Safety of Spent Fuel Storage — 1

Safety of Spent

 Fuel Storage

Safety of Spent Fuel Storage — 1

What Is Spent Fuel?

Nuclear reactors use uranium fuel rods 

bundled into fuel assemblies to generate the 

heat that turns generators. These generators 

produce electricity that powers people’s homes.

As it burns in the reactor, this fuel becomes 

very hot and very radioactive. After about 

5 years, the fuel is no longer useful and is 

removed. Reactor operators have to manage 

the heat and radioactivity that remains in this 

spent fuel. 

In the United States, every reactor site 

has at least one pool on site for spent fuel 

storage. Plant personnel move the spent 

fuel underwater from the reactor to the pool. 

Over time, spent fuel in the pool cools as the 

radioactivity decays away.

These pools were intended to provide 

temporary storage. The idea was that after 

a few years, the spent fuel would be shipped 

offsite to be reprocessed, or separated so usable portions could be recycled into new fuel. 

But reprocessing did not succeed in the United States, and the pools began to fill up.

In the early 1980s, reactor operators began to look for ways to increase the amount of 

spent fuel they could store onsite. They began to place fuel in dry casks that could be 

stored in specially built facilities on their sites. Most nuclear plants today use dry storage. 

Fuel 

Assembly

Nuclear

Reactor

Fuel 

Rods

Uranium

Fuel Pellets

Fuel

Rod

Coolant

Uranium

Fuel 

Pellet

Fuel

Rod

Fuel 

Assembly

Fuel 

Rods

Spent fuel pool
2 — U.S. Nuclear Regulatory Commission

Dry Cask Storage—The Basics

A dry cask storage system is a cylinder that operators 

lower into the pool and fill with spent fuel. They raise 

the cylinder, drain, and dry it, before sealing and 

placing it outdoors on a concrete pad. There are many 

varieties of spent fuel storage casks. They all need to:

• Maintain confinement of the spent fuel

• Prevent nuclear fission (the chain reaction that 

allows a reactor to produce heat)

• Provide radiation shielding

• Maintain the ability to retrieve the spent fuel, 

if necessary

• 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 up to 

14 feet long, and they can weigh up to 150 tons—as much as 50 midsize cars. Plants may 

need a special crane that can handle heavy loads to be able to lift a loaded cask full of 

water out of the pool for drying. After the casks are dried, robotic equipment is used to 

seal them closed to keep doses to workers as low as possible. 

Two basic designs are in wide use today. Welded, canister-based systems feature an 

inner steel canister that contains the fuel surrounded by 3 feet or more of steel and 

concrete. The canisters may be oriented either vertically or horizontally. In bolted cask 

systems, there is no inner canister. Bolted casks have 

thick steel shells, sometimes with several inches of 

radiation shielding inside.

Plants use special transporters to move the loaded 

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 allowed to someone standing at the 

fence for a full year is about the dose someone would 

receive going around the world in an airplane. The 

actual dose at the site boundary is typically much 

lower. 

Dry cask storage has proven to be a safe technology 

over the 30 years it has been used. Since the first 

casks were loaded in 1986, dry storage has released 

no radiation that affected the public or contaminated 

the environment. As of January 2017, more than 

2,400 casks have been loaded and are safely storing 

100,000 spent fuel assemblies. Tests on spent fuel and 

cask components after years in dry storage confirm 

that the systems continue to provide safe storage.

At least 23 feet of water covers the fuel assemblies 

in the spent fuel pool of Unit 2 at the Brunswick 

Nuclear Power Plant in Southport, NC. 

(Courtesy: Matt Born/Wilmington Star-News)

Loading spent fuel cask under water. 

(Courtesy: Holtec International)
Safety of Spent Fuel Storage — 3

The U.S. Nuclear Regulatory Commission (NRC) 

analyzed the risks from loading and storing spent 

fuel in dry casks. Two separate studies 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 the nuclear industry.

What We Regulate and Why

The NRC oversees the design, manufacturing, and use of dry casks. 

This oversight ensures licensees and designers are following safety 

and security requirements, meeting the terms of their licenses, and 

implementing quality assurance programs.

Cask designers must show that their systems meet the NRC’s 

regulatory requirements. The NRC staff reviews cask applications 

in detail. The agency will only approve a system that meets NRC 

requirements and can perform safely. NRC inspectors visit cask 

designer offices, fabricators and spent fuel storage facilities to 

ensure they are meeting all our regulations. Cask design applications, 

the NRC’s documentation of reviews, and NRC inspection reports are 

available to the public on the agency website at www.nrc.gov.

There are strict security requirements in place to protect the stored fuel. Security has multiple 

layers, including the ability to detect, assess, and respond to an intrusion. Our general 

security requirements for dry cask storage are in 10 CFR Part 73 (https://www.nrc.gov/

reading-rm/doc-collections/cfr/part073/). The specific requirements in NRC orders and the 

licensee’s security plans are not available to the public, as they could give an adversary the 

ability to defeat the security measures and compromise the safety systems. There have been 

no known or suspected attempts to sabotage cask storage facilities.

The NRC’s requirements for dry cask storage can be found in 10 CFR Part 72 (https://www.

nrc.gov/reading-rm/doc-collections/cfr/part072/), which requires all structures, systems, 

and components important to safety to meet quality standards for design, fabrication, and 

testing. Part 72 and related NRC guidance on casks and storage facilities also detail specific 

engineering requirements.

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. The agency 

will only approve a storage cask design if 

these experts are satisfied that all the specific 

safety requirements in each discipline have 

been met.

The NRC’s regulations appear 

in Chapter 10 of the Code 

of Federal Regulations, also 

known as 10 CFR.

Cask transporter moves loaded spent fuel storage cask 

to storage pad.

Workers prepare to load an AREVA-TN 

NUHOMS canister into a concrete storage 

module at the Calvert Cliffs Nuclear Power 

Plant in Lusby, MD. (Courtesy: Exelon)
4 — U.S. Nuclear Regulatory Commission

The following sections discuss technical 

evaluations the NRC conducts during 

technical 

reviews of dry cask storage.

Materials

Materials—the stuff of which 

everything is made. In every case—the 

metal in a car door, the plastic used 

in airplane windows, or the steel used 

in elevator cables—the selection of 

appropriate materials is critical to safety. 

Systems that transport and store spent 

nuclear fuel and other radioactive substances are made of a variety of materials. All of them 

are reviewed to confirm that those systems can protect the public and environment from 

the effects of radiation. The NRC does not dictate what materials are used. Rather, the NRC 

evaluates the choice of materials proposed by applicants. What makes a material “appropriate” 

to transport and store radioactive substances depends on a number of factors.

First, materials must be adequate for the job. In other words, the mechanical and physical 

properties of the materials have to meet certain requirements. For example, the steel chosen 

for a storage cask has to withstand possible impacts such as from tornadoes or earthquakes.

Next, when making a complex metal system, parts often are welded together—that is, partially 

melted—in a way that ensures that the joints themselves are adequate. The welder actually 

creates a new material at the joint with its own unique properties. That is why the NRC looks at 

how this is done, including the selection of weld filler metals, how heat is controlled to ensure 

good welds, and the use of examinations and testing to verify that no defects are present.

Finally, the NRC considers how materials degrade over time. Reviewers must take into 

account a material’s chemical properties, how it was manufactured, and how it reacts with its 

environment. Just as iron rusts and elastic materials become brittle over time, all materials 

can degrade. This degradation and its impact must be well understood. Materials must be 

selected based on their present condition and their projected condition throughout their 

lifetimes.

Best practices for appropriately selecting 

materials and the processes used to join 

them often can be found in consensus codes 

and standards. These guidelines are typically 

developed over many years of operational 

experience, and through industrywide and 

government technical discussions and 

agreement. The NRC also relies on both 

historical operating experience and the latest 

materials performance and testing data.

NUHOMS horizontal spent fuel storage system under construction 

at the Calvert Cliffs Nuclear Power Plant in Lusby, MD.

Loaded vertical HI-STORM 100 casks are storing spent fuel 

at the Diablo Canyon Power Plant in Avila Beach, CA.
Safety of Spent Fuel Storage — 5

Managing Heat

Keeping the spent fuel from getting too hot is 

one way to ensure casks will be safe. The NRC 

requires the cask and fuel to remain within a 

certain temperature range. These requirements 

protect the cladding (the metal tube that holds 

the fuel pellets). As the fuel cools, heat is 

transferred from inside the cask to the outside. 

NRC experts examine how that heat will move 

through the cask and into the environment.

The method used to remove heat has to be 

reliable and provable. It must also be passive—

that is, without the need for electrical power or 

mechanical device. Casks use conduction, convection, and radiation to transfer the heat to 

the outside.

Conduction transfers heat from a burner through a pot to the handle. The process of heat 

rising (and cold falling) is known as convection. The heat coming from a hot stove is known as 

radiant heat. 

These methods work the same way in a storage cask. Where the structure containing the fuel 

touches the fuel assemblies, it conducts heat toward the outside of the cask. Most casks have 

vents that allow outside air to flow naturally into the cask and around the canister to cool it 

(convection). And most casks would feel warm to the touch from radiant heat, much like a 

home radiator.

The NRC also confirms that the pressure inside a cask is below the design limit so it will not 

impact the structure or operations. Technical experts review applications for cask designs 

carefully to verify that the fuel cladding and cask component temperatures and the internal 

pressure will remain below specified limits.

Each storage cask is designed to withstand the effects from a certain amount of heat. This 

amount is called the heat load. The NRC reviews whether the designer correctly considered 

how the heat load will affect cask component and fuel temperatures, and how this heat 

load was calculated. Cask designs must show that heat from spent fuel can be effectively 

transferred to the outside of the cask.

The NRC’s review also verifies that the cask designer looked at all the environmental 

conditions that can be expected to affect cask components and fuel temperatures. These 

conditions may include windspeed and direction, temperature extremes, and a site’s elevation. 

To make sure the right values are considered, the NRC verifies that they match the historical 

records for a site or region.

NRC reviewers consider all of the methods used to prove that the storage system can 

handle the specified heat loads. They verify computer codes, making sure they are the latest 

versions and have been endorsed by experts. They look at the values used in the codes, such 

as for material properties, and confirm calculations for temperature and pressure. The NRC 

might run its own analysis using a different computer code to see if those results match 

the application.

Radiant

Conduction

Convection

Three different methods transfer heat.
6 — U.S. Nuclear Regulatory Commission

Making Sure Casks 

Will Hold Up 

In its application, the cask 

designer must provide an 

evaluation that shows the system 

will be strong and stable enough 

to perform its safety functions 

even after experiencing a load, 

such as if the cask were dropped. 

NRC reviewers examine the 

structural design and analysis 

of the system under all credible 

loads for normal conditions—

that is, planned operations and 

environmental conditions that 

can be expected to occur often 

during storage. They also look 

at accidents, natural events, and 

conditions that can be expected to 

occur from time to time, but not 

regularly. 

The NRC review looks at whether 

the cask designer evaluated 

the proper loading conditions. 

It will also ensure the designer 

evaluated the system’s response 

to those loads accurately and 

completely. Reviewers must 

verify whether the resulting 

stresses in the material meet 

the acceptance criteria in the 

appropriate code. The NRC’s 

review also looks at several 

different realistic combinations of 

loads. These cases are analyzed 

to determine the stresses 

placed on the material used to 

construct the cask system. To be 

conservative, the NRC and the 

designers overestimate loads and 

underestimate material strength. 

Doing this enhances the NRC’s 

assurance that the 

design is adequate.

Cutaway of spent fuel storage cask shows spent fuel assemblies 

surrounded by steel and thick concrete shielding.
Safety of Spent Fuel Storage — 7

Confinement

The cask design must prevent the release of 

radioactive material. This role is performed 

by the confinement boundary, which usually 

includes a metal canister with a lid that has 

at least two closures. Some casks have two 

separate lids that are each welded closed. 

Others are bolted and have two separate seals. 

Having both closures provides an extra layer of 

protection to ensure the radioactive materials 

remain confined.

The design must also keep the fuel assemblies in a protected, or “inert,” environment. This is 

important to keep the fuel cladding from degrading. Once the water is removed from inside the 

cask, it is filled with a gas such as helium that will not react with fuel cladding.

Cask users must monitor the confinement boundary. The monitoring requirements depend on 

whether a cask is bolted or welded. Bolted confinement boundaries with O-ring seals need to 

have alarms to alert the user if a seal starts to leak. In that case, the seal would need to be 

repaired or replaced to ensure the cask continues to have redundant confinement. Our experts 

review the proposed monitoring programs to make sure they are adequate. Welded closures 

do not need to be monitored in the same way. This is because the welds are examined closely 

after they are made to ensure they do not leak. 

The NRC’s review of a cask’s confinement boundary looks at the “source term.” This is the 

inventory of radioactive material inside the cask. While the redundant closures and other 

requirements ensure the material will remain safely confined, the NRC requires cask 

designers to look at the dose rates in case some material were to come out. They also need to 

analyze how those dose rates compare to the NRC’s regulatory limits. 

Finally, cask designers must 

provide an analysis of how the 

confinement boundary works. 

Casks must be designed and 

tested to meet criteria approved 

by the American National 

Standards Institute, or ANSI. The 

ANSI standard for leak tests on 

radioactive materials packages 

was put together by a committee 

of experts and went through 

a lengthy review and approval 

process before it was adopted. 

Loaded spent fuel storage casks are in place on 

storage pad at the Haddam Neck Plant in Meriden, CT. 

(Courtesy: Connecticut Yankee)

Loaded spent fuel storage cask on transporter is moved from the fuel 

handling building at the Surry Power Station in Surry, VA.
8 — U.S. Nuclear Regulatory Commission

Criticality Safety

The nuclear chain reaction used 

to create heat in a reactor is 

known as fission. In this process, 

uranium atoms in the fuel break 

apart, or disintegrate, into smaller 

atoms. These atoms cause other 

atoms to split, and so on. Another 

word for this process is criticality.

The potential for criticality is an 

important thing to consider about 

reactor fuel throughout its life. 

Fuel is most likely to go critical 

when it is fresh. The longer the 

fuel is in the reactor, the less 

likely it is to go critical. This is 

why it is removed from the reactor 

after several years—it loses 

energy and will no longer easily 

support a self-sustaining chain reaction. Once fuel is removed from the reactor, the NRC 

requires licensees to ensure it will never again be critical. This state is referred to as 

“subcriticality.” 

Subcriticality is required whether the fuel is stored in a pool or a dry cask. It is required 

for both normal operating conditions and any accident that could occur at any time.

Many methods help to control criticality. The way spent fuel assemblies are positioned 

is an important one. How close they are to each other and the burnup of (or amount of 

energy extracted from) nearby assemblies all have an impact. This method of control is 

referred to as fuel geometry.

Certain chemicals, such as boron, can also slow down a chain reaction by absorbing 

neutrons released during fission, and keeping them from striking other uranium atoms. 

Casks have strong baskets to maintain fuel geometry. They also have solid neutron 

absorbers, typically made of aluminum and boron, between fuel assemblies. A cask 

application must include an analysis of all the elements that contribute to criticality 

safety during both normal and accident conditions.

NRC technical experts review this analysis to verify several things: 

• The factors that could affect criticality have been identified.

• The models address each of these factors in a realistic way.

• Any assumptions used in the models are conservative—they result in more 

challenging conditions than would actually be expected.

Neutrons cause uranium-235 atoms to split in a nuclear chain reaction.
Safety of Spent Fuel Storage — 9

Radiation Shielding

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. Shielding is necessary to 

block this radiation and protect workers and 

the public.

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.

But 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 for the two main types of dry storage casks is configured in slightly different ways. 

For welded, canister-based systems, the thick steel-reinforced concrete vault that surrounds 

an inner canister provides shielding for both neutron and gamma radiation. Shielding in bolted 

cask systems comes from their thick steel shells that may have several inches of lead gamma 

shielding inside. These systems have a neutron shield on the outside consisting of low-density 

plastic material, typically mixed with boron to absorb neutrons.

The NRC’s reviews ensure that dry cask designs meet regulatory limits on radiation doses at 

the site boundary, under both normal and accident conditions, and that dose rates in general 

are kept as low as possible. 

Every applicant must provide 

a radiation shielding analysis. 

This analysis uses a computer 

model to simulate how radiation 

penetrates through the fuel and 

into thick shielding materials 

under normal operating 

and accident conditions. 

Reviewers ensure the 

analysis has identified all the 

important radiation-shielding 

parameters and models them 

conservatively, in a way that 

maximizes radiation sources 

and external dose rates. 

Different types of radiation have different properties.

At right, a dry storage cask recently loaded with spent fuel is lifted from 

a horizontal transporter to be placed on a specially designed storage 

pad. (Courtesy: Sandia National Laboratories)
10 — U.S. Nuclear Regulatory Commission

Inspections

As part of its oversight function, the NRC 

inspects the companies that design and fabricate 

dry storage casks and the facilities that use 

them. Inspectors from NRC headquarters 

and the four regional offices conduct these 

inspections and issue their findings in publicly 

available reports.

Cask designers are responsible for ensuring that 

the fabricated cask components comply with the 

design as approved by the NRC. To do this, they 

are required to have a quality assurance program 

that meets the 18 criteria described in NRC 

dry storage regulations. The NRC reviews and 

approves these programs.

The designers must make sure their quality assurance programs are properly 

implemented during both design and fabrication. The NRC conducts periodic safety 

inspections to independently assess and verify that the designers are doing so. Some 

inspections look at design activities carried out at corporate offices. At fabrication 

facilities, both in the United States and overseas, NRC inspectors look at controls for 

fabrication, the process for verifying that the fabricated components comply with the 

approved design, and how the designer ensures that the fabricator meets its quality 

assurance program. 

Each licensee is responsible for ensuring that its storage facility meets NRC 

regulations during construction and operation. NRC inspectors verify that the licensees 

are properly implementing the regulations. These inspections cover the design and 

construction of the concrete pad or modules that support the storage casks, 

preoperational testing (also referred to as dry runs), cask loading, and routine monitoring 

of operating dry storage facilities.

Inspectors examine dry storage casks containing 

spent nuclear fuel.

Transportable spent fuel storage casks sit on a storage pad. 

(Courtesy: Holtec International)
Safety of Spent Fuel Storage — 11

Managing Aging

Cutting-edge robotic technology 

is making it easier to inspect 

inside spent fuel dry cask storage 

systems. 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.

The 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.

Robots are being developed to apply these NDE techniques inside casks. These robots need to 

fit into small spaces and withstand the heat and radiation inside the cask. The state-of-the-art 

robot technology is evolving quickly.

The Electric Power Research Institute and cask manufacturers have successfully 

demonstrated robotic inspection techniques to NRC staff several times at different reactor 

sites. These demonstrations are helping to refine the robots’ designs.

In one demonstration, a robot inside a spent fuel storage cask maneuvered a camera with 

a fiber optic probe, which meets the industry code for visual examinations. The robot 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 were analyzed for atmospheric 

deposits that could cause corrosion.

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.

Prototype robotic delivery system. 

(Courtesy: EPRI/RTT)

Cutaway mockup of 

NAC International 

MAGNASTOR cask 

system at Palo 

Verde Nuclear 

Generating Station 

in Wintersburg, AZ. 

(Courtesy: EPRI/APS)
12 — U.S. Nuclear Regulatory Commission

For more information on spent fuel and 

dry cask storage, visit the NRC’s website:

https://www.nrc.gov/waste/spent-fuel-storage.html

Cover Photos:

Top: Massive storage casks loaded with spent nuclear 

fuel sit on a concrete pad inside a secure storage facility.

Middle: A transportable spent fuel storage system is 

moved to a storage pad at the Peach Bottom Atomic 

Power Station in Delta, PA. (Courtesy: AREVA)

Bottom: A horizontal spent fuel storage system sits 

behind a secure fence at the Calvert Cliffs Nuclear 

Power Plant in Lusby, MD.

U.S. Nuclear Regulatory Commission

NUREG/BR-0528

April 2017

For Additional Information Contact:

Office of Public Affairs

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Phone: (301) 415-8200

Email: OPA.resource@NRC.GOV

Internet Home Page: http://www.nrc.gov