Dry Casks 101: Managing Heat

CASK_101finalCaylee Johanson
Mechanical Engineer

In this series we’ve been talking about storing spent nuclear fuel in dry casks. One major function of these casks is to cool the fuel. Keeping the spent fuel from getting too hot is one way to ensure casks will be safe. As the fuel cools, heat is transferred from inside the cask to the outside.

Our experts look at how the cask will perform this function. We require the cask and fuel to remain within a certain temperature range. Our review looks at four main areas:

Spent fuel releases heat as a result of its radioactive decay. This is called decay heat. A key function of dry storage casks is to move the decay heat from the cask to the outside environment to ensure the fuel and cask components do not get too hot. Our experts look at how that heat will move through the cask and into the environment.

The method used to remove heat has to be reliable and provable. Heat must also be removed in a way that is passive—meaning no electrical power or mechanical device is needed. Casks use conduction, convection and radiation to transfer the heat to the outside.

Heat Radiation Transparent 2The graphic shows the three heat transfer methods. As you can see, conduction transfers heat from the burner through the pot to the handle. The process of heat rising (and cold falling) is known as convection. And the heat you feel coming off a radiator, or a hot stove, is known as radiant heat.

These methods work the same way in a storage cask. Where the canister or metal structure containing the fuel touches the fuel assemblies, heat is conducted toward the outside of the cask. Most casks have vents that allow outside air to flow naturally into the cask (but not into the canister) and cool the canister containing the fuel (convection). And most casks would be warm from radiant heat if you stood next to them. (The heat generated by a loaded spent fuel cask is typically less than is given off by a home-heating system.)

We limit how hot the cask components and fuel materials can get because we want to protect the cladding, or the metal tube that holds the fuel pellets. Limiting the heat is one important way we can ensure the cladding doesn’t degrade. The cask must  keep spent fuel cladding below 752 degrees Fahrenheit during normal storage conditions—a limit that, based on the material properties of the cladding, will prevent it from degrading. The fuel must also remain below 1058 degrees in off-normal or accident conditions (such as if a cask were dropped while it is being positioned on the storage pad, or if a flood or snow were to block the vents).

We also confirm the pressure inside is below the design limit to make sure the pressure won’t impact the structure or operations. Our experts review applications for new cask designs carefully to verify 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. We look at whether the designer correctly considered how the heat load will affect cask component and fuel temperatures. We review how this heat load was calculated.

We also verify that the cask designer looked at all the environmental conditions that can be expected because these will also affect the cask component and fuel temperatures. These may include wind speed and direction, temperature extremes, and a site’s elevation (which can affect internal pressure). To make sure the right values are considered, we verify they match the historical records for a site or region.

We review all of the methods used to prove that the storage system can handle the specified heat loads. We also verify any computer codes used in the analysis and the values that were plugged in. For example, we look at the material properties for cask components used in the code. We look at calculations for temperatures and pressure. We make sure the computer codes are the latest versions.

And we only allow designers to use codes that have been endorsed by experts. We might run our own analysis using a different computer code to see if our results match the application.

The analysis and review allow us to see whether and how the dry cask will meet the temperature limits. Our review ensures the temperature is maintained and the cladding is protected. Finally, our review confirms the cask designer used acceptable methods to analyze or test the system and evaluate the thermal design. If we have any questions or concerns, we ask the designer for more information.

Only when we are satisfied that our requirements are met will we approve the thermal analysis in a cask application.

REFRESH — Transporting Spent Nuclear Fuel: How Do We Know It’s Safe?

Mark Lombard
Director, Division of Spent Fuel Storage and Transportation

refresh leafAs the country wrestles with how to manage the highly radioactive fuel left over from generating nuclear power, one question often comes up: “how do we know we can transport it safely from reactor sites to other locations for storage, testing or disposal.” For one thing, we periodically assess the risks. For another, spent fuel shipments are strictly regulated and have not released any radioactive materials since they began more than 30 years ago.

Our most recent risk assessment, published in 2014, confirmed that NRC regulations for spent fuel transport are adequate to ensure safety of the public and the environment. As more data become available and computer modeling improves, these studies allow the NRC to better understand the risks.

Both the NRC and the U.S. Department of Transportation oversee radioactive material transport. DOT regulates shippers, vehicle safety, routing and emergency response. The NRC certifies shipping containers for the more hazardous radioactive materials, including spent fuel.

To be certified, a container must provide shielding, dissipate heat and prevent a nuclear chain reaction. It must also prevent the loss of radioactive contents under both normal and accident conditions. Containers must be able to survive a sequence of tests meant to envelope the forces in a severe accident. These tests include a 30-foot drop onto an “unyielding” surface (one that does not give, so the cask absorbs all the force) followed by a 1,475-degree Fahrenheit fire that engulfs the package for 30 minutes.

The 2014 Spent Fuel Transportation Risk Assessment modeled the radiation doses people might receive if spent fuel is shipped from reactors to a central facility. The study found:

  • Doses along the route would be less than 1/1000 the amount of radiation people receive from background sources each year
  • There is a 1 in 1 billion chance that radioactive material would be released in an accident
  • If an accident did release radioactive material, the dose to the most affected individual would not cause immediate harm

The Spent Fuel Transportation Risk Study examined how three NRC-certified casks would behave during both normal shipments and accidents. It modeled a variety of transport routes using population data from the 2000 census, as updated in 2008. It used actual highway and rail accident statistics. It considered doses from normal shipments to people living along transportation routes, occupants of vehicles sharing the route, vehicle crew and other workers, and anyone present at a stop. And it used state-of-the-art computer models.

The 2014 study builds on earlier studies of transportation risks. It uses real-world data and equipment in place of generic designs and conservative assumptions. The first study, done in 1977, allowed the NRC to say that its transport regulations adequately protect public health and safety. Other studies done in 1987 and 2000 found the risks were even smaller than the 1977 study predicted. Together with analyses we perform on major transportation accidents, these studies give the NRC confidence in the safety of spent fuel shipments.

For more information on how the NRC regulates spent fuel transportation, click on our backgrounder.

REFRESH is an occasional series where we revisit previous posts. This originally ran in September 2013