Dry Cask Storage of Spent Nuclear Fuel 

Nuclear plants were originally designed to provide temporary onsite storage of used nuclear 

fuel. Known as “spent fuel,” these bundles of fuel rods must be replaced from time-to-time because 

they lose efficiency. About one-third of the nuclear fuel in a reactor is removed and replaced with 

fresh fuel at each refueling. The spent fuel, which generates considerable heat and radiation, is 

placed into deep pools of water at the reactor site, where it can be stored safely. 

Reactor designers expected spent fuel to be 

stored in pools for a few years before it was shipped 

offsite to be “reprocessed.” A reprocessing plant 

would separate portions that could be recycled into 

new fuel; unusable portions would be disposed as 

waste. But commercial reprocessing never succeeded 

in the United States, so pools began to fill up. 

Pools Reach Capacity

As spent fuel accumulated in the pools in the early 1980s, utilities began to look at options 

for increasing the amount they could store. The pools are robust structures that cannot be enlarged. 

So utilities needed to fit more fuel into their pools. Current regulations permit two options for 

expanding pool capacity: re-racking (replacing storage racks to decrease the distance between 

assemblies) and fuel rod consolidation (removing fuel rods from the assemblies to pack them more 

densely for storage). The NRC must review and approve any such changes. But pool capacity can 

only be expanded so far. 

Enter Dry Storage 

Utilities began turning to dry storage to manage their spent fuel onsite. After a few years in 

the pool, the fuel has cooled and its radioactivity decreased enough to allow it to be removed. 

Moving spent fuel into dry casks frees up space in the pool to store spent fuel newly removed from 

the reactor.

Dry casks typically have a sealed metal cylinder to contain the spent fuel enclosed within a 

metal or concrete outer shell to provide radiation shielding. In some designs, casks are set vertically 

on a concrete pad or below grade within a concrete pad; in others, they are placed horizontally. 

Dry cask storage is safe for people and the environment. Cask systems are designed to 

contain radiation, manage heat and prevent nuclear fission. They must resist earthquakes, projectiles, 

tornadoes, floods, temperature extremes and other scenarios. The heat generated by a loaded spent 

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fuel cask is typically less than is given off by a home-heating system. The heat and radioactivity 

decrease over time without the need for fans or pumps. The casks are under constant monitoring and 

surveillance.

Licensing Dry Storage

The NRC developed cask licensing requirements through a public process to provide a sound 

basis for ensuring protection of public health and safety and the environment. NRC staff conducts 

thorough reviews and only approves designs that meet those requirements. Utilities can choose from 

two licensing options, both of which allow for public input: 

• A site-specific license allows a specific cask 

design to be used at a specific location and offers the 

opportunity for a hearing before the NRC grants the 

license.

• A general license allows a reactor site to use any 

cask certified by the NRC, as long as the site meets the 

conditions specified in the certificate. The public can 

comment on cask designs before the NRC certifies them.

The NRC periodically 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. 

Since the first casks were loaded in 1986, dry 

storage has released no radiation that affected the 

public or contaminated the environment. There have 

been no known or suspected attempts to sabotage 

cask storage facilities. Tests on spent fuel and cask 

components after years in dry storage confirm that the 

systems are providing safe and secure 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 small. 

Additional information on the storage of spent 

nuclear fuel is available on the NRC’s website. 

January 2023

Horizontal storage system

Vertical casks