Nuclear Fuels

PWR fuels :

Pressurized water reactor (PWR) fuel elements are made of uranium oxide pellets sheathed in Zircaloy tubes of about 1 cm diameter. These fuel elements are arranged in 14x14 or 17x17 formation and are about 4 meters in length. The fuel cladding gap is filled with helium gas to improve the conduction of heat from the fuel to the cladding. There are about 179-264 fuel rods per fuel bundle and about 121 to 193 fuel bundles are loaded into a reactor core. The fuel bundles are usually enriched . The uranium oxide is dried before inserting into the tubes to try to eliminate moisture in the ceramic fuel that can lead to corrosion and hydrogen embrittlement. The Zircoloy tubes are pressurized with helium to try to minimize pellet cladding interaction (PCI) which can lead to fuel rod failure.

CANDU fuel :

The fuel bundles are about a half meter long and 10 cm dia. They consist of sintered (UO2) pellets in Zirconium alloy tubes, welded to Zirconium alloy end plates. Each bundle is roughly 20 kg, and a typical core loading is on the order of 4500-6500 bundles, depending on the design. The bundle has typically have 37 identical fuel pins radially arranged used. The CANFLEX bundle has 43 fuel elements, with two element sizes. It is also about 10 cm (four inches) in diameter, 0.5 m (20 inches long) and weighs about 20 kg (44 lbs) and replaces 37-pin standard bundle. It has been designed specifically to increase fuel performance by utilizing two different pin diameters. Current CANDU designs do not need enriched uranium to achieve criticality (due to their more efficient heavy water moderator), however, some newer concepts call for low enrichment to help reduce the size of the reactors.

BWR Fuel:
It is similar to PWR fuel except that it is canned to prevent density changes near the fuel as the same can affect the nuclear reactions and thermal hydraulics of the reactor.The no of fuel pin per assembly is of the order of 90's varying to design to design. The no of assemblies depend on the size of the core.

Magnox Fuel :The Metallic fuel is used In Magnox reactor which are gas cooled reactors operating in UK. The size varied from 50MWEe to ~500MWe. They were the precursors of the Advanced Gas cooled reactor. Unenriched Uranium is cladded with an alloy of Mg-Al and other metals in small amounts. The main disadvantage of this fuel is limit on max fuel pin temp hence the efficiency of the plant and reactivity of Magnesium with water prevents long term under water storage.

TRISO Fuel: It consists of a fuel kernel composed of uranium oxide (sometimes Uranium carbide or UCO), coated with four layers of three isotropic materials.

1. The first layer is a porous buffer layer made of carbon.
2. The second layer pyrolytic carbon (PyC).
3. The third ceramic layer of Siilicon Carbide retains the fission products and gives the TRISO particle structural integrity.
4. The outer layer of is of PyC.

TRISO fuel particles are designed not to crack at temperatures beyond 1600°C(due to differential thermal expansion or released fission gas pressure). They can contain the fuel in the worst accident scenario in a properly designed reactor. Two such reactor designs are pebble bed modular reactor (PBMR), in which thousands of TRISO fuel particles are dispersed into graphite pebbles, and a prismatic-block gas cooled reactorin which the TRISO fuel particles are fabricated into compacts and placed in a graphite block matrix. Both of these reactor designs are high-temperature gas-cooled reactors (HTGR), which is a type of very high temperature reactors (VHTR).