Security has multiple layers, including the ability to detect and respond to an intrusion. There have been no known or suspected attempts to sabotage cask storage facilities.

Since the first casks were loaded in 1986, dry storage has released no radiation that affected the public or contaminated the environment. Tests on spent fuel and cask components after years in dry storage confirm that the systems are providing safe storage.

The NRC also analyzed the risks from loading and storing spent fuel in dry casks. That study found the potential health risks are very, very small. To ensure continued safe dry storage of spent fuel, the NRC is further studying how the fuel and storage systems perform over time. The NRC is also staying on top of related research planned by the Department of Energy and nuclear industry.

We’ll talk about “high burnup spent fuel,” which is receiving a lot of attention at shutdown reactor sites, in an upcoming blog post.

 

When Problems Are a Sign of Success

Chris Allen
Project Manager
Division of Spent Fuel Storage and Transportation
 

Can a problem show that our regulatory system works? If you don’t think so, read on.

Two weeks ago, the NRC published an “information notice” about moisture causing problems for dry spent fuel storage casks. Information notices are one way the NRC communicates formally with licensees. We send these notices when we want all licensees to be aware of a particular problem found with just one or only a handful of licensed facilities or equipment so they can prevent similar problems.

Spent fuel dry casks
Spent fuel dry casks

The problem in this case centers on dry spent fuel storage casks that store used nuclear fuel after it’s been cooled for several years in spent fuel pools. The NRC reviews the designs of these casks to make sure they will safely cool the fuel and contain the radiation it emits.

In this case, two different sites using two different storage designs had unanticipated problems on the outside of the system caused by moisture. The structural integrity of the systems was never compromised and the radiation levels at both sites remained very low.

The first problem dates to 2007 at a facility in Idaho that stores spent fuel debris from the damaged Three Mile Island reactor. The system uses thick concrete for shielding and protection from earthquakes and other natural forces. The operator saw that cracks in the concrete—originally thought to be cosmetic and trivial—were spreading. The licensee’s evaluation found water had entered bolt holes on top of the casks, froze, thawed and cracked the concrete. The evaluation also identified repairs, ways to prevent more water from getting in and a program for monitoring cracking.

The second problem, at the Peach Bottom reactor site in Pennsylvania, was identified on October 11, 2010, when an alarm sounded. That alarm was designed to be an early warning that the helium inside might be leaking. On examination, the licensee found rust beneath a metal weather cover and moisture around the bolts holding the cask lid in place. An outer lid seal was leaking more helium than the NRC license allowed. An inner seal kept the spent fuel and radioactivity confined inside the cask.

From the time these issues were discovered, we made information available through licensee event reports, NRC inspection reports, letters and other communications with licensees. Our licensees and some trade publications that follow NRC activities closely knew of the issues.

The licensees talked with one another as well at industry-wide workshops and conferences. And our inspectors, who also talk with one another, always look for evidence that dry storage casks are in good condition.

So how does this mean the process worked?

Alarms like the one at Peach Bottom and follow-up evaluations like the one in Idaho are examples of the monitoring and periodic examinations that the NRC requires all cask users to perform. These provide warnings long before a problem could develop that might affect public health and safety or the environment. We also require periodic examinations of dry storage casks so any potential issues can be identified early.

The NRC stayed up-to-date as the licensees learned more about the cause of their problems, how to prevent such problems in the future, and how to fix the problems on their existing systems. In this case, the NRC took the extra step of issuing the information notice even though communication between the NRC and licensees as well as among licensees meant that, when the information notice came out last week, it was actually “old news.”

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