Mixed oxide fuel

Mixed oxide fuel. Commonly known as MOX fuel, it is a type of nuclear fuel composed of a blend of plutonium and uranium oxides. It is primarily fabricated using plutonium recovered from the reprocessing of spent nuclear fuel from conventional reactors, mixed with depleted, natural, or reprocessed uranium. The utilization of MOX fuel forms a key component of the nuclear fuel cycle, particularly in strategies aimed at recycling fissile materials and reducing stocks of separated plutonium. Its deployment occurs in several types of commercial nuclear reactors, most notably light-water reactors and fast-neutron reactors.

Composition and fabrication

The typical composition of MOX fuel consists of approximately 5-10% plutonium dioxide (PuO₂) blended with 90-95% uranium dioxide (UO₂). The plutonium used is not weapons-grade but is reactor-grade, isotopically containing significant fractions of plutonium-240 and plutonium-242 alongside the fissile plutonium-239. The uranium component is often depleted uranium, a byproduct from uranium enrichment processes, though natural or reprocessed uranium can also be used. Fabrication begins at specialized plants, such as the Melox facility in France or the Sellafield site in the United Kingdom, where the oxide powders are thoroughly mixed, pressed into pellets, and sintered at high temperatures. These pellets are then loaded into fuel rods made of zirconium alloy cladding, which are assembled into final fuel assemblies for reactor use. The process requires stringent safeguards and security measures due to the presence of plutonium.

Nuclear properties and performance

The nuclear properties of MOX fuel differ from those of standard low-enriched uranium fuel due to the presence of plutonium isotopes. Plutonium-239 has a higher neutron absorption cross-section and a different fission product yield spectrum compared to uranium-235. This alters the neutronics of the reactor core, typically resulting in a flatter power distribution and a harder neutron spectrum. The fuel's performance in-reactor is characterized by its burnup, the amount of energy extracted per unit mass, which is comparable to that of uranium fuel. However, MOX fuel has a lower delayed neutron fraction, which influences reactor control dynamics. The thermal conductivity of MOX is slightly lower than that of UO₂, and it exhibits different fission gas release behavior, which must be accounted for in fuel rod design and safety analyses conducted by organizations like the Institut de radioprotection et de sûreté nucléaire.

Use in nuclear reactors

MOX fuel is primarily used in commercial light-water reactors, including both pressurized water reactors and boiling water reactors. Countries like France, Japan, and Germany have licensed and operated LWRs with partial MOX fuel cores, often in one-third of the core positions. Its most historically significant application is in fast-neutron reactors, such as the Phénix and Superphénix reactors in France, where the hard neutron spectrum is ideal for fissioning plutonium and breeding new fissile material. The BN-800 reactor in Russia also uses MOX fuel. Research into advanced applications includes its potential use in Canada deuterium uranium reactors and in proposed Generation IV reactor concepts like the Sodium-cooled fast reactor, which aim for more efficient plutonium recycling.

Advantages and disadvantages

A primary advantage of MOX fuel is the reduction of stockpiles of separated civilian plutonium from nuclear reprocessing, thereby contributing to non-proliferation goals by consuming this material. It enhances the utilization of the energy content in mined uranium by recycling plutonium, effectively increasing fuel resources. Economically, it can offer an alternative to uranium fuel, though fabrication costs are higher. Disadvantages include the high cost and complexity of fabrication facilities, which require heavy shielding and security. The use of MOX in LWRs introduces different safety and control characteristics, necessitating specific licensing and core management strategies. There are also persistent public and political concerns regarding the proliferation risks associated with the plutonium fuel cycle and the challenges of managing spent MOX fuel, which has higher heat output and radiotoxicity than spent uranium fuel.

Production and global status

Global production of MOX fuel has been dominated by France, through the Orano subsidiary's Melox plant, and to a lesser extent by the United Kingdom. Japan has constructed the Rokkasho Reprocessing Plant and a MOX fabrication facility, though their operational status has been delayed. Russia produces MOX for its fast reactor program at facilities like the Mining and Chemical Combine. The United States has pursued MOX fuel fabrication for disposition of weapons-origin plutonium, a project historically associated with the Savannah River Site, though this program has faced significant challenges and policy shifts. Other countries like Belgium, Switzerland, and the Netherlands have used MOX fuel in their reactors, sourcing it primarily from French and British fabrication services. The global industry is influenced by policies of the International Atomic Energy Agency and national decisions regarding the closed nuclear fuel cycle. Category:Nuclear fuels Category:Nuclear reprocessing