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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

 

 

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.

Testing Spent Fuel Transport Casks Using Scale Models

Bernard White
Senior Project Manager
Division of Spent Fuel Storage and Transportation

Before casks can be used to transport the most radioactive cargo—including spent nuclear fuel—the NRC requires them to undergo a thorough safety evaluation. Casks are evaluated for their ability to withstand vibration, water spray, free fall, stacking, penetration and fire. A cask must be able to contain and shield the spent fuel and keep it in a safe configuration under both normal and accident conditions. Typically, spent fuel casks are certified through a combination of engineering analyses and scale model or component testing.

People often ask why the NRC allows designers to test scale models instead of requiring tests on full-sized casks. The bottom line is scale-model testing provides the necessary information for the NRC staff to know that a cask loaded with spent fuel can be transported safely, even in the event of an accident.

scalemodel2First, it is important to understand what information comes out of these tests. Test casks are fitted with sensors to measure acceleration. These accelerometers are similar to the ones used in smart phones, video game remotes and pedometers to respond to the movements of the user. Knowing the cask’s acceleration allows designers and the NRC to understand the forces different parts of the cask will experience in different types of impacts. The design engineer generally calculates these impact forces first by hand or by computer. Tests on a scale model can be used to check the accuracy of these analyses.

Engineers follow a similar process to safety-test airplanes, ships, bridges, buildings and other large structures. Scale-model testing is a proven and accepted practice across engineering disciplines, and may be one of the oldest engineering design tools. (Ancient Egyptian, Greek, and Roman builders are known to have built small models to assist in planning structures.) Today, models allow oversized structures to be examined in wind tunnels, under different weight loads and on shake tables to provide key inputs into design and safety reviews.

Cost savings is a factor, but not the most important one. The biggest reason for using scale models is practicality. Transport casks for spent nuclear fuel are typically in the 25-ton to 125-ton range. There are very few testing facilities in the world that can put a 125-ton cask through the required tests.

For example, during 30-foot drop test, the test cask must strike the surface in the position that would cause the most severe damage. Cask designers often perform several drops to ensure they identify the correct position. After the 30-foot drop, the cask is dropped 40 inches onto a cylindrical puncture bar, then placed in a fully-engulfing fire for 30 minutes. Casks are also immersed in water to ensure they don’t leak. Measurements from these tests are plugged into computer programs that analyze the cask structure in great detail.

This analysis can determine the stresses placed on cask closure bolts, canisters and baskets that hold the spent fuel in place, and the spent fuel assemblies themselves. Computer simulations can be run for different scenarios, providing maximum flexibility to designers in understanding how best to design different parts of a cask’s structure.

In addition, NRC regulations specify that in the 30-foot drop test, the cask must hit an “unyielding” surface. This means the cask itself, which may be fitted with “impact limiters,” has to absorb all the damage. The impact limiters work much like the bumper that protects a car in a collision. The target surface cannot dent, crack or break in any way. In a real-world accident, a 125-ton cask would damage any surface significantly. It requires considerably more engineering work to achieve an unyielding surface for a full-sized cask than for a scale model, with no measurable advantage. The rule-of-thumb for testing is the impact target should be 10 times the mass of the object that will strike it. So a 125-ton cask would need to hit a 1,250 ton surface. A 30-ton cask would only need a 300-ton target.

Scale models are easier to handle and can be used efficiently for many drop orientations to meet the multiple test requirements. If a test needs to be run again, it can be done much more easily with a scale model. Design changes are also more easily tested on models. Together with extensive analyses of a cask’s ability to meet our regulatory requirements, the information from these tests allows the NRC to decide whether a cask can safely transport the radioactive contents.

NRC Science 101 – About Spent Nuclear Fuel Part II

Greg Casto
Branch Chief
Division of Reactor Safety Systems
science_101_squeakychalkOur last post talked about the fuel that powers nuclear reactors. Today, we’ll talk about what happens to that fuel when it’s removed from a reactor.

You’ll recall that fuel becomes very hot and very radioactive as it is used in the reactor core to heat water. After about five years, the fuel is no longer useful and is removed. Reactor operators have to manage the heat and radioactivity that remains in the “spent fuel” after it’s taken out of the reactor. In the U.S., every reactor has at least one pool on the plant site where spent fuel is placed for storage. Plant personnel move the spent fuel underwater from the reactor to the pool. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away.

These pools contain an enormous quantity of water—enough to cover the fuel by about 20 feet. The water serves two purposes: it cools the fuel and shields workers at the plant from radioactivity. Having 20 feet of water above the fuel means there is a lot more water than is needed for cooling and shielding the workers. Also, because of the extra water and the simple design of the pool, there is a lot of time for plant personnel to add water to the pool if needed for any reason.

fuelpoolThe pools are built to meet strict NRC safety requirements. They have very thick, steel-reinforced concrete walls and stainless-steel liners, and are protected by security personnel. There are no drains that would allow the water level to drop or the pool to become empty. The plants have a variety of extra water sources and equipment to replenish water that evaporates over time, or in case there is a leak. Plant personnel are also trained and prepared to quickly respond to a problem. They keep their skills sharp by routinely practicing their emergency plans and procedures.

When the plants were designed, the pools were intended to provide temporary onsite storage. The idea was for the spent fuel to sit in the pool for a few years to cool before it would be shipped offsite to be “reprocessed,” or separated so usable portions could be recycled into new fuel. But reprocessing didn’t end up being an option for nuclear power plants and the pools began to fill up.

In the early 1980s, nuclear plants began to look for ways to increase the amount of spent fuel they could store at the plant site. One way was to replace spent fuel storage racks in the pools with racks containing a special material that allowed spent fuel to be packed closer together. Another way was to place older, cooler and less radioactive fuel in dry storage casks that could be stored in specially built facilities at the plant site. We’ll talk more about dry spent fuel storage in future blog posts.

Most plants today use both re-designed storage racks and dry storage facilities to store spent fuel. All storage methods must be reviewed in detail and approved by the NRC before a plant is allowed to change storage methods.

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