Some positions in a PWR fuel assembly are occupied by guide tubes into which control rods can be inserted.

Fuel pellets are made of uranium dioxide (UO₂). These pellets cannot be inserted directly into a reactor. They are loaded into fuel rods, which are arranged into fuel assemblies that facilitate fuel handling during refueling and maintain the required geometry of the reactor core. The arrangement of the fuel rods also allows coolant to flow between them and remove the heat produced by nuclear fission. The fuel rod cladding prevents direct contact between the fuel and the coolant and forms an important barrier against the release of radioactive fission products into the coolant.

Each type of reactor requires its own type of fuel. Thus, made-to-measure fuel assemblies are manufactured for each specific reactor.

During its several years in a PWR core, a fuel assembly must withstand coolant temperatures of around 300 °C, pressures of about 15 MPa and an intense neutron flux, while temperatures inside the fuel pellets can exceed 1,000 °C.

PWR Fuel Assembly

The most commonly used type of power reactor is the PWR, fuelled by slightly enriched uranium and cooled and moderated by ordinary water. PWR fuel consists of cylindrical uranium dioxide pellets typically about 1 cm long and 8 mm in diameter, each weighing about 5 grams.

The pellets are inserted into cylindrical cladding tubes made of zirconium alloy. Zirconium alloys contain small amounts of other elements that improve their mechanical properties and corrosion resistance. The cladding prevents direct contact between the cooling water and the fuel while absorbing as few neutrons as possible. The narrow gap between the pellets and the cladding is filled with helium, which improves heat transfer. The gap also provides space to accommodate dimensional changes in the fuel during irradiation and the accumulation of gaseous fission products.

Model of a VVER fuel assembly.

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VVER model fuel assembly

A stack of pellets enclosed in zirconium-alloy cladding forms a fuel rod, typically about 4 metres long and containing several hundred pellets. Fuel rods are arranged in a rigid fuel assembly and supported by spacer grids that maintain their position while allowing coolant to flow between them. A typical large PWR fuel assembly contains around 200—300 fuel rods and is approximately 20 cm wide.

Some positions in the assembly are occupied by guide tubes into which control rods can be inserted. A large PWR core typically contains between about 120 and 200 fuel assemblies. Western PWR fuel assemblies generally have a square cross-section, while those used in Russian-designed VVER reactors have a hexagonal cross-section.

A typical PWR fuel assembly can weigh around 600 kg.

Spherical fuel pebbles for HTGR reactors.

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Spherical fuel pebbles for HTGR reactors

Fuel for Other Types of Reactors

BWR

BWR (reactor cooled and moderated by ordinary water, which boils inside the reactor vessel). BWR fuel assemblies are similar to those used in PWRs. Each assembly is surrounded by a fuel channel that guides the flow of coolant through the assembly.

CANDU

CANDU (Canadian reactor using heavy water as both coolant and moderator). CANDU fuel assemblies, known as bundles, are cylindrical, about half a metre long and 10 cm in diameter. A standard CANDU 6 fuel bundle contains 37 fuel elements arranged in concentric rings and weighs about 20 kg. Because the bundles are relatively short, several are placed end-to-end in each horizontal fuel channel. A CANDU 6 reactor has 380 fuel channels, each containing 12 fuel bundles, or about 4,560 bundles in the reactor core.

Magnox

Magnox (gas-cooled reactor using graphite as a moderator). Magnox reactors use natural uranium metal fuel clad in a magnesium alloy known as Magnox. A typical fuel element is about 1 metre long and contains approximately 10—12 kg of uranium. Several fuel elements are stacked end-to-end in each vertical fuel channel in the reactor core.

AGR

AGR (advanced gas-cooled reactor using carbon dioxide as a coolant and graphite as a moderator). AGR fuel consists of slightly enriched uranium dioxide pellets contained in stainless-steel cladding, which allows operation at higher temperatures than the magnesium-alloy cladding used in Magnox reactors. A fuel assembly contains 36 fuel pins, weighs approximately 43 kg and is surrounded by a graphite sleeve.

RBMK

RBMK (water-cooled, graphite-moderated reactor developed in the former Soviet Union). An RBMK fuel bundle consists of 18 fuel rods approximately 3.65 metres long. Two bundles are joined end-to-end, giving an active fuel length of about 7 metres. Together with the supporting and end structures, the complete fuel assembly is approximately 10 metres long and is inserted into a vertical pressure channel in the reactor core.

HTR a PBMR

HTR/PBMR (high-temperature gas-cooled reactor using helium as a coolant and graphite as a moderator). In pebble-bed designs, uranium fuel is contained in tiny TRISO particles, each consisting of a fuel kernel surrounded by several layers of carbon and silicon carbide. Thousands of TRISO particles containing a total of about 9 grams of uranium are embedded in a graphite sphere about 6 cm in diameter, approximately the size of a tennis ball. The fuel pebbles are loaded into the eactor core from above and can be gradually removed from the bottom during operation.

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A single uranium fuel pellet weighing only about 5 grams can release approximately as much energy during its use in a reactor as one tonne of coal.