The Power Of Pressurized Water Reactors: Fuel Choice

Pressurized water reactors (PWRs) are a type of light-water nuclear reactor that uses water as a neutron moderator and coolant fluid for the reactor core. PWRs constitute the majority of the world's nuclear power plants. The core of a PWR contains uranium dioxide (UO2) fuel pellets, which are loaded into hollow zirconium fuel tubes called fuel rods. The fission of atoms in the fuel heats the water in the reactor core, which is then pumped into a steam generator to create steam and drive steam turbines linked to an electric generator.

Uranium dioxide fuel pellets

Uranium is the main fuel for nuclear reactors. Uranium dioxide (UO2) fuel pellets are a key component of a pressurised water reactor (PWR). These pellets are created by enriching uranium and converting it into a powder, which is then pressed to form small, hard ceramic pellets. These pellets are about the size of a sugar cube and contain as much energy as a tonne of coal.

The uranium dioxide powder is created by first mining and refining uranium ore. The uranium is then enriched through a process called conversion, where uranium oxide is converted into a gas (uranium hexafluoride). This gas is then fed into centrifuges, which separate the uranium-235 from the uranium-238. The enriched uranium is then transported to a fuel fabrication plant, where it is converted into uranium dioxide powder.

The powder is pressed into pellets and heated to form a hard ceramic material. These pellets are then inserted into thin tubes called fuel rods, which are grouped together to form fuel assemblies. Each pellet is nominally 0.375 inches in diameter and 0.5 inches long. A fuel rod typically contains a column of 200 to 300 fuel pellets, with a length of 12 feet. The fuel rods are then inserted into a reactor core, which typically contains 120 to 193 fuel assemblies.

The fission of uranium dioxide fuel in the reactor core is the heat source in a nuclear reactor. The uranium-235 isotope undergoes nuclear fission, releasing a large amount of energy. This heat is transferred to the water in the primary coolant loop, which then flows to a steam generator to produce steam and, ultimately, electricity.

The uranium dioxide fuel pellets have a crucial role in the functioning of pressurised water reactors, providing the fuel source for the nuclear reaction and generating the heat necessary to produce electricity.

Fuel assembly design

Uranium dioxide (UO2) is the fuel used in a pressurised water reactor (PWR). The uranium dioxide is formed into small ceramic pellets

Fission of uranium dioxide

Uranium dioxide (UO2) is a black, radioactive, crystalline powder that occurs naturally in the mineral uraninite. It is the most commonly used nuclear fuel material in reactors today. Uranium dioxide is produced by reducing uranium trioxide with hydrogen.

UO2 is used in the form of small ceramic pellets, which are inserted into thin tubes called fuel rods. These rods are then grouped together to form a fuel assembly. Uranium dioxide is resistant to radiation damage and has a high melting point of 2800°C, making it suitable for high-temperature applications.

During the fission process in a nuclear reactor, the UO2 fuel pellets are exposed to various damage sources, including neutrons, alpha particles, and fission fragments. This results in changes to the macroscopic properties of the fuel, known as radiation effects. One of the critical aspects of UO2 fuel is its low thermal conductivity, which can lead to localized overheating within the fuel pellets.

The uranium dioxide fuel pellets are typically 0.375 inches in diameter and 0.5 inches long. Approximately 200 to 300 of these pellets are loaded into hollow zirconium fuel tubes, known as fuel rods. Each fuel rod contains a column of pellets, typically 12 feet long. The fuel rods are then assembled into bundles, forming the core of the reactor.

The fission of uranium dioxide fuel in the reactor core generates heat, which is the primary source of energy in a nuclear reactor. The heat produced by the fission process turns the water in the reactor core into steam, which drives the turbines to generate electricity.

The uranium dioxide fuel typically operates in the reactor for 3 to 5 years. During this period, around 3 to 5% of the uranium atoms undergo fission, leaving fission products in the fuel. At the end of its operational life, the spent fuel is removed from the reactor and replaced with fresh fuel. The spent fuel, also known as "spent nuclear fuel" or "spent fuel," remains radioactive for extended periods and is carefully managed and stored.

Fuel replacement

Uranium dioxide (UO2) is the fuel used in a pressurised water reactor (PWR). The uranium is enriched and processed into small ceramic pellets, which are then stacked together into sealed metal tubes called fuel rods. These fuel rods are bundled together to form a fuel assembly, and a reactor core is made up of a few hundred of these assemblies.

At the end of its power-generating life, the fuel is discharged from the reactor and replaced with fresh fuel. The discharged fuel is called "spent nuclear fuel" or "spent fuel". It remains radioactive for very long periods of time, generating decay heat and emitting penetrating radiation in the form of high-energy gamma rays and beta particles.

The spent fuel is stored at reactor sites for several years, typically five or more years, before being shipped to a nuclear fuel reprocessing facility or a high-level radioactive waste repository for disposal.

Fuel rods

The uranium dioxide (UO2) powder is fired in a high-temperature sintering furnace to create hard, ceramic pellets of enriched uranium dioxide. The cylindrical pellets are then sealed inside the fuel rods. Each fuel rod contains a column of around 200 to 300 fuel pellets. The rods are around 4 metres in length, with the fuel column being 12 feet (144 inches) long. The nominal 6-inch length of the fuel rod that does not contain fuel pellets is called the "plenum". The plenum provides space to accommodate fuel pellet axial thermal expansion during operation and the fission product gases that are released during power operations.

A spring is inserted into the top of the fuel rod to hold the fuel pellets in place during transportation to the reactor, and end caps are welded onto the upper and lower ends of the rod. The finished fuel rods are then grouped into fuel assemblies, called fuel bundles, which are used to build the core of the reactor. A typical PWR has fuel assemblies of 200 to 300 rods each, and a large reactor would have about 150–250 such assemblies with 80–100 tons of uranium in total. Generally, the fuel bundles consist of fuel rods bundled 14 x 14 to 17 x 17.

Control rods can be inserted into the reactor core to reduce the reaction rate or withdrawn to increase it. Control rods are made of boron carbide or a silver-indium-cadmium mixture. They are mounted by electromagnets so that, in the event of a power failure, the current interruption causes the rods to fall and shut down the primary nuclear reaction.

Refuelling for most commercial PWRs is done on an 18–24-month cycle, with approximately one-third of the core replaced each time. However, some more modern refuelling schemes may reduce the refuelling time to a few days and allow refuelling to occur more frequently.

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