Spent Fuel Casks 101 — What We Regulate and Why

Mark Lombard
Director, Division of Spent Fuel Management

CASK_101finalWe talked back in March about dry casks for storing spent nuclear fuel and how they work. Today we want to introduce you to the different things the NRC looks at each time we review a cask application.

To recap: spent fuel is placed into cooling pools at reactor sites when it can no longer efficiently sustain a nuclear reaction. Dry casks give utilities an alternate way to store their spent fuel, freeing up space in the pools. They were first developed back in the 1980s because space in the pools – designed for temporary storage – was growing short.

Our requirements for dry cask storage can be found in 10 CFR Part 72. All structures, systems and components important to safety must meet quality standards for design, fabrication and testing. And they must be structurally able to withstand wind, rain, snow and ice, temperature extremes, hurricanes and tornadoes, earthquakes, and fires and explosions.

Fuel pellets, rods, and casks_r9Part 72 and related NRC guidance on casks and storage facilities also detail specific engineering requirements. Casks must be designed to keep water out so the fuel can’t have a chain reaction, as it would in a reactor. The casks must also shield workers and the public from radiation. They must safely remove the heat remaining in the spent fuel. And the materials used in dry casks and their physical properties must be well-understood and analyzed.

The NRC has dozens of experts in different scientific and engineering disciplines whose job is to review cask applications (which can be hundreds of pages long) and the detailed technical designs they contain. We will explain in more detail in later blog posts what our experts look for and how they go about approving a cask design.

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.