Nuclear Fusion vs. Fission: A Physicist Clarifies the Distinction

Nuclear power generates about 10% of the world’s electricity, with countries like France relying on it for nearly 70%. Tech giants like Google are also turning to nuclear energy to power their demanding data centers.

The energy for nuclear power comes from atomic binding energy, released through two primary processes: fission and fusion. Fission splits heavy atoms into lighter ones, while fusion combines light atoms into heavier ones. Both methods yield significant energy; for instance, a single fission reaction of U-235, commonly used in power plants, releases over 6 million times more energy than a chemical reaction with coal.

What is Fission?

Fission powers current nuclear plants and occurs when a neutron strikes a uranium atom, splitting it and releasing more neutrons. This initiates a chain reaction, producing substantial energy. To convert this energy into electricity, heat exchangers turn water into steam, driving turbines.

Controlling fission involves regulating neutron supply through “control rods” that absorb neutrons. Accidents like Chernobyl have occurred when these rods fail or coolant circulation stops. Third-generation reactors improve safety with passive features that operate without active controls, relying instead on natural physical principles. The first of these, the Kashiwazaki 6 and 7 reactors in Japan, exemplify this advancement.

However, a major challenge remains: fission byproducts are radioactive for thousands of years, and reprocessed fuel can potentially be used for nuclear weapons. Fission technology is scalable, with plants ranging from the massive Kashiwazaki-Kariwa Nuclear Power Plant at 7.97 gigawatts to smaller reactors generating about 150 megawatts, as seen in nuclear submarines.

What is Fusion?

Fusion, the process that powers the Sun, occurs when atoms fuse together. The easiest fusion reaction involves isotopes of hydrogen, deuterium, and tritium, producing four times more energy per unit mass than U-235 fission. Deuterium is abundant, but tritium is rare and radioactive, requiring a “lithium blanket” in fusion plants to generate it.

Currently, creating a fusion reaction outside of a lab is challenging due to the extreme temperatures needed—around 150 million degrees Celsius. At these temperatures, fuel exists as plasma, and the byproduct is helium, a non-radioactive gas.

The leading method for achieving sustained fusion is toroidal magnetic confinement, which uses a doughnut-shaped magnetic field to contain plasma at high temperatures. Unlike fission, the main hurdle is not an uncontrolled meltdown but rather initiating the fusion reaction itself.

A key challenge for toroidal magnetic confinement fusion, which is the focus of much research, is demonstrating a burning self-heated plasma. This occurs when the heating power from the reaction itself becomes the primary energy source. This is the goal of the publicly funded ITER project, the largest fusion experiment globally, and the privately funded SPARC experiment at MIT.

However, many scientists agree that fusion won’t be commercially viable until at least 2050.

A Climate Solution?

Many ask whether nuclear power can mitigate climate change. I have colleagues in climate science, including my late wife, a renowned climate scientist.

The consensus is clear: it’s too late to completely halt climate change. The world must urgently reduce carbon dioxide emissions to minimize catastrophic impacts—a task that should have begun decades ago.

In this context, fission is part of the global solution, alongside the widespread adoption of renewable energy sources like wind and solar. In the long run, fusion could potentially replace fission, as it offers a more abundant fuel supply, significantly smaller waste volume and longevity, and a technology that cannot be weaponized.

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