Fission

(Mass of the separate protons + neutrons) − (mass of the bound nucleus). The nucleus is the **lighter** one.

The energy equivalent of the mass defect (E = mc²) — the energy needed to **pull the nucleus apart** into separate nucleons.

Binding energy ÷ number of nucleons (A). It lets you **compare the stability** of different nuclei fairly.

$E = mc^{2}$ — mass-energy equivalence (given in the data booklet). Here m is the mass defect.

Multiply Δm (in u) by **931.5**, because 1 u = 931.5 MeV c⁻².

**More tightly bound = more stable.** The curve peaks near iron (A ≈ 56), the most stable nuclei.

It moves **up** the steep left side of the curve — the product is more tightly bound — so energy is released.

It moves **up** the gentle right side of the curve toward iron — the products are more tightly bound — so energy is released.

Iron (around **A ≈ 56**) — the most tightly bound, most stable nucleus.

**Fusion** — it climbs the steep light-nuclei side, giving several times more MeV per nucleon than fission.

E = 0.030 × 931.5 ≈ 28 MeV total, then 28 ÷ 4 ≈ **7 MeV per nucleon**.

A **large nucleus splits** into two smaller nuclei, releasing **energy** and a few spare **neutrons**.

Fission **triggered** by a nucleus **absorbing a neutron**, which makes it unstable so it splits (not happening on its own).

Each fission releases neutrons that go on to cause **more** fissions — one fission triggers the next.

The chain **keeps itself going** without any extra neutrons being added from outside.

About **2 or 3** (plus two daughter nuclei and a lot of energy).

On average **exactly one** neutron per fission goes on to cause the **next** fission.

**N − 1.** One continues the chain; the rest must be lost or absorbed.

More than one continues, so the rate **grows** — the reaction is **supercritical**.

Fewer than one continues, so the reaction **dies out** — it is **subcritical**.

The products are slightly **lighter** than the original — that tiny **mass defect** becomes energy via **E = mc²**.

Subcritical = dying out; **critical = steady**; supercritical = growing. Set by how many neutrons continue per fission.

**Fuel**, **moderator**, **control rods** and **heat exchanger**.

It **slows down the fast neutrons** so they are more likely to cause the next fission.

They **absorb spare neutrons** to keep the chain reaction steady (or shut it down).

It is the material (e.g. **uranium-235**) that **undergoes fission** and releases the energy.

It **carries heat out of the core** to boil water into steam, which drives a turbine.

**Water** or **graphite** (both slow neutrons effectively).

**Boron** or **cadmium** (both strongly absorb neutrons).

A **slow** neutron is **much more likely** to be absorbed by U-235 and cause fission than a fast one — so slowing them keeps the chain reaction going.

More neutrons are **absorbed**, so the chain reaction **slows down**. Raising them speeds it up.

Both act on neutrons: the moderator **slows** them (helps fission); the control rods **absorb** them (limit fission).

Fission heats the core → the heat exchanger makes **steam** → steam spins a **turbine** → the turbine drives a **generator**.

$E = mc^{2}$ — the lost mass (mass defect) times the speed of light squared.