# Chap 43: Energy from the Nucleus

## Sections

| Sec | Topic |
|-----|------|
| 43-1 | [Nuclear Fission](43-1-nuclear-fission.ipynb) |
| 43-2 | [The Nuclear Reactor](43-2-the-nuclear-reactor.ipynb) |
| 43-3 | [A Natural Nuclear Reactor](43-3-a-natural-nuclear-reactor.ipynb) |
| 43-4 | [Thermonuclear Fusion: The Basic Process](43-4-thermonuclear-fusion-the-basic-process.ipynb) |
| 43-5 | [Thermonuclear Fusion in the Sun and Other Stars](43-5-thermonuclear-fusion-in-the-sun-and-other-stars.ipynb) |
| 43-6 | [Controlled Thermonuclear Fusion](43-6-controlled-thermonuclear-fusion.ipynb) |

## Review & Summary

:::{glossary}
Nuclear Fission
  A heavy nucleus can split into lighter fragments (**fission**), releasing energy because the binding energy per nucleon is greater for medium-mass nuclei. **Fissile** nuclides such as $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ undergo fission when struck by slow (thermal) neutrons. A **chain reaction** occurs when neutrons from one fission trigger further fissions. The **critical mass** is the minimum amount of fissile material needed for a sustained chain reaction.

The Nuclear Reactor
  A **nuclear reactor** uses controlled fission to produce heat, which is converted to electricity. A **moderator** (e.g., water, graphite) slows neutrons to thermal energies where the fission cross section is large. **Control rods** (e.g., boron, cadmium) absorb neutrons to regulate the power level. A **coolant** removes heat from the core. **Breeder reactors** convert fertile material (e.g., $^{238}\mathrm{U}$) into fissile material ($^{239}\mathrm{Pu}$), producing more fuel than they consume.

A Natural Nuclear Reactor
  The Oklo deposit in Gabon: about 2 billion years ago, when $^{235}\mathrm{U}$ was more abundant (~3% vs ~0.7% today), geological conditions allowed a natural chain reaction. Evidence includes anomalous isotope ratios of fission products.

Thermonuclear Fusion: The Basic Process
  Light nuclei can fuse to form heavier ones, releasing energy when the product lies below iron on the binding-energy-per-nucleon curve. The **deuterium-tritium** reaction has the largest cross section at achievable temperatures:

  $$
  d + t \to {}^4\mathrm{He} + n + 17.6\,\mathrm{MeV}
  $$ (eq-43-dt-fusion)

  where the energy is shared as kinetic energy of the products.

Thermonuclear Fusion in the Sun and Other Stars
  The **proton-proton chain** and **CNO cycle** fuse hydrogen to helium in stars. **Gravitational confinement** compresses and heats the plasma, sustaining the high temperature ($\sim 10^7$ K) and density needed for fusion.

Controlled Thermonuclear Fusion
  **Tokamaks** (magnetic confinement) and **inertial confinement** (laser or particle-beam compression) aim to achieve fusion on Earth. **Ignition** requires the **Lawson criterion:** $n\tau T$ above a threshold, where $n$ is the particle density, $\tau$ is the confinement time, and $T$ is the temperature. Fusion offers abundant fuel (deuterium from seawater) and produces less long-lived radioactive waste than fission.
:::