Fusion and Fission

The wiki articles for these two topics contain a lot of great information, written in complex English and giving textbook vibes. Let’s break down their highlights to understand them!

Nuclear Fusion (Wikipedia link)

Usually deuterium and tritium (hydrogen variants) or helium (variants?)
The mass changes from reactants to products
- Observed as energy absorbing or being released during reaction
- Exothermic: reactions releasing energy
  - Usually what occurs when producing nuclei lighter than iron-56 or nickel-62 (presumably the same mass)
- Endothermic: reactions absorbing energy
  - Usually what occurs when the resulting product nuclei are very large
- In a supernova, nuclei are formed heavier than iron

Fusion History

First self-sustaining fusion in November 1052 hydrogen bomb (H-bomb / thermonuclear bomb)
- Reaction must be self-sustained and also controlled to use fusion as an energy source
US Dept of Energy in December 2022: first fusion reaction reported where energy succeeded in breaking even during human-caused fusion (2.05 megajoules 15 MJ)
Private fusion companies funded with $2.1B during 2021

Fusion Process

Caused by interaction of nuclear force and Coulomb force
- Nuclear force: holds protons and neutrons together in nucleus (due to strong interaction)
  - Most effective at short distances
- Coulomb force: positively-charged protons in nucleus repel each other
- When lighter atomic nuclei are pushed closely together, the Nuclear force overwhelms the Coulomb force, and all excess energy is released
  - Less effective as nuclei get larger, as Coulomb force starts to match or overwhelm Nuclear force (due to Nuclear force decreasing)
- Nucleosynthesis is when (most) elements are produced by fusion in stars
  - Our sun is a “main-sequence star”
    - Fuses hydrogen nuclei into helium
    - 6% of mass is exerted using alpha particles or EM radiation or other energy
  - Energy released is much larger than exothermic chemical reactions, because binding energy of nucleus is much more than energy holding electrons to a nucleus (different by factor of ~1 million)
  - Sounds like overall fusion reactions produce more energy per unit mass, while the individual fission reactions produce more energy
  - Fusion in stars
    - Mostly fusion of hydrogen to form helium (proton-proton chain reaction)
      - Fuses four protons into one alpha particle, with release of two positirons and two neutrinos (also two protons are turned into two neutrons?) and releases energy
    - In heavier stars, other processes such as the Carbon-Nitrogen-Oxygen cycle are important
    - As fraction of hydrogen remaining is reduced, heavier elements begin to be produced
      - Heaviest produced in a giant supernova at end of a star’s life
    - Artificial fusion
      - Thermonuclear
      - Beam-beam (beam-target)
      - Muon-catalyzed
        
        At ordinary temps
        
        High energy required to create muons, so likely will not produce net energy increase

Nuclear Fission (Wikipedia link)

Otto Hahn and Fritz Strassman discovered in 1938
Usually uses heavier elements (uranium, thorium, and plutonium) as inputs
In comparison with fusion, where atoms are fused together, in fission we see large atoms split up into smaller ones
- In addition to smaller atoms, fission produces gamma photons and energy (kinetic energy of particles and EM radiation both produced) (exothermic)
A form of “nuclear transmutation”: resulting fragments different elements from original parent atoms
Most fissions produce 2 charged fragments, occasionally 3
“Unpredictable composition”: nuclear power and nuclear weapons (“self-sustaining nuclear chain reactions”) do not break apart in a perfectly predictable way
Lots more energy released per atom and per mass and per reaction than methane and fossil fuels.
Products: some products are more radioactive than products in other reactions, which are dangerous in the long-term
- Some strategies to reduce their harm
Mechanism
- Radioactive decay: random, happens “spontaneously”, but at generally consistent rates over time
  - Used in isotopic geochemistry, which I will discuss in a future article
- Nuclear reactions
  1. An incident particle fuses with a target, which forms a “compound system”
  2. One of a few types of fission reactions occur
    - Usually binary fission, where 2 atoms (plus neutrons) are produced
      - Assuming that atoms are what is referred to as fragments
    - 2-4% of time we get ternary fission, where 3 fragments are produced

Nuclear fuels: chemical element isotopes which can sustain a fission chain reaction
- “Fissile”
- Most common fuels: Uranium-235 and Plutonium-239
- These two fuels and U-233 can be used as both fissile nuclear fuels and can undergo spontaneous fission (radioactive decay)
- Most neutrons usually escape rapidly from fuel, becoming a “free neutron” in the air, and decay into protons and beta particles in ~15 minutes
  - If, instead of escaping, those neutrons hit other fuel atoms, they can continue inducing other reactions
  - This will happen if there is a lot of fuel in one place, or if the neutrons are adequately contained (physically)
  - Fun fact: freshly created neutrons from fission move ~7% the speed of light
- Critical mass: a mass of an element which could support a sustained nuclear chain reaction
- Critical assembly: an assembly of an element which could support a sustained nuclear chain reaction
Fission reactors
- Usually only built with of 1 of 3 goals:

Power reactors: produce heat for nuclear power
1. Either in a generating station or a nuclear submarine
Research reactors: produce neutrons / activate radioactive sources for scientific / research purposes
Breeder reactors: produce nuclear fuels in bulk
- Fission bombs
  - One class of nuclear weapons (different from fusion bombs)
  - Generates as much energy as possible from before reactor explodes, which stops the chain reaction
  - Fundamental physics same as those in the chain reactions of reactors
    - But goals are different: fission bombs release all their energy at once, while fission reactors want to release energy slowly and steadily for power generation
      - Overheating of a reactor can and has led to metdowns
      - Will never have as much destructive power as a weapon because of “low Uranium enrichment”