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.