Changing Fuel Rods: Maintaining The Heart Of A Nuclear Reactor

Nuclear fuel rods are an essential component of nuclear reactors, and their timely replacement is crucial for maintaining the reactor's performance and safety. The process of changing fuel rods involves several steps and requires strict adherence to safety protocols. Here's an overview of the procedure:

Firstly, it's important to understand that nuclear fuel rods are not simply used up until they are fully depleted. Reactor operators typically replace the rods while they still contain a significant amount of fissile material to ensure the reactor operates within a stable regime. This replacement process is typically done gradually, with one-third of the reactor core being replaced every 12 to 24 months.

The process of changing fuel rods begins with the removal of the spent fuel assemblies, which have become highly radioactive. These assemblies are then submerged in a pool of water, known as a spent fuel pool, to cool down for several years. This step is crucial as the spent fuel continues to generate heat due to the decay of radioactive elements produced during the nuclear reaction.

Once the spent fuel assemblies have cooled sufficiently, they may be transferred to dry cask storage containers, which are often air-conditioned concrete or steel structures located at the power plant site. This interim storage step is necessary until a permanent underground repository for high-level nuclear waste becomes available.

The next step is the preparation of fresh fuel rods. Uranium ore is mined and refined into uranium concentrate, also known as yellowcake. This concentrate undergoes a series of processes, including conversion, enrichment, and fuel fabrication, to produce the final fuel rods. The enriched uranium is formed into small ceramic fuel pellets, which are then sealed inside metal tubes to create the fuel rods.

The freshly fabricated fuel rods are then transported to the reactor site and stored in fresh fuel storage bins until required. When it's time for a replacement, the new fuel assemblies are carefully inserted into the reactor core, replacing the spent fuel assemblies. This process is done gradually and is controlled to maintain the desired power level in the reactor.

It's important to note that changing fuel rods in a nuclear reactor is a complex and highly regulated procedure. The entire process is carefully monitored and managed to ensure the safety of the personnel involved and the surrounding environment.

The nuclear fuel cycle

The Front End

The front end of the nuclear fuel cycle involves preparing uranium for use in nuclear reactors. This includes the following steps:

• Mining and Milling: Uranium is mined from the ground using surface (open-pit) or underground mining techniques, depending on the depth of the deposit. The uranium ore is then milled to extract the uranium and convert it into uranium oxide (U3O8) concentrate, also known as "yellowcake".
• Conversion: The uranium oxide is refined into uranium dioxide, which can be used as fuel for certain types of reactors. For most reactors, however, the uranium oxide needs to be converted into uranium hexafluoride (UF6), a gas that can be enriched.
• Enrichment: The enrichment process separates the UF6 gas into two streams: one with a higher concentration of the fissile isotope U-235 (3-5%) and the other with depleted uranium ("tails"). This is done using centrifuges or laser-based technologies.
• Fuel Fabrication: The enriched uranium is converted back into a solid form and then processed into ceramic fuel pellets. These pellets are sealed into metal tubes to form fuel rods, which are then bundled into fuel assemblies ready for use in nuclear reactors.

The Back End

The back end of the nuclear fuel cycle involves managing, preparing, and disposing of spent nuclear fuel, which is highly radioactive. The steps include:

• Interim Storage: After the fuel assemblies are used in a reactor, they are highly radioactive and must be stored in a spent fuel pool to cool down and allow the radiation levels to decrease. This can take several months or years.
• Reprocessing (not permitted in the US): Spent fuel can be reprocessed to recover usable uranium and plutonium for recycling into new fuel assemblies. This reduces the amount of waste that needs to be disposed of.
• Long-Term Storage and Final Disposal: The final step is to collect the spent fuel assemblies from interim storage sites and transport them to a permanent underground repository for long-term disposal.

Uranium conversion and enrichment

Uranium Conversion:

Uranium conversion is a complex process that takes place in several stages and across multiple sites. The first step is the extraction of natural uranium ore through underground or open-pit mining methods. The uranium ore is then sent to a conversion plant, where it undergoes a series of chemical reactions to transform it into a usable form. The specific steps in this process can vary, but one common conversion process is the "wet process" used in Canada, France, and Russia. Here is a general overview of the wet process:

• Uranium oxide concentrate is dissolved in nitric acid, creating uranyl nitrate.
• The uranyl nitrate undergoes purification, evaporation, and thermal decomposition to form uranium trioxide powder.
• The uranium trioxide is converted into uranium dioxide through a kiln process.
• Uranium dioxide is reacted with hydrogen fluoride to produce uranium tetrafluoride.
• Uranium tetrafluoride is fed into a fluidized bed reactor and reacted with gaseous fluorine to produce uranium hexafluoride.
• The uranium hexafluoride undergoes further refinement to remove impurities.

The conversion process is crucial as it transforms the uranium ore into a form that can be used in subsequent enrichment and fuel fabrication steps.

Uranium Enrichment:

Uranium enrichment is the process of increasing the concentration of the fissile isotope uranium-235. While natural uranium contains only about 0.7% of this isotope, most nuclear reactors require enriched uranium with concentrations ranging from 3.5% to 4.5% uranium-235. There are several methods for uranium enrichment, but the most common one today is the gas centrifuge method. Here is an overview of the process:

• Uranium hexafluoride gas is fed into an evacuated cylinder containing a rotor.
• The cylinder is spun at high speeds, causing the heavier uranium-238 molecules to collect near the walls while the lighter uranium-235 molecules collect near the central axis.
• The enriched product is then drawn off, and the process is repeated in a series of centrifuges (a cascade) to achieve the desired level of enrichment.

The gas centrifuge method is highly efficient and requires much less energy compared to older methods like gaseous diffusion. It also allows for smaller enrichment plants due to the shorter distance the gas needs to travel.

In summary, uranium conversion and enrichment are essential steps in the nuclear fuel cycle. Conversion purifies and transforms uranium ore, while enrichment increases the concentration of the fissile isotope to make the uranium suitable for use in nuclear reactors. These processes are highly technical and require advanced industrial facilities to ensure safety and environmental sustainability.

Fuel assembly and transportation

Nuclear fuel assemblies are specifically designed for particular types of reactors and are made to exacting standards. The standard fuel form comprises a column of ceramic pellets of uranium oxide, clad and sealed into zirconium alloy tubes. The tubes containing the fuel pellets are sealed: these tubes are called fuel rods. The finished fuel rods are grouped into fuel assemblies that are used to build up the core of a power reactor.

The fabrication of fuel structures – called assemblies or bundles – is the last stage of the front end of the nuclear cycle. There are three main stages in the fabrication of the nuclear fuel structures used in light-water reactors (LWRs) and pressurised heavy water reactors (PHWRs):

Producing pure uranium dioxide (UO2)

Uranium arrives at a fuel manufacturing plant in one of two forms, uranium hexafluoride (UF6) or uranium trioxide (UO3), depending on whether it has been enriched or not. It needs to be converted to uranium dioxide (UO2) prior to pellet fabrication. Most fabrication plants have their own facilities for effecting this chemical conversion.

Producing high-density, accurately shaped ceramic UO2 pellets

The UO2 powder may need further processing or conditioning before it can be formed into pellets. Conditioned UO2 powder is fed into dies and pressed biaxially into cylindrical pellet form using a load of several hundred MPa. These ‘green’ pellets are then sintered by heating in a furnace at about 1750°C under a precisely controlled reducing atmosphere. The pellets are then machined to exact dimensions.

Producing the rigid metal framework for the fuel assembly

Pellets meeting QA specifications are loaded into tubes made from an appropriate zirconium alloy, referred to as the ‘cladding’. The filled tube is flushed with helium and pressurised with tens of atmospheres of this gas before the ends are sealed at each end by precision welding. The completed fuel rods are then fixed into the prefabricated framework structures that hold the rods in a precisely defined grid arrangement.

Inserting the fuel rods

• Fuel Rod Preparation: Before insertion, the fuel rods must be carefully fabricated. Uranium hexafluoride (UF6) gas is converted into uranium dioxide (UO2) powder. This powder is then compressed and formed into small ceramic pellets, which are stacked and sealed inside metal tubes to create the fuel rods. These tubes are typically made of zirconium alloy and measure about 1 centimetre in diameter.
• Fuel Assembly: The fuel rods are then bundled together to form a fuel assembly. The number of rods in each assembly varies depending on the reactor type, ranging from 179 to 264 fuel rods per assembly.
• Transportation: Once the fuel assemblies are fabricated, they are transported to the reactor sites using trucks, ensuring safe handling and security.
• Storage: Upon arrival at the reactor site, the fuel assemblies are stored in fresh fuel storage bins until it is time for them to be loaded into the reactor. At this stage, the uranium emits only mild radioactivity, and the radiation is contained within the metal tubes.
• Insertion: The insertion process involves placing the fuel assemblies into the reactor core, which is a cylindrical arrangement encased in a steel pressure vessel. The core typically holds between 121 and 193 fuel assemblies.
• Control and Monitoring: Control rods, made of materials such as boron, cadmium, silver, or hafnium, are used to regulate the nuclear chain reaction and control the power output. These rods are inserted or removed from the core to maintain the desired power level. The number of control rods and the distance to which they are inserted are critical factors in controlling the reactivity of the reactor.
• Maintenance and Replacement: Over time, the fuel rods deplete and need to be replaced. Typically, reactor operators replace about one-third of the reactor core (40 to 90 fuel assemblies) every 12 to 24 months. This replacement ensures the reactor maintains optimal performance and prevents the risk of fuel failure.

Removing and storing spent fuel

Removal of Spent Fuel:

• Cooling the Fuel Assemblies: After their removal from the reactor, the highly radioactive fuel assemblies are submerged in a pool of water, known as a spent fuel pool, for several years. This process allows the assemblies to cool down, as they continue to generate heat due to the decay of radioactive elements formed during nuclear fission.
• Transfer to Dry Cask Storage: Once the spent fuel has cooled sufficiently, typically within a few years, it is transferred from the spent fuel pool to dry cask storage containers. These containers are usually air-conditioned and made of concrete or steel to provide additional protection.

Long-Term Storage and Final Disposal:

• Interim Storage: The spent fuel assemblies are then stored at interim storage sites, which can be at the power plant site or other designated locations. This step ensures the safe storage of the highly radioactive material until a permanent disposal solution is available.
• Final Disposition: The final step in the nuclear fuel cycle is the collection and transportation of the spent fuel assemblies from the interim storage sites to a permanent underground repository for long-term disposal. However, it is important to note that the United States currently lacks a permanent underground repository for high-level nuclear waste.

It is crucial to handle and store spent nuclear fuel with utmost care to prevent accidental criticality and potential radioactive contamination. The procedures outlined above aim to safely manage and isolate the spent fuel until its radioactivity naturally decreases over time.

Frequently asked questions