How can we fuel nuclear energy for a cleaner future?

Nuclear energy is obtained by the huge amount of energy that is released by atoms in the processes of fusion or fission. Fusion is when two lightweight atoms (like hydrogen) are joined to form a larger atom. On the other hand, fission requires splitting large and heavy atoms (like uranium) into smaller ones. Both processes release large amounts of energy which can be used to efficiently power cities without directly contributing to greenhouse gas emissions. One of the key challenges is fueling these processes safely and sustainably.

Nuclear energy processes

Both the fission and fusion processes have their own sets of challenges. While nuclear fission is a process that has been honed over the years, nuclear fusion is still a relatively new advancement. When the hydrogen particles bond together, it creates helium gas and free-floating neutrons that transfer their kinetic energy as heat. The heat can be used to boil water for steam, which powers turbines to produce electrical energy.

Related: Nuclear fusion is the newest advancement in clean energy

Fusion requires high amounts of energy to conduct and control. This is because particles have to be heated to immensely high temperatures. Once that occurs, they can be forced to fuse despite their extremely repulsive electrostatic forces. While the sun and other stars do this every second, it was unknown if these conditions could be replicable on Earth until very recently.

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One of the downsides of fusion is that there is very little exploration in this field because of technological limitations and costs. Advances in equipment and methods to carry out fusion have only recently been made and progress is slow. However, there is an increasing interest in fusion energy because of its incredible efficiency. In fact, it is one of the most efficient renewable energy sources because, after the first fusion process, the energy obtained can be used to power more reactions down the line.

Conversely, fission is easier to carry out because larger atoms are easier to split. To do this, a neutron collides with a large atom and splits it into multiple parts, releasing energy through heat and radiation. Though fission is an easier process to carry out, it produces weapons-grade radioactive waste that is harmful to living organisms. This is because high exposures to radioactive material can alter DNA at the cellular level, which can impact overall health and reproduction.

Energy fuels for fusion

Deuterium and tritium are forms of hydrogen that can be used as fusion fuels and are prominent in nature. Deuterium can easily be distilled from seawater or freshwater and is very efficient. There is enough deuterium naturally available to sustain humans for 150 billion years! Though tritium is rarer, it can be produced synthetically. It can be extracted from lithium, which is also abundant in nature.

Using tritium in conjunction with deuterium in nuclear reactions can be extremely useful. In fact, just a few grams of each reactant can produce one terajoule (one trillion joules) of energy. That is enough to meet the energy needs of an individual in a developed country for 60 years.

Since fusion uses two hydrogen atoms that fuse into helium, a non-toxic gas, the process is quite safe. Fusion also produces very minimal radioactive material. The radioactive waste produced is also short-lived and can be recycled within 100 years or so, unlike that of fission, which can take several centuries.

Fueling fission

U-235

Heavy atoms like uranium and less commonly plutonium or thorium, are required to fuel fission. Nuclear power plants mainly use a specific isotope of uranium, known as U-235, because its atoms are easy to divide. Though uranium is a common element, U-235 is extremely rare, occurring in only 0.7% of natural uranium.

Uranium can be found in rocks and seawater and the concentrate is separated from the ore at mills or a slurry through in-situ leaching. In the past, traditional uranium mining would cause major ground disturbance and the lack of regulations posed risks to the environment and workers onsite. These hazards are now limited through the use of contemporary in-situ leaching methods.

In-situ leaching uses water injected with oxygen that circulates through boreholes to extract uranium ore. This uranium solution is then pumped to the surface and dried to become uranium oxide concentrate before it can be enriched and processed for fuel.

Once uranium oxide concentrate is obtained, it must be converted into uranium hexafluoride gas, which is enriched in centrifuges to obtain U-235. This is then converted into uranium dioxide powder and compressed into fuel pellets. These uranium fuel pellets are roughly the size of a sugar cube. However, despite their tiny size, they are incredibly powerful. Each pellet contains as much energy as one metric tonne (1.1 U.S. tons) of coal!

Though there are mines across the globe, 85% of the uranium used for energy is produced in Australia, Canada, Kazakhstan, Namibia, Niger and Russia. Uranium mines tend to have similar environment and health-related hazards to metalliferous mines, but with additional radiation-related risks. If not managed appropriately, uranium mining can pose risks such as air pollution through radioactive dust and contamination of surrounding groundwater.

Molten salt reactors

Molten salt reactors (MSRs) were first tested in the 1960s. These machines would use a combination of liquid salts for fuel. Using MSRs is potentially safer than typical nuclear reactors because they require lower pressures and have in-built safety measures for overheating.

For the fission process, radioactive thorium in the core heats the molten salt. This turns water into steam that activates a turbine to generate electricity. If at any point the core does overheat, the salt drops into a containment vessel and solidifies.

In recent years, MSRs are being developed in countries such as China, the US and Denmark. They are being explored to commercialize for ships. This way, the vessels will be able to cruise electrically by producing their own electricity supply and never have to rely on fossil fuel-based energy.

Triso fuel

In recent years, TRISO fuels have been gaining speed. These are TRi-structural ISOtropic particle fuels. Each particle is made up of an oxygen, uranium and carbon kernel which is encased in three layers of ceramic-based materials which prevent the release of radioactive material. These kernels are only about the size of a poppy seed but are very robust. In fact, they have even been deemed the most robust nuclear fuel on earth because of their resistance to high temperatures, neutron irradiation, corrosion and oxidation.

These fuel kernels are also very versatile. Once they are fabricated into billiard ball-sized pebbles, they can be used in MSR or high-temperature gas reactors. Because of TRISO fuels‘ unique properties, they can be used multiple times, making them sustainable as they require less frequent manufacturing. Presently, TRISO fuels are still being developed for maximum efficiency. They are also being explored for use in both large nuclear reactors and smaller modular reactors/micro-reactors.

In conclusion…

Depending on the type of nuclear reaction taking place, a variety of fuels can be used for power. For fusion, deuterium and tritium are particularly useful because of their abundance and ease of production, respectively. For fission, slowly moving away from uranium may be a safer and more sustainable approach. This way, newer developments of MSRs and TRISO fuels can be utilized for their optimized safety and efficiency.

Via U.S. Energy Information Administration