Fusion vs. Fission

Fusion and fission are alike in that both extract the energy stored inside atomic nuclei, yet the direction of the reaction, the fuel, and the nature of their safety are completely different. On this page, we place the two side by side and compare them, so that you can understand step by step, from intuition to the research frontier, why fusion is expected to be the energy source of the next generation.

Start with Intuition (High School)

An atomic nucleus is made of protons and neutrons packed tightly together. How stable this packing is depends on the type of nucleus, and it is most stable right in the middle, around iron.

Picture this as a slope. Iron sits at the bottom of a valley, with slopes spreading out on either side. On the left slope are light nuclei like hydrogen and helium; on the right slope are heavy nuclei like uranium. Whenever a nucleus rolls down the slope toward the iron at the bottom, it releases the leftover energy to the outside.

Fusion is the reaction that brings light nuclei on the left slope together to form a slightly heavier nucleus, moving closer to the valley bottom. The Sun shining by turning hydrogen into helium is exactly this. Fission is the reaction that splits a heavy nucleus on the right slope into two medium-sized nuclei, again moving closer to the valley bottom. This is what nuclear power plants use.

Even though both are motions “toward greater stability,” you won’t get confused if you remember that fusion climbs up from the light side while fission slides down from the heavy side.

The picture of safety is contrasting as well. In fission, a neutron flung out by one reaction splits the next nucleus, which splits yet another, and this chain keeps releasing energy. Because the reaction sustains itself like falling dominoes, you must always keep a mechanism working to stop it. Fusion, on the other hand, only continues if you forcibly maintain the special state of ultra-high-temperature plasma. It is like a gas stove whose flame goes out the moment you let go: if the device is disturbed, the reaction stops on its own.

Understand the Physics (Undergraduate)

The starting point for comparison is the binding energy per nucleon curve. If you take the mass number $A$ on the horizontal axis and the binding energy per nucleon $B/A$ on the vertical axis, you get a mountain-shaped curve with a maximum of about $8.8$ MeV near iron ($A \approx 56$). The larger this $B/A$, the more stable the nucleus.

Whether you fuse light nuclei or split heavy ones, if $B/A$ increases across the reaction, that difference is released as energy. The released energy is given through the mass defect $\Delta m$ by

$$E = \Delta m\, c^2$$

where $c$ is the speed of light, expressing how a tiny mass difference turns into an enormous amount of energy.

Let’s compare with representative reactions. In fusion, the reaction of deuterium $D$ and tritium $T$

$$D + T \rightarrow {}^4\mathrm{He}\ (3.5\ \mathrm{MeV}) + n\ (14.1\ \mathrm{MeV})$$

releases $17.6$ MeV per reaction. In fission, the reaction in which uranium-235 absorbs a neutron and splits releases about $200$ MeV per event.

If the absolute value per reaction is more than ten times larger for fission, why is fusion said to have “high energy density”? The key is the number of nucleons involved in the reaction. Fusion produces $17.6$ MeV from a system of mass number 5, while fission produces about $200$ MeV from a system of mass number 235, so per nucleon this comes to about $3.5$ MeV for fusion and about $0.85$ MeV for fission. From the same mass of fuel, fusion can extract roughly four times as much energy.

Let’s also look at the difference in fuel resources quantitatively. Uranium, the fission fuel, mainly uses ${}^{235}\mathrm{U}$, which makes up only $0.7\%$ of natural uranium, so its recoverable resources are estimated at several decades to about a century at current consumption. Deuterium, the fusion fuel, exists in seawater at a proportion of about $0.015\%$ of hydrogen and is effectively inexhaustible. Tritium barely exists in nature, but it can be produced inside the reactor by striking lithium with neutrons, as in

$${}^6\mathrm{Li} + n \rightarrow {}^4\mathrm{He} + T$$

(tritium breeding). Lithium resources, including those in seawater, can also be supplied over the long term.

The physical difference in safety lies in the reaction’s self-sustaining mechanism. Fission operates by maintaining criticality, that is, a state in which on average one of the neutrons produced by a fission triggers the next fission. Control rods and other means manage this neutron balance, keeping the effective multiplication factor $k_{\mathrm{eff}}$ near 1. The $D$-$T$ fusion reaction has no such neutron chain, and its reaction rate is determined by the density $n$, temperature $T$, and confinement time $\tau_E$. If the device is disturbed, the plasma cools and the reaction stops naturally, so a runaway increase in output cannot occur in principle.

Deepen the Theory (Graduate)

A more essential quantity that separates the safety of fission and fusion is decay heat. In fission, the fission fragments produced are unstable radioactive nuclides that continue to generate heat through beta and gamma decay even after operation stops. This decay heat reaches a few $\%$ of the rated thermal output right after shutdown and decays over time, but if it is not removed the core is damaged. This is why forced cooling over a long period after shutdown is indispensable, and loss of cooling function becomes the main scenario for a severe accident.

In a fusion reactor as well, structural materials irradiated by neutrons become activated and produce decay heat, but its magnitude is orders of magnitude smaller than the core decay heat of a fission reactor, and in many designs it is at a level that can be handled by passive natural cooling. The fuel present inside the reactor is only a few grams at any time, and the absence of any mechanism that chain-amplifies even as burning continues fundamentally simplifies the safety design.

The nature of radioactive waste also becomes clear when understood from its formation mechanism. Fission waste divides into two categories. One is the fission fragments themselves, which include a variety of medium-mass nuclides. The other is the actinides produced when uranium absorbs neutrons, such as plutonium and americium, which include long-lived nuclides with half-lives ranging from thousands to tens of thousands of years. These high-level radioactive wastes presuppose deep geological disposal, isolating them deep underground for tens of thousands of years.

Fusion waste has a different origin. The direct product of the $D$-$T$ reaction is helium, which is a harmless, stable nucleus. The source of radioactivity is almost entirely the activation caused when the $14.1$ MeV fast neutrons strike the structural materials of the reactor wall and blanket. Therefore the amount and lifetime of the waste change greatly depending on which structural materials you choose. This is an important design degree of freedom in fusion. If you use materials such as reduced-activation ferritic/martensitic steel, which remain only short-lived nuclides even when activated, the waste is estimated to decay to a reusable level within several decades to about a century. The fact that actinides are not produced in principle is a decisive difference from fission.

From the standpoint of the theoretical framework, the two deal with fundamentally different physical systems. The design of a fission reactor centers on neutronics based on the neutron transport equation, solving for criticality and neutron flux distributions. The design of a fusion reactor centers on plasma physics dealing with populations of charged particles, with equilibrium and stability analysis via magnetohydrodynamics (MHD), and transport and heating analysis based on kinetic theory, being the essentials. Even within the same “nuclear energy,” the languages of the theories required are entirely different.

Placing fusion alongside other renewable energies makes its position even clearer. Solar and wind power require no fuel and emit no carbon during operation, but they are variable sources whose output depends on the weather and time of day, and they must be considered together with large-scale energy storage and supply-demand balancing. Fusion, like solar and wind, emits no carbon dioxide during operation and produces neither the long-lived high-level waste nor the actinides of fission, yet it differs in that it can become a baseload source supplying large, stable output independent of the weather. In other words, fusion can be understood as a technology aiming for a middle position between the two: compensating for the variability that is the weakness of renewable energy, while avoiding the long-lived waste and runaway risk that are the weaknesses of fission.

Research Frontier (PhD)

Toward the practical realization of fusion, a number of challenges are being researched in real time. One is the problem of reactor materials. The $14.1$ MeV neutrons knock atoms out of place and damage the material structure, and produce helium and hydrogen through transmutation. Research is underway to find materials that endure this radiation damage over long periods, and to demonstrate low-activation materials. In the specialist literature, indicators such as displacement per atom (dpa) and neutron fluence appear frequently.

Another is the tritium balance. To operate a reactor self-sufficiently, it must produce more tritium in the blanket than it consumes, and the question is whether the tritium breeding ratio (TBR) can be kept greater than 1. The design and demonstration of the breeding blanket is an important research theme.

On the core plasma side, the main challenges being researched are reconciling confinement performance with steady-state operation, the exhaust of heat and particles from the edge plasma (divertor exhaust), and the avoidance or mitigation of disruptions, in which confinement is suddenly lost.

From the standpoint of comparison, an indicator that has drawn attention in recent years measures how far fusion has come. Results are often discussed in terms of the energy gain $Q$, that is, the ratio of output energy to input energy, and in December 2022 the U.S. NIF reported achieving ignition in inertial confinement fusion, obtaining a fusion output exceeding the input laser energy. In magnetic confinement, ITER is under construction with a target of $Q \geq 10$. Whereas fission is an established baseload source, fusion is at the stage of how high and how sustainably this $Q$ can be achieved, and this goal is being pursued by different methods in each device.

Check Your Understanding

Q1. On which side of the binding energy curve, and in which direction, do fusion and fission move the nuclei?

A Fusion combines nuclei lighter than iron and moves them toward the heavy side, while fission splits nuclei heavier than iron and moves them toward the light side; both approach the peak at iron B Both fusion and fission split nuclei heavier than iron C Fusion combines nuclei heavier than iron, and fission splits nuclei lighter than iron D Both move away from iron and release energy

Q2. The energy released per reaction is larger for fission, yet fusion has higher energy density per unit mass. Why?

A Because fusion has a higher reaction temperature B Because the speed of light acts more strongly in fusion C Because far fewer nucleons are involved in fusion, so on a per-nucleon basis fusion comes out ahead D Because in fission most of the released energy is trapped in the waste

Q3. Why is cooling still indispensable in a fission reactor even after operation stops?

A Because the neutron chain reaction continues naturally even after shutdown B Because the fission fragments are radioactive and keep producing decay heat after shutdown, so forced cooling is needed C Because the plasma is kept at high temperature for a long time D Because uranium fuel ignites when it contacts air

Q4. Why are the amount and lifetime of fusion's radioactive waste less severe than fission's?

A Because fusion produces no radioactive substances at all B Because all of the waste produced can be reused as fuel C Because the direct product helium is harmless and no actinides are produced, and the source of radioactivity is limited to neutron activation of structural materials D Because all of the waste is gaseous and disperses into the atmosphere

Q5. How does fusion compare to renewable energies such as solar and wind power?

A Like solar and wind, it is a variable source dependent on the weather B It shares the trait of emitting no carbon dioxide during operation, but differs in that it can become a baseload source supplying large, stable output independent of the weather C It differs from renewable energy in emitting large amounts of carbon dioxide during operation D It differs from renewable energy in producing long-lived high-level waste

Related Topics

• What Is Fusion: Learn the principle of fusion itself from the ground up.
• Fusion Safety: A detailed explanation of why the absence of a chain reaction and the small decay heat lead to safety.
• Radioactive Waste: A deeper look at the nature of fusion waste and its differences from fission.