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).

Magnox Reactors

Magnox reactors are pressurised, carbon dioxide-cooled, graphite-moderated reactors using natural uranium (i.e. not enriched) as fuel and magnox alloy as fuel cladding. Boron-steel control rods were used.

On power fuelling was an economically essential part of the design, to maximise power station availability by eliminating refuelling downtime. This was particularly important for Magnox as the unenriched fuel had a low burn-up, requiring more frequent changes of fuel than most enriched uranium reactors.

Early reactors have steel pressure vessels, while later units (Oldbury and Wylfa) are of reinforced concrete; some are cylindrical in design, but most are spherical.

Technical Features:

Steam Quality: There is very little difference in the steam conditions between the American light water reactor and the European gas cooled reactors. Both produced saturated steam at approximately the same temperature and pressure.
In gas cooled and pressurized water reactors, the steam systems were separate, non-radioactive systems, a feature that was a good selling point to customers concerned about the unknown dangers of radioactive contamination.



1. there was a view that gas reactors would eventually provide better steam conditions as material knowledge improved and as inert gas coolants like helium became more available.


2. Construction Costs:The Magnox reactors had low maximum fuel temperatures and low coolant heat transfer capability thereby were several times larger than a LWR with the same power output.


3. Magnox reactors required construction of large, high purity graphite structures with tight tolerances and very large, high quality pressure vessels. Being very large to transport were built at site.


4. LWR imposed different constraints. The reactor internals were also carefully manufactured components with tight tolerances, but were small enough to be produced in a factory for later transport . The pressure vessel that enclosed the reactor internals was a challenging component and required a large investment in specialized manufacturing equipment, but the final product was small enough to be transported provided there were rail or water routes available. So the manufacturers were interested in a large no of deals to get back their investment.


5. The fuel used in the Magnox reactors was natural uranium metal clad with Magnox alloy. Initially maximum burn-up obtainable was about 3000 MWD/ Te of heavy metal, but it improved to about 6000 MWD/ Te ton . In 1960, the cost per kilogram of Natural uranium $18.00.


6. The fuel for the light water reactors was uranium oxide with a U-235 concentration of 3 percent clad with either stainless steel or zirconium alloy. At first, the maximum burn-up for this fuel was about 5000 MW days per ton, but it improved to about 25,000 MW days per ton within a few years. In 1962, the cost per kilogram of 3 percent enriched uranium hexafluoride (the direct product of the enrichment plants) was listed by the AEC as $254.00.


7. Disposal Costs: The natural uranium reactors produced a larger volume of high/medium level waste because of larger reactors with lower burn-up fuel . This tended to raise the cost estimates for decommissioning those reactors. This factor was countered by longer plant life estimates based on the lower stresses and lower neutron irradiation of the pressure vessel. The waste volume could also be reduced by fuel material and moderator recycling.


8. Large PWR have a significant cost disadvantage compared to gas cooled reactors as the pressure vessels are more highly contaminated and normally had to be cut up before disposal. The barges and rail lines that delivered the vessel were frequently at their capacity limits in moving an empty vessel, there is little space or weigh capacity left for adding the shielding.


9. Gas cooled vessels will also have to be dismantled, but it is far easier to cut a steel wall that is <> vessels.