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!