Nuclear Fuel: What do we do with that stuff?

All nuclear power reactors use uranium fuel to generate electricity. This fuel consists of small ceramic pellets stacked into metal fuel rods, which are bound together into fuel assemblies that are eight to 12 feet in length.

Pellet

Once fuel has been used in a reactor, it needs to be stored. Currently, all used nuclear fuel is stored at the plant site in either used fuel pools or in dry storage.  The Nuclear Energy Institute estimates that all the used nuclear fuel produced by the U.S. nuclear energy industry in nearly 50 years—if stacked end to end—would only cover an area the size of a football field to a depth of less than 10 yards.

Jason Smith, a lead engineer specializing in nuclear fuel management for Duke Energy, says that safe used fuel storage is a priority at all nuclear power plants. “Currently, every site is responsible for storing its used fuel. There’s a tremendous amount of work and planning that goes into managing used fuel. Whether it is stored in fuel pools or dry cask storage, it poses no risk to the public or our workers,” Smith said.

Diagram of a typical dry cask storage system.

Diagram of a typical dry cask storage system.

On average, nuclear fuel spends about four years in a reactor. Once removed from the reactor, the fuel is stored in the used fuel pool for a minimum of five years, which allows the fuel to cool and lowers radioactivity levels. The used fuel remains in the pool until it meets certain requirements necessary for it to be transferred to dry cask storage.

Dry cask canisters are concrete and stainless steel. To transfer fuel from the pool to dry storage, workers carefully place the canisters into the fuel pool and load the used fuel. Once the fuel is loaded into the canister, the water is removed and it is filled with helium and welded shut. At this point, the canister is transported into a storage module for long-term storage.

The fuel will continue to give off heat and radiation, but it is safely contained within the dry storage cask. “Once the fuel is placed in the concrete storage module, the exposure is very low and poses no threat,” Jason Smith said. “Of course, we continuously monitor it.” At the Robinson Nuclear Plant, the used fuel on site dates back to the beginning of operations in 1971.

Containers used to store and carry used fuel are extremely robust and provide additional layers of protection beyond that provided by the fuel rods themselves. The Nuclear Regulatory Commission requires thorough tests and analyses prior to certifying used fuel containers.

Facilities such as Sandia National Laboratories have tested containers under extreme circumstances to ensure they will protect the public in the unlikely event of an accident during transport. Tests have proven that containers can withstand high-speed crashes, extremely hot and long-lasting fires, puncture, and submersion in water.

Dry casks being monitored by plant workers.

Dry casks being monitored by plant workers.

The approved containers are massive, weighing 25 to 40 tons for truck shipments and 75 to 125 tons for rail shipments. Multiple layers of steel and other materials confine the radioactivity. Typically, for every ton of fuel, there is more than three tons of protective shielding.

There is still a need for a sustainable, long-term solution for storing used nuclear fuel. However, dry cask storage remains a viable, safe and security storage option for the foreseeable future.

Want to learn more about the disposal of nuclear fuel, click on the links below:

NEI – Transporting Used Nuclear Fuel

 NEI – Safely Managing Used Nuclear Fuel

The Fuel in the Reactor Core

In this the second-part of three-part series where we examine the phases of the nuclear fuel cycle. To read the first part of our series, click here.

Part 2

From Fabrication Facility to Reactor

After nuclear fuel assemblies are built at the fuel fabrication facility, they are transported to a nuclear power plant. Different types of nuclear reactors require different designs of fuel assemblies, so each shipment is tailored to the specific nuclear plant receiving the assemblies. 

Fuel assemblies arrive at Duke Energy’s nuclear fleet by truck. During transport, the fuel assemblies are housed in robust containers. When the trucks arrive on site, the fuel shipping containers are inspected by Security and unloaded into the used fuel pool building.

Trained and experienced site personnel use remote-controlled cranes to lift each fuel assembly out of the container. They then meticulously inspect the assembly for any imperfections. Once the fuel assembly passes inspection, the fuel team inserts it into the used fuel pool or new fuel storage racks where it is stored until it is loaded into the reactor core. To load an assembly into the reactor core, it is laid-down horizontally and transferred underwater through a transfer tube to the containment building. The fuel assemblies are then up-ended and inserted into the reactor vessel by manipulator cranes, where they are used to generate heat (through fission) for 54 months until they are replaced with new fuel assemblies. 

Generating Heat through Fission 

Once the fuel is in the reactor, it undergoes the process of fission. Fission is a nuclear reaction and is the plant’s source of energy. In the fission process, uranium in the fuel assemblies is bombarded with neutrons, which split the uranium atoms. Heat energy and neutrons are produced in the fission process. The energy heats the water in the reactor and the neutrons split other atoms, thus perpetuating the nuclear chain reaction.

 

Refueling 

One of the great advantages of producing energy at a nuclear plant is that refueling occurs only once every 18-24 months. As mentioned in a previous article, when a unit shuts down for refueling, the reactor cavity is flooded with water and about one-third of the fuel assemblies (the oldest assemblies in the reactor) are taken out of the core and placed in the used fuel pool. The remaining fuel is carefully relocated in the reactor to maximize the energy use from each fuel assembly, and the new fuel assemblies are loaded into the core.

During refueling, the top of the reactor, or head, is removed from the reactor vessel. The oldest fuel assemblies are removed from the reactor using manipulator cranes and transported through the transfer canal to the used fuel pool, where they are stored until they are cool enough for dry storage in large, robust concrete containers. After the fuel assemblies have been placed into the reactor to form the new reactor core, the reactor head is reinstalled. 

Be sure to stop by next week to read the third part of the nuclear fuel cycle series. In the last installment, we will examine the storage of used fuel.

Further Reading:

Terrapower and the Traveling Wave Reactor

One of the goals of this space is to shed light on big ideas in the industry- even if they aren’t Duke Energy’s ideas. One of the newest designs in nuclear energy technology is the traveling wave reactor, featured in today’s post.

The traveling wave reactor (TWR) is a small glimpse into a big future for nuclear energy. Recently introduced by Bill Gates at the TED conference in 2010, the TWR has enough depleted uranium to produce full power for 60 years or more. One major benefit of these reactors is that the TWRs can get the most out of their fuel without reprocessing and could eliminate the need to enrich uranium. 

Bill Gates is a primary investor in TerraPower, a nuclear reactor design company, which is taking the lead on the development of the TWR. TerraPower plans to have a TWR prototype built by 2020, which will produce electricity for the grid in the several-hundred megawatt capacity range. Watch the video below for more information.