9.6 Nuclear Fission and Fusion

Nuclear fission and fusion are powerful energy-producing processes that reshape atoms. Fission splits heavy nuclei, while fusion combines light ones. Both convert mass to energy, releasing way more power than chemical reactions.

Fission uses uranium in nuclear plants, creating radioactive waste. Fusion, still in development, could offer cleaner energy using abundant hydrogen. Understanding these processes is key to our energy future and tackling climate change.

Nuclear Fission vs Fusion

Fundamental Processes and Energy Release

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• Nuclear fission splits heavy atomic nuclei (uranium-235) into lighter nuclei releasing energy and neutrons
• Nuclear fusion combines light atomic nuclei (hydrogen isotopes) to form heavier nuclei releasing energy
• Both processes convert mass to energy following Einstein's equation $E = mc^2$
• Fission and fusion release significantly more energy per unit mass than chemical reactions (nuclear power plants)
• Fission reactions controlled in nuclear reactors while fusion requires extreme temperatures and pressures
• Fusion overcomes electrostatic repulsion between nuclei at millions of degrees Celsius

Fuel Sources and Byproducts

• Fission typically uses heavy elements as fuel (uranium-235, plutonium-239)
• Fusion utilizes light elements as fuel (deuterium, tritium)
• Fission produces radioactive waste products requiring long-term storage and management
• Fusion generally results in less radioactive byproducts (primarily short-lived tritium)
• Fusion fuel more abundant and widely available (deuterium from seawater) reducing geopolitical tensions

Critical Mass in Fission

Chain Reaction Dynamics

• Critical mass defines minimum amount of fissile material needed to sustain nuclear chain reaction
• Neutrons released from one fission event trigger subsequent fissions in nearby nuclei
• Subcritical mass insufficient to sustain chain reaction
• Supercritical mass leads to exponential increase in reaction rate (nuclear weapons)
• Control rods in nuclear reactors absorb excess neutrons maintaining steady controlled chain reaction

Factors Influencing Critical Mass

• Critical mass depends on type of fissile material (uranium-235, plutonium-239)
• Material purity affects critical mass (higher purity lowers required mass)
• Geometry of fissile material impacts critical mass (spherical shape most efficient)
• Presence of neutron moderators reduces critical mass (heavy water, graphite)
• Neutron reflectors decrease critical mass by redirecting escaping neutrons (beryllium)
• Precise engineering and safety measures prevent uncontrolled reactions in nuclear power plants

Harnessing Nuclear Fusion

Plasma Confinement Approaches

• Magnetic confinement fusion uses powerful magnetic fields to contain plasma (tokamaks, stellarators)
• Inertial confinement fusion uses lasers or particle beams to compress fusion fuel (National Ignition Facility)
• Achieving sustained fusion reaction producing more energy than consumed known as "breakeven"
• ITER project represents international collaboration to demonstrate fusion power feasibility
• Alternative fusion concepts explored (magnetic mirror, z-pinch, magnetized target fusion)

Materials and Engineering Challenges

• Developing materials withstanding intense fusion conditions drives advanced materials science
• Plasma-facing components must endure extreme heat fluxes and neutron bombardment
• Superconducting magnets required for efficient magnetic confinement (high-temperature superconductors)
• Tritium breeding and handling systems needed for fuel self-sufficiency
• Neutron shielding essential to protect surrounding structures and personnel
• Advanced diagnostics and control systems required for plasma stability and performance optimization

Energy Yields and Impacts of Fission vs Fusion

Energy Output and Efficiency

• Fusion reactions release several times more energy per unit mass than fission reactions
• Fission of uranium-235 releases about 200 MeV per atom
• Fusion of deuterium and tritium releases about 17.6 MeV per reaction
• Fusion fuel energy density significantly higher than fission fuel (1 kg fusion fuel equivalent to millions of kg of fossil fuels)
• Fission reactors currently achieve thermal efficiencies around 33-37%
• Potential fusion reactor efficiencies estimated to be 40-50% or higher

Environmental and Safety Considerations

• Fission produces long-lived radioactive waste requiring storage for thousands of years
• Fusion primarily produces short-lived radioactive tritium decaying relatively quickly
• Neither fission nor fusion directly produce greenhouse gas emissions during energy generation
• Fission reactors carry risk of meltdowns and radiation leaks (Chernobyl, Fukushima)
• Fusion reactors pose no risk of meltdown due to different operational principles
• Fission technology mature and widely used while fusion still in research and development phase
• Fusion potential to provide nearly limitless clean energy source solving global energy and climate challenges