Nuclear Fission and Fusion

In fission, a large unstable nucleus (typically uranium-235 or plutonium-239) absorbs a SLOW (thermal) neutron and becomes highly unstable. It then splits roughly in half into two smaller daughter nuclei, releases 2–3 more neutrons, and releases a large amount of energy (mostly as kinetic energy of the daughter nuclei).

$^{235}_{92}\text{U} + {}^1_0 n \to {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3\,{}^1_0 n + \text{energy}$

(One example — daughter nuclei vary.)

Chain reaction. The neutrons released can be absorbed by other U-235 nuclei, each causing another fission, releasing more neutrons. If on average more than one neutron from each fission causes another fission, the reaction grows exponentially — uncontrolled chain reaction (nuclear bomb).

In a reactor, control rods (e.g. boron, cadmium) absorb excess neutrons so that EXACTLY ONE neutron from each fission causes another. The reaction is critical and steady — controlled chain reaction. Heat from the daughter nuclei heats water → steam → drives a turbine → electricity.

In fusion, two LIGHT nuclei join together to form a heavier nucleus, releasing energy. Example (in the Sun):

$^2_1\text{H} + {}^3_1\text{H} \to {}^4_2\text{He} + {}^1_0 n + \text{energy}$

(Deuterium + tritium → helium-4 + neutron.)

Fusion requires extreme temperature (~10⁷ K in the Sun's core, ~10⁸ K in lab reactors) and pressure to overcome the electrostatic repulsion between the two positively charged nuclei.

Where it occurs:

• Inside the Sun and stars — main energy source of the universe.
• In hydrogen bombs (uncontrolled).
• In experimental fusion reactors (ITER, JET) — research goal is commercial electricity by 2050+.

Why is fusion attractive?

• Fuel (hydrogen isotopes) extremely abundant.
• Products (helium) are not radioactive long-term.
• Releases more energy per kg than fission.
• No long-lived radioactive waste.
• No risk of meltdown.

Why is it not yet commercial?

• Sustaining the high T and P stably is extremely hard.
• Containing 100-million-K plasma without melting reactor walls requires magnetic confinement (tokamaks) or inertial confinement.

Feature	Fission	Fusion
Nucleus size	Large (U-235)	Small (H isotopes)
Process	Splits	Joins
Conditions	Slow neutron triggers	Extreme T and P
Waste	Long-lived radioactive	Mostly non-radioactive
Fuel	Uranium, plutonium (limited)	Hydrogen isotopes (abundant)
Status	Commercial (UK ~14 % electricity)	Experimental
Example reaction	$^{235}_{92}U + n \to ^{141}_{56}Ba + ^{92}_{36}Kr + 3n$	$^2_1H + ^3_1H \to ^4_2He + n$

Both release energy because the products have slightly less total mass than the reactants — the 'missing mass' has become energy via Einstein's $E = mc^2$ (NOT assessed at GCSE numerically).