MOX fuel: What is it, and how is it used?

The reactor core at a typical nuclear power plant with 1,000 MWe installed power is composed of approximately 150 fuel elements. As they generate energy via the chain fission reaction, they lose efficacy due to a reduction in the fissionable material and the accumulation of fission products. 

For this reason, it is necessary to replace approximately one third of the fuel elements with new ones. This operation is known as refueling outage, and is usually carried out in nuclear power plants every 12, 18 or 24 months.

When the irradiated fuel is removed from the reactor, only 5% of the energy that was previously contained in the fresh fuel has been used. Thus, there is still a large amount of excess energy that can be reused in other reactors.

Once the fuel has been used for three operation cycles - approximately five years - it still has 95% of the enriched uranium it initially had. 1% is plutonium generated during the chain fission reactions, and the rest is composed of minor actinides, long-lived and short-lived fission products and stable products.

Fue refabrication or recycling

Some countries have considered reusing the excess U-235 and PU-239 that is generated. The fuel is refabricated or recycled, and then used in other specifically-designed nuclear power plants, since it still has over 90% of its initial power capacity.

This operation separates the two elements from the fission products, which constitute the high-level waste. This option where the spent fuel is reused and recycled is known as a closed cycle, and is one of the possible alternatives in the second part of the nuclear fuel cycle.

The refabricated fuel is known as Mixed Oxide fuel (MOX), and is composed of a mix of natural uranium oxide, reprocessed or depleted uranium and plutonium oxide.

The reprocessed uranium is the aforementioned, and the depleted uranium is a subproduct of the natural uranium enrichment stage in the fabrication of conventional fuel.

Plutonium oxide is obtained from the plutonium extracted from spent conventional fuel, as indicated above. It can also be obtained from plutonium recovered from deactivated military weapons. At the end of the cold war, the United States and the ex Soviet Union started to dismantle thousands of nuclear warheads, an initiative which produced large quantities of plutonium. The international community did not want this material to be used again to build new nuclear bombs, and it had to be safely stored and managed to protect people and the environment.

The US Department of Energy proposed to irradiate plutonium, which, in combination with uranium, began to be used in MOX fuel fabrication. This would hinder the use of plutonium for any other non-pacific purpose. Once removed from the nuclear reactor, the MOX fuel would not be recycled nor reused. The proportion of plutonium in MOX fuel is 3% to 10%.

MOX fuel behaves in a similar way to that of low-enrichment uranium fuel, which most light water reactors in operation around the world right now were designed for, both the pressurized water reactors (PWR) and the boiling water reactors (BWR).

MOX fuel represents approximately 5% of the new nuclear fuel currently used. Since it was first used in a German reactor in 1972, MOX fuel has fed 44 reactors all over the world.

Source: Orano

How is MOX fuel fabricated?

These are the stages of MOX fuel fabrication from spent conventional fuel:

1. The first step is to separate the plutonium from the remaining uranium (approximately 96% of the spent fuel) and the fission products, along with the rest of waste (another 3% in total)
2. The plutonium oxide is mixed with depleted uranium, which is part of the waste of a uranium enrichment plant (enriched uranium is used to fabricate new conventional nuclear fuel) with 7% plutonium.
3. The MOX fuel can be manufactured by grinding the uranium oxide (UO₂) and the plutonium oxide (PuO₂) together and then compacting the mix to form pellets, but this process has the disadvantage that it creates large quantities of radioactive dust. An alternative is to mix a solution of uranium nitrate and plutonium nitrate in citric acid. This mix can then be solidified using an alloy. The solid is sintered to create a mix of uranium and plutonium oxides, which is then used to make pellets and introduced into the reactor rods.

Advantages of MOX fuel

MOX fuel helps conserve natural uranium resources. Also, its fabrication reduces the amount of final waste produced in the nuclear industry.

Additionally, it helps address the problem of highly radioactive plutonium. In fact, one of its main advantages is that it can be used to eliminate part of the plutonium from military programs, thus eliminating the need to store it and contributing to non-proliferation.

Storage and management of spent MOX fuel

After MOX fuel is used in nuclear reactors, there are no significant differences in the way it is stored, as compared to conventional spent fuel.

The spent MOX fuel elements are directly introduced into special containers licensed by the corresponding nuclear regulator, and then transported to a radioactive waste storage facility.

MOX and the future

MOX fuel will allow countries that produce nuclear energy to manage their plutonium inventories and supply a growing number of reactors. All third-generation reactors, such as the European Pressurized Reactor (EPR), can use it. In the longer term, the fourth generation of reactors that is currently at the design stage, such as the Molten Salt Reactor, will be able to use plutonium in an optimal way and get the most out of the MOX fuel. Take a look at the monograph on new reactor designs.

Sources:

NRC

Orano

WNA