Changing Nuclear Reactor Fuel: A Step-By-Step Guide

Nuclear reactors require regular refuelling to maintain power output. Uranium fuel is typically replaced every 18 to 24 months, depending on the enrichment of the Uranium-235 in the fuel assemblies. The process of changing the fuel involves removing spent fuel rods from the reactor core and replacing them with fresh fuel assemblies. This is done during a scheduled refuelling outage, which typically occurs every 12 to 24 months. During this time, the reactor is shut down, and the used fuel is transferred to a spent fuel pool for cooling and temporary storage.

Uranium mining and milling

Uranium mining is the process of extracting uranium ore from the ground. Uranium is mined by in-situ leaching (57% of world production) or by conventional underground or open-pit mining of ores (43% of production). During in-situ mining, a leaching solution is pumped down drill holes into the uranium ore deposit where it dissolves the ore minerals. The uranium-rich fluid is then pumped back to the surface and processed to extract the uranium compounds from the solution. In conventional mining, ores are processed by grinding the ore materials to a uniform particle size and then treating the ore to extract the uranium by chemical leaching.

The milling process commonly yields a dry powder-form material consisting of natural uranium, called "yellowcake". The ore is crushed, pulverized, and ground into a fine powder. Chemicals are added to the fine powder, which causes a reaction that separates the uranium from the other minerals. Uranium mills are often located near uranium mines so that the ore can be easily transported to the mill for processing.

The milling process results in either black oxide (uranium oxide, U3O8) or soda salt (sodium diuranate, Na2U2O7). From this point, the uranium concentrate undergoes a number of chemical processes in various refineries, plants, and laboratories to prepare it for use as nuclear fuel. The different uranium mining facilities produce different grades and forms of uranium, and therefore require slightly different milling processes.

Conversion and enrichment

Conversion

Uranium oxide, obtained from uranium mills or in-situ leaching facilities, needs to be converted into a gas before enrichment. This is achieved by refining uranium oxide into uranium dioxide, which can be used as fuel for reactors that don't require enriched uranium. However, most of the uranium oxide is then converted into uranium hexafluoride, a gas that can be processed at relatively low temperatures. This conversion step is crucial as it transforms the uranium into a form suitable for the enrichment process.

Enrichment

The enrichment process aims to increase the concentration of the U-235 isotope in the uranium hexafluoride gas. The commercial enrichment process typically involves using centrifuges to separate the U-235 and U-238 isotopes. The centrifuges spin at incredibly high speeds, utilizing the slight mass difference between the two isotopes to separate them. This process increases the concentration of U-235 to the desired level, typically between 3.5% and 5% for most reactor types.

The enriched uranium hexafluoride is then reconverted to produce enriched uranium oxide, which serves as the feedstock for the next stage of the nuclear fuel cycle—fuel fabrication. This stage involves forming the enriched uranium oxide into fuel pellets and assembling them into fuel rods and assemblies, ready for use in nuclear reactors.

The enrichment process is strategically sensitive and capital-intensive, with only a few commercial enrichment suppliers operating worldwide. It is also subject to tight international control due to its potential proliferation concerns.

Fuel fabrication

The dry method involves heating UF6 into a vapour and introducing it into a two-stage reaction vessel, where it is first mixed with steam to produce solid uranyl fluoride (UO2F2). This powder then moves through the vessel to be reacted with H2 (diluted in steam), which removes the fluoride and chemically reduces the uranium to a pure microcrystalline UO2 product.

Wet methods involve injecting UF6 into water to form a UO2 particulate slurry. Either ammonia or ammonium carbonate is added to this mixture, and the UO2F2 reacts to produce ammonium diuranate or ammonium uranyl carbonate. In both cases, the slurry is filtered, dried, and heated in a reducing atmosphere to create pure UO2.

The UO2 powder may need further processing or conditioning before it can be formed into pellets. Powders may need to be blended to ensure uniformity in particle size distribution and specific surface area. U3O8 may be added to ensure satisfactory microstructure and density for the pellets. Other fuel ingredients, such as lubricants, burnable absorbers, and pore-formers may also be added.

The conditioned UO2 powder is then fed into dies and pressed biaxially into cylindrical pellet form. These 'green' pellets are then sintered by heating in a furnace at about 1750°C under a precisely controlled reducing atmosphere to consolidate them. This also decreases their volume. The pellets are then machined to exact dimensions. For most reactors, pellets are just under one centimetre in diameter and a little over one centimetre long.

The pellets are then stacked and sealed into long metal tubes (about 1 centimetre in diameter) to form fuel rods. A spring is usually put into the space left between the top of the pellet stack and the welded end-plug, called the 'plenum' space, to accommodate thermal expansion of the pellets and some fission product gases. The space also allows for the application of a compressive force on the pellet stack to prevent its movement.

The completed fuel rods are then fixed into the prefabricated framework structures that hold the rods in a precisely defined grid arrangement. The physical structures for holding the fuel rods are engineered with extremely tight tolerances. They must be resistant to chemical corrosion, high temperatures, large static loads, constant vibration, fluid and mechanical impacts, and as neutron-transparent as possible. Assembly structures comprise a strong framework made from steel and zirconium, upon which are fixed numerous grid support pieces that firmly hold rods in their precise lattice positions. These are made from zirconium alloy and must permit (and even enhance) the flow of coolant water around the fuel rod.

The final step is to bundle the fuel rods together to make up a fuel assembly. Depending on the reactor type, each fuel assembly has 179 to 264 fuel rods. A typical reactor core holds 121 to 193 fuel assemblies.

Power generation and burn-up

Nuclear power plants use a specific type of uranium, U-235, for nuclear fission as its atoms are easily split apart. However, U-235 is relatively rare, making up just over 0.7% of natural uranium. The concentration of the fissile U-235 isotope needs to be increased to between 3.5% and 5% U-235. This process is called isotope separation, and it physically concentrates one isotope relative to others.

The uranium hexafluoride gas produced in the converter facility is called natural UF6 as the original concentrations of uranium isotopes are unchanged. It is then sent to an enrichment plant where the individual uranium isotopes are separated to produce enriched UF6. This enriched UF6 is then converted into nuclear fuel.

At a nuclear fuel fabrication facility, the solid UF6 is heated to a gaseous form, and then chemically processed to form uranium dioxide (UO2) powder. This powder is then compressed and formed into small ceramic fuel pellets. These pellets are then stacked and sealed into long metal tubes to form fuel rods. The fuel rods are then bundled together to make up a fuel assembly.

The core of a reactor is made up of several hundred fuel assemblies. For a reactor with an output of 1000 MWe, the core would contain about 75 tonnes of low-enriched uranium. In the reactor core, the U-235 isotope fissions or splits, producing a lot of heat in a continuous process called a chain reaction. The heat is then used to produce steam to drive a turbine and an electric generator.

Fuel burn-up is measured in gigawatt-days (thermal) per tonne, and its potential is proportional to the level of enrichment. Burn-up levels have been limited to about 40 GWd/t, requiring around 4% enrichment. With advancements in equipment and fuel assemblies, 55 GWd/t is now possible (with 5% enrichment), and 70 GWd/t is in sight (though this would require 6% enrichment).

In the context of nuclear power technology, burnup, or fuel utilisation, is a measure of how much energy is extracted from a primary nuclear fuel source. It is typically measured as the fraction of fuel atoms that underwent fission in %FIMA (fissions per initial metal atom) or %FIFA (fissions per initial fissile atom). It can also be computed by multiplying the thermal power of the plant by the time of operation and dividing by the mass of the initial fuel loading.

In a power station, high fuel burnup is desirable as it reduces downtime for refuelling, the number of fresh nuclear fuel elements required, and the potential for diversion of plutonium from spent fuel for use in nuclear weapons. It is also desirable that burnup is as uniform as possible within individual fuel elements and from one element to another within a fuel charge.

Interim storage and final disposal

After being used in a reactor, fuel assemblies become highly radioactive and must be removed and submerged in a pool of water for several years at the reactor site. The water in the spent fuel pool serves to cool the fuel and block the release of radiation. The fuel may then be transferred to naturally-ventilated dry storage, generally on-site. Depending on the policies of particular countries, some used fuel may be transferred to central storage facilities.

There is a clear incentive for interim storage, but ultimately, the used fuel must either be reprocessed to recycle most of it or prepared for permanent disposal. The longer it is stored, the easier it is to handle due to the progressive decrease of radioactivity.

There are two options for used fuel:

• Reprocessing to recover and recycle the usable portion of it.
• Long-term storage and final disposal without reprocessing.

Used fuel still contains about 96% of its original uranium, of which the fissionable U-235 content has been reduced to less than 1%. About 3% of the used fuel comprises waste products, and the remaining 1% is plutonium (Pu) produced while the fuel was in the reactor. Reprocessing separates uranium and plutonium from waste products (and from the fuel assembly cladding) by cutting up the fuel rods and dissolving them in acid. It enables the recycling of uranium and plutonium into fresh fuel and produces a significantly reduced amount of waste.

The general consensus favours the placement of nuclear waste into deep geological repositories, about 500 metres down, initially recoverable before being permanently sealed. Deep geological disposal is widely agreed to be the best solution for the final disposal of the most radioactive waste produced.

Frequently asked questions