Fission and Fusion

Three nuclear processes release energy:

• Fission — a large nucleus splits.
• Fusion — small nuclei join.
• Radioactive decay — α, β, γ emission from unstable nuclei (covered in the Radioactivity subtopic).

U-235 fission step-by-step (spec 7.18, 7.19).

1. A SLOW (thermal) neutron strikes a U-235 nucleus.
2. The neutron is absorbed → temporary U-236 nucleus, which is highly unstable.
3. The unstable nucleus SPLITS into TWO daughter nuclei (commonly Ba-141 + Kr-92, or Sr-90 + Xe-143).
4. A small number (typically 2 or 3) of fast neutrons are released.
5. Most of the energy released appears as KINETIC ENERGY of the daughter nuclei and neutrons. This kinetic energy is rapidly transferred to thermal energy of the reactor fuel and coolant.

The daughter nuclei are themselves RADIOACTIVE (spec 7.19) — they continue to emit radiation for years, which is why spent nuclear fuel is dangerous and must be carefully stored.

Balancing the equation. Nucleon AND proton numbers are conserved.

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

• Nucleons: $235 + 1 = 141 + 92 + 3 \times 1 = 236$. ✓
• Protons: $92 + 0 = 56 + 36 + 0 = 92$. ✓

Chain reaction (spec 7.20). Each fission releases 2-3 neutrons. If at least ONE of these strikes another U-235 nucleus and causes ANOTHER fission, the reaction sustains itself.

• One neutron sustains a fission → ONE neutron triggers next fission → STEADY power.
• More than one → reactions multiply → EXPONENTIALLY GROWING (uncontrolled) → atomic bomb in a worst-case scenario.
• Less than one → reactions die out.

The reactor designer's job is to keep this number at exactly 1.

Moderator (spec 7.21) — graphite or water. Fission neutrons emerge FAST. Fast neutrons are poorly absorbed by U-235. The moderator surrounds the fuel rods; fast neutrons collide with the moderator's nuclei and SLOW DOWN to thermal speeds at which U-235 readily absorbs them. So the moderator's job is to MAKE THE CHAIN REACTION POSSIBLE.

Control rods (spec 7.21) — boron or cadmium. These materials strongly ABSORB neutrons. Operators raise or lower the rods between the fuel rods:

• Rods LOWER → more neutrons absorbed → fewer reach U-235 → REACTION SLOWS.
• Rods RAISE → fewer neutrons absorbed → reaction SPEEDS UP.
• Rods FULLY LOWERED → reactor shut down.

Control rods are the ON/OFF and throttle for the reactor.

Shielding (spec 7.22). Thick concrete (often 3-5 m), sometimes combined with lead, surrounds the reactor core. Absorbs neutrons and gamma rays that escape from the fuel, protecting operators and the environment from harmful radiation.

Cooling. Not on the spec but worth noting: the kinetic energy of fission products heats the coolant (water, gas, liquid sodium), which is used to drive a turbine and generate electricity.

Fusion vs fission (spec 7.23). Opposite processes.

	Fission	Fusion
Starts with	One LARGE unstable nucleus	TWO LIGHT nuclei
Process	SPLITS	JOIN
Products	2 daughters + neutrons	1 heavier nucleus
Energy released	Yes (kinetic energy of products)	Yes (mass→energy via $E=mc^2$)
Usable on Earth?	Yes — nuclear power plants	Not currently practical

Fusion mechanism (spec 7.24). Two LIGHT nuclei (typically isotopes of hydrogen — deuterium $^2_1 H$ and tritium $^3_1 H$) JOIN to form a heavier nucleus (helium $^4_2 He$) plus a free neutron:

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

The TOTAL MASS of the products is SLIGHTLY LESS than the total mass of the reactants. This mass difference is converted to ENERGY by Einstein's $E = mc^2$.

Fusion in stars (spec 7.25). This is how the Sun and other stars produce their energy. Hydrogen nuclei fuse in the core to form helium. Massive stars also fuse helium → carbon → oxygen → ... → iron, producing all the elements up to iron during their lifetimes. Heavier elements come from supernovae (the explosive deaths of massive stars).

Why fusion is hard on Earth (spec 7.26). All atomic nuclei are POSITIVELY CHARGED (they contain protons). When you try to bring two nuclei together to fuse, the ELECTROSTATIC REPULSION between them is enormous at close range. To overcome it, the nuclei need:

• High temperature (typically 10⁷-10⁸ K) — high kinetic energy so they collide hard enough to fuse despite the repulsion.
• High pressure / high density — so the nuclei are close together and collisions are frequent.

These are the conditions in the Sun's core. Replicating them in a contained reactor on Earth is the great unsolved problem of energy science — tokamak fusion reactors like ITER are getting closer but commercial fusion power is still decades away.

Why fusion is attractive. Fuel is abundant (deuterium in sea water, tritium can be bred from lithium), no long-lived radioactive waste (the helium product is inert), no possibility of meltdown.