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Fission & Fusion: Similarities and Differences
What is the difference between fusion and fission energy?
Many people are posing this question as both are drawing attention as clean, firm energy sources, but they are in fact opposite processes.
Fission is the process that powers existing power plants around the world. It’s commonly known as nuclear power. During fission, heavy atoms (such as uranium) split into multiple smaller ones.
Fusion is what powers the stars. There, light atoms (such as hydrogen) combine to make heavier atoms (such as helium) and release an enormous amount of energy.
Fusion
Energy source = combining atoms
Commercial fusion power plants will create the precise conditions needed for fusion to happen on Earth and ultimately generate power. High temperatures and pressures create and sustain a plasma, bringing the atoms close enough to fuse.
The challenges of achieving and maintaining these conditions are what make fusion so hard, but they also ensure its safety. The fusion process can be stopped at any time and will simply cease on its own when conditions aren’t perfect. A chain reaction is physically impossible in fusion, which means there is no risk of a runaway chain reaction or meltdown.
Fission
Energy source = splitting atoms
Splitting a heavy atom creates lighter atoms plus neutrons. When other atoms absorb the neutrons, they split and release more neutrons. This creates a chain reaction that must be carefully controlled in a nuclear reactor.
Even when the reactor is shut down, the smaller atoms continue to radioactively decay, generating continued heat that safety systems must dissipate to avoid a fuel meltdown.
Fuel = from water, minerals and fusion devices
Fusion can be productively performed with different fuels. Three potential fuel combinations are proton and boron, deuterium and tritium, or deuterium and helium-3.
Protons and deuterium are both found in water and are naturally abundant. Boron is also naturally abundant— from existing surface mines in the U.S. and other countries.
Tritium, which requires special handling, is a byproduct of other nuclear processes. In the short term, it can come from fission power plants. As commercial fusion scales, fusion companies and researchers are developing ways to produce this fusion fuel in a fusion machine from other elements.
Helium-3 can be produced from deuterium- deuterium fusion.
Fuel = needs mining and refinement
Uranium, the main fuel for fission, is common, though the uranium isotope U-235 required for power generation is rare and requires mining and enrichment before it can be used as fuel.
Processing the raw materials into more concentrated U-235 fuel for power generation requires a regulated facility and specialized handling.
Regulations = similar to medical
facilities
Fusion facilities will be regulated like particle accelerators, thousands of which are already operating in hospitals and other facilities around the world. They require appropriate shielding to protect workers, the public, and the environment from radiation.
Standard industrial safety processes will be in place for fusion. Fusion is inherently safe as there is no risk of meltdowns or runaway chain reactions and no presence of high- level radioactive materials to create and sustain the fusion reaction.
Regulations = complex national and international systems
Every fission facility requires a lengthy and costly licensing and permitting process to ensure that the plant will operate safely.
Byproducts = short-term, low-level
waste storage
Fusing atoms together creates new atoms and releases energy, either as charged particles or as neutrons and charged particles. The resulting atoms are most commonly isotopes of helium, an inert gas familiar to us through balloons.
Many fusion approaches will use and/or produce tritium, a low-level radioactive byproduct that requires shielding and storage. These fusion approaches will use tritium in a closed fuel cycle, meaning that the tritium will be produced and consumed in the plant.
Some other fusion approaches do not use any tritium, which significantly reduces the quantity of radioactive byproducts.
Neutrons from the fusion process activate the walls of the plasma vessel, which must be safely stored at the end of its operational life. These fusion-activated materials are low-level radioactive waste and pose a much lower risk than high-level radioactive materials from fission power plants. Radiation levels from fusion waste decrease significantly faster, too. Scientists and companies are researching newer materials to reduce activation levels as well as ways to recycle these materials after some period of time to be reused in fusion power plants.
Byproducts = special, long-term, high-level waste storage
Some of the byproducts fission creates are highly radioactive, requiring shielding and long-term storage. They are contained within the nuclear fuel and taken out of the reactor as spent fuel, which is high-level radioactive waste.
Nuclear waste from fission must be safely stored for a long time, up to one million years. It requires specialized management. It can be recycled into more fuel, but that requires additional processing.