9.6 Nuclear Fission and Fusion

Nuclear Fission vs Fusion

Fundamental Processes and Energy Release

Nuclear fission and fusion are the two nuclear reactions that convert mass into energy. Fission splits heavy nuclei apart; fusion joins light nuclei together. Both follow Einstein's mass-energy equivalence, $E = mc^2$, and both release far more energy per unit mass than any chemical reaction.

• Nuclear fission occurs when a heavy nucleus (like uranium-235) absorbs a neutron and splits into two lighter nuclei, releasing energy and additional neutrons.
• Nuclear fusion occurs when light nuclei (like the hydrogen isotopes deuterium and tritium) combine to form a heavier nucleus, releasing energy in the process.

Fission can be controlled at relatively modest conditions inside a reactor. Fusion is a different story: the nuclei you're trying to combine are both positively charged, so they repel each other through the Coulomb (electrostatic) force. Overcoming that repulsion requires temperatures on the order of $10^7$ to $10^8$ K, which is why fusion is so much harder to achieve on Earth.

Fuel Sources and Byproducts

• Fission fuels are heavy elements: uranium-235 and plutonium-239.
• Fusion fuels are light elements: deuterium ($^2H$) and tritium ($^3H$).
• Fission produces radioactive waste products (fission fragments) that remain hazardous for thousands of years and require long-term storage.
• Fusion produces far less radioactive waste. The main radioactive byproduct is tritium, which has a half-life of about 12.3 years, much shorter than typical fission waste isotopes.
• Deuterium can be extracted from ordinary seawater, making fusion fuel extremely abundant compared to the mined uranium that fission requires.

Critical Mass in Fission

Chain Reaction Dynamics

When a uranium-235 nucleus undergoes fission, it releases 2 or 3 neutrons. If at least one of those neutrons triggers another fission event, the reaction sustains itself. Critical mass is the minimum amount of fissile material needed to make this happen.

• Subcritical: Too few neutrons find other nuclei. The chain reaction dies out.
• Critical: Exactly one neutron per fission event, on average, causes another fission. The reaction rate holds steady. This is the target state in a nuclear reactor.
• Supercritical: More than one neutron per fission triggers additional fissions, causing an exponential increase in the reaction rate. This is the principle behind nuclear weapons.

In a reactor, control rods made of neutron-absorbing materials (like cadmium or boron) are inserted or withdrawn to keep the reaction at criticality.

Factors Influencing Critical Mass

Several variables determine how much fissile material you need to reach criticality:

• Material type: Plutonium-239 has a smaller critical mass than uranium-235.
• Purity (enrichment): Higher concentrations of the fissile isotope lower the required mass. Weapons-grade uranium is enriched to over 90% U-235, while reactor-grade is typically 3–5%.
• Geometry: A sphere minimizes surface area relative to volume, reducing the fraction of neutrons that escape. Spherical shapes therefore have the lowest critical mass.
• Neutron moderators: Materials like heavy water ($D_2O$) or graphite slow neutrons down, increasing their probability of causing fission and effectively lowering the critical mass.
• Neutron reflectors: A shell of material like beryllium surrounding the fissile core reflects escaping neutrons back in, also reducing the critical mass.

Reactor design relies on precise engineering of all these factors to maintain a safe, controlled chain reaction.

Harnessing Nuclear Fusion

Plasma Confinement Approaches

At fusion temperatures, matter exists as plasma, a state where electrons are stripped from nuclei. The central challenge is confining this plasma long enough and at high enough density for fusion reactions to occur. Two main approaches exist:

1. Magnetic confinement uses powerful magnetic fields to hold the plasma in place. The most studied device is the tokamak, a doughnut-shaped chamber where magnetic field lines spiral around to keep the plasma from touching the walls. Stellarators are a related design with a more complex, twisted geometry. The ITER project in France is an international tokamak designed to demonstrate that fusion can produce more energy than it consumes.

2. Inertial confinement uses intense lasers or particle beams to rapidly compress a tiny pellet of fusion fuel, heating and squeezing it until fusion occurs. The National Ignition Facility (NIF) in the U.S. uses this approach and achieved fusion ignition (energy output exceeding laser energy delivered to the fuel) in late 2022.

The goal for any fusion device is breakeven: producing at least as much energy from fusion as was put in to heat and confine the plasma.

Materials and Engineering Challenges

Even once plasma confinement works, building a practical fusion reactor involves serious engineering problems:

• Plasma-facing components must withstand extreme heat fluxes and constant bombardment by high-energy neutrons.
• Superconducting magnets are needed for efficient magnetic confinement. Recent progress with high-temperature superconductors has enabled stronger, more compact magnet designs.
• Tritium breeding: Tritium doesn't occur naturally in useful quantities. Reactor designs plan to breed it by surrounding the plasma with a lithium blanket, where neutrons from the fusion reaction convert lithium into tritium.
• Neutron shielding is essential to protect the reactor structure and personnel from the 14.1 MeV neutrons produced in deuterium-tritium fusion.
• Plasma control requires advanced diagnostics and real-time feedback systems to prevent instabilities that can disrupt the plasma.

Energy Yields and Impacts of Fission vs Fusion

Energy Output and Efficiency

Here's where the numbers can seem contradictory, so pay attention. A single fission event releases about 200 MeV per nucleus, while a single D-T fusion event releases about 17.6 MeV per reaction. So fission releases more energy per reaction.

However, fusion releases more energy per unit mass of fuel. Why? Because hydrogen isotopes are far lighter than uranium atoms. Gram for gram, you get many more fusion reactions than fission reactions, and the total energy output per kilogram of fuel is several times higher for fusion. To put it in perspective, 1 kg of fusion fuel could release energy equivalent to burning roughly 10 million kg of coal.

Current fission reactors achieve thermal efficiencies of about 33–37% (similar to fossil fuel plants, limited by thermodynamic constraints). Projected fusion reactors are estimated at 40–50% efficiency or higher, partly because of the higher operating temperatures involved.

Environmental and Safety Considerations

Fusion's safety profile is fundamentally different from fission's. A fusion reactor contains only a tiny amount of fuel at any moment, and if anything goes wrong, the plasma cools within seconds and the reaction stops on its own. There's no equivalent of a meltdown scenario.

Neither process emits $CO_2$ during energy generation, making both relevant to addressing climate change. Fission is available now and already provides about 10% of global electricity. Fusion, if successfully commercialized, could provide a nearly limitless, clean energy source, but significant engineering challenges remain before that becomes reality.