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.

A Bit of NRC Myth Busting — Part I

Eric Stahl
Acting Public Affairs Officer

Facebook1We’ve taken a few of the interesting comments we’ve received on our Facebook page and posed them to our experts for their take on the question, suggestion or assertion. Here are their responses.

One Facebook user suggested that nuclear waste could be “encased in thick high strength concrete, then dropped into a churning volcano. It would sink into the magma and over a time it would disperse.” (We took the liberty of cleaning up the typos.)

Spent fuel must be handled and stored with care due to its radioactivity. The only way radioactive waste becomes harmless is through decay, which can take hundreds of thousands of years. As a result, the waste must be stored and disposed of in a way that provides protection to the public for a very long time.

PrintDropping spent fuel into an active volcano would run counter to this idea. Radioactive material could be released into the atmosphere, causing a hazard to people and the environment.

Another Facebook user, on our post about renewing licenses for nuclear power plants beyond the original license renewal, wrote this: “Beyond 60 years They are about to blow now you idiots.”

Contrary to what Hollywood often presents in television and movies, U.S. nuclear reactors are designed with numerous safety features, including containment buildings that continue to protect people and the environment. The nuclear fuel can’t explode, and many reinforcing safety systems would prevent or control the buildup of flammable gases during an accident.

NRC inspectors spend more than 6,000 hours (on average) performing inspection-related activities at each reactor site. In addition, the NRC has a robust aging management program to ensure that the country’s oldest reactors continue to operate safely. Keep in mind that regardless of the age of any reactor, the NRC has authority to address safety issues at any time.

Another Facebook commenter had concerns about the current dry cask storage system. He writes: “All that nuclear waste is being stored in the ground in what is supposed to be 5000 year containers, what if an earthquake hit the storage facility?”

All nuclear waste storage containers, known as “casks,” that are used to store spent fuel in the United States undergo a thorough safety review by the NRC before they’re certified for use. All casks licensed by the NRC must demonstrate their ability to withstand earthquakes and other natural hazards. Once the casks are put into use, they’re continuously monitored for leaks and periodically inspected by the NRC.

Come back tomorrow for Part II!

 

WCS Sends NRC Interim Storage Application

Mark Lombard
Director, Division of Spent Fuel Management

You may have heard that the NRC has received an application today for a centralized storage facility for spent nuclear fuel. We thought this would be a good time to talk about what that facility would do, and how we will review the application.

First some background. “Spent fuel” is the term we use for nuclear fuel that has been burned in a reactor. When spent fuel is removed from a reactor, it is very hot, so it is put immediately into an onsite pool of water for cooling. Initially, the plan in the ‘70s had been to send the spent fuel for “reprocessing” prior to final disposal, so usable elements could be removed and made into fresh fuel. But reprocessing fell out of favor in the United States in the ‘80s.

Officials from Waste Control Specialists deliver its application to construct and operate a consolidated interim storage facility to Joel Munday, Acting Deputy Director of the NRC’s Office of Nuclear Material Safety and Safeguards.
Officials from Waste Control Specialists deliver its application to construct and operate a consolidated interim storage facility to Joel Munday, Acting Deputy Director of the NRC’s Office of Nuclear Material Safety and Safeguards.

To manage their growing inventory, nuclear utilities turned to dry storage. The idea behind dry storage casks is to cool the fuel passively, without the need for water, pumps or fans. The first U.S. dry storage system was loaded in 1986. In the past 30 years, dry storage has proven to be safe and effective.

Against this backdrop, a Texas company, Waste Control Specialists (WCS), filed an application with us today for a dry cask storage facility to be located in Andrews County. WCS plans to store spent fuel from commercial reactors; initially, from reactors that have permanently shut down.

The application discusses utilizing dry storage casks that have previously been approved by the NRC. The spent fuel would arrive already sealed in canisters, so the handling would be limited to moving the canisters from transportation to storage casks.

Ever since Congress enacted the first law for managing spent nuclear fuel in 1982, federal policy has included some centralized site to store spent fuel before final disposal in a repository. Congress made DOE responsible for taking spent fuel from commercial reactors. It gave NRC the responsibility to review the technical aspects of storage facility designs to ensure they protect public health and safety and the environment.

We conduct two parallel reviews – one of the safety and security aspects, the other of potential environment impacts.

But before those reviews get underway, we will review the application to see if it contains enough information that is of high enough quality to allow us to do the detailed reviews. If it doesn’t, WCS will have a chance to supplement it. If we find the application is sufficient and accept it, we will publish a notice in the Federal Register. This notice will alert the public that we have accepted the application for technical review, and offer an opportunity to ask for a hearing.

Then we begin our reviews. At the beginning of our safety and security review, NRC staff will hold a public meeting near the site to answer questions about our process. We’ll also have public meetings with WCS as needed so the staff can ask questions about the application. We will document this review in a Safety Evaluation Report.

Once we get public and stakeholder input on the scope of our environmental review, we will conduct the review and document the results in a draft Environmental Impact Statement (EIS). We’ll ask the public and stakeholders to comment on the draft. After considering those comments, we’ll finalize it.

We expect the review process to take us about three years, assuming WCS provides us with good information in a timely way during our review.

If interested parties ask for a hearing, and their petition is granted by our Atomic Safety and Licensing Board, then the board will consider specific “contentions,” or challenges to our reviews of the safety, security or environmental aspects of the proposed facility. The board will hold a hearing on any contentions that cannot be resolved. We can’t predict how long this hearing process would take.

The Safety Evaluation Report, the EIS and the hearing need to be complete before the NRC staff can make a licensing decision. If the application meets our regulations, we’re legally bound to issue a license. We don’t consider whether there’s a need for the facility or whether we think it’s a good idea. Our reviews look at the regulatory requirements, which are carefully designed to ensure public health and safety will be protected, and at the potential environmental impacts and applicant’s plans for mitigating them.

Incidentally, we are expecting an application for a second centralized interim storage facility Nov. 30. This one, to be filed by Holtec International, will be for a site in New Mexico. We’ll follow the same process in reviewing that application.

Dry Cask 101: Storage and Transport – The Right Materials for the Job

John Wise
Materials Engineer

CASK_101finalMaterials – the stuff of which everything is made. You might not give much thought to the materials around you: the metal in the door of your car, the plastic used in airplane windows, or the steel from which elevator cables are made. Yet, in each of these cases, the selection of appropriate materials is critical to our 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 that want NRC approval of systems to transport or store radioactive substances.  We typically refer to these substances as radioactive materials, but that might make this discussion much too confusing.

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 transportation canister has to withstand possible impacts in a transport accident.  Neutron-absorber materials need to block the movement of neutrons to control nuclear reactions in spent nuclear fuel.

Next, when making complex metal system, parts often are fused together by partially melting, or welding, them in a way that ensures that the joints themselves are adequate for the job. It may not be obvious, but during the welding process, the welder is creating a new material at the joint with its own unique properties.  That’s 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.

Horizontal storage systems under construction.
Horizontal storage systems under construction.

Finally, the NRC considers how materials degrade over time. In other words, we must take into account a material’s chemical properties – how it reacts with its environment. We’re all familiar with how iron rusts when it gets wet or how old elastic materials (e.g., rubber bands) become brittle. Often such degradation is not important. But sometimes it can cause concern. Thus, 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 experience and through industry-wide and government agreement.  But such guidelines may not cover all aspects of material selection. So we also rely on both historical operating experience and the latest materials testing data.

The NRC has a team of materials experts that reviews every application we receive for approval of spent fuel storage and transportation systems. These experts must be satisfied that every material and the processes used to join them are up to the job. The materials review is one part of a comprehensive review the NRC does on every application. We will focus on other parts of our reviews in upcoming blog posts.

Dry Cask 101 – Criticality Safety

Drew Barto
Senior Nuclear Engineer

CASK_101finalIn earlier Science 101 posts, we told you about how nuclear chain reactions are used to generate electricity in reactors. In a process known as fission, uranium atoms in the fuel break apart, or disintegrate, into smaller atoms. These atoms cause other atoms to split, and so on. This “chain reaction” releases large amounts of heat and power. 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. It is removed from the reactor after several years (typically four to six) because it will no longer easily support a self-sustaining chain reaction. This “spent fuel” is placed into a storage pool. After cooling for some time in a pool, the fuel may be put into dry storage casks.

Here, a spent fuel assembly is loaded into a dry storage cask.
Here, a spent fuel assembly is loaded into a dry storage cask.

When fuel is removed from the reactor, we require licensees to ensure it will never again be critical. This state is referred to as “subcriticality.” Preventing an inadvertent criticality event is one safety goal of our regulations. Subcriticality is required whether the fuel is stored in a pool or a dry cask. We require it for both normal operating conditions and any accident that could occur at any time.

There are many methods that 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. Known as a “neutron absorber,” boron captures neutrons released during fission, and keeps them from striking uranium atoms. Fuel burnup is another factor. As we said above, after some time in the reactor it is harder for fuel to sustain a chain reaction. The longer the fuel is in the reactor, the less likely it is to go critical. However, high burnup fuel generates greater heat loads and radiation, which must be taken into account.

Spent fuel storage cask designs often rely on design features to make sure the fuel remains subcritical. When we review a cask design, this is one of the key elements the NRC looks at in detail.

Casks have strong “baskets” to maintain fuel geometry. They also have solid neutron absorbers, typically made of aluminum and boron, between fuel assemblies. The applications that we review must include an analysis of all the elements that contribute to criticality safety. Part of the analysis is a 3-D model that shows how the fuel will act in normal and accident conditions.

A dry storage canister is loaded into a horizontal storage module.
A dry storage canister is loaded into a horizontal storage module.

Our technical experts review this analysis to make sure the factors that could affect criticality have been identified. We check to see that the models address each of these factors in a realistic way. In cases where the models require assumptions, we make sure they are conservative. That means they result in more challenging conditions than we would actually expect. We also create our own computer models to confirm that the design meets our regulatory requirements. We will only approve a storage cask design if, in addition to meeting other safety requirements, our criticality experts are satisfied that our subcriticality safety requirements have been met.

Our reviewers look at several other technical areas in depth any time we receive an application for a spent fuel storage cask. We will talk about some of the others—materials, thermal, and shielding—in future posts.

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.