Nuclear Science Fundamentals

The Building Blocks of Matter

Atoms are incredibly small—about 100 million could fit across a single centimetre. Yet each atom has an internal structure with three types of particle.

Three Types of Particle

Protons are in the nucleus and carry a positive charge (+1). The number of protons defines the element—uranium always has 92, carbon always has 6. This is called the atomic number.

Neutrons are also in the nucleus but have no charge. They help hold the nucleus together by counteracting the repulsion between positively charged protons.

Electrons orbit the nucleus and carry a negative charge (−1). In a neutral atom, the number of electrons equals the number of protons—the positive and negative charges balance out, giving the atom no overall charge.

Ions: When Electrons Don't Balance

When protons and electrons are equal, their charges cancel out and the atom is electrically neutral. But atoms can gain or lose electrons, creating an imbalance:

Ions are chemically reactive because they "want" to return to a balanced state. This charge imbalance is different from nuclear instability (which we'll cover shortly) — ions are electrically unstable but their nuclei can be perfectly stable.

💡 Try adding/removing electrons in the atom builder above to create ions!

What Makes an Element an Element?

Here's the key rule: the number of protons defines which element an atom is. Full stop.

Change the number of protons, and you've got a completely different element. Add one proton to gold (79 protons) and you get mercury (80 protons)—a totally different substance.

So What Are Isotopes?

While protons define the element, neutrons can vary. You can add or remove neutrons without changing what element the atom is. These variations are called isotopes.

How Do We Name Isotopes?

Isotopes are named by their mass number—the total count of protons + neutrons. For example:

All three are carbon (6 protons each), but they have different masses because of the different neutron counts.

Where Do Isotopes Come From?

Every isotope on Earth was created in one of these ways:

Why Are Some Isotopes Stable and Others Aren't?

Inside every nucleus, there's a constant tug-of-war between two forces: Electromagnetic and Strong.

Think of it like magnets. Protons are all positively charged (+), and just like when you try to push two magnet ends with the same pole together, they repel each other. So inside a nucleus, all those protons are constantly trying to fly apart.

So what holds them together? The Strong Force. This force acts like incredibly powerful glue at very short range, pulling protons and neutrons together and overpowering the electrical repulsion. Without it, no nucleus with more than one proton could exist.

So why are some isotopes stable? It comes down to the neutron-to-proton ratio. Neutrons help hold the nucleus together—they're bound by the Strong Force just like protons, but they don't repel each other because they have no charge. They act as "buffers" between the repelling protons. For small atoms, a roughly 1:1 ratio of neutrons to protons works. For larger atoms, you need more neutrons to help overcome the increasing repulsion between all those protons—uranium needs about 1.5 neutrons per proton.

Get the ratio wrong in either direction, and the nucleus is unstable:

📍 Alpha particles and beta decay will be explained in the Radiation section. For now, just understand that stability depends on getting the neutron-to-proton ratio right.

Isotopes in Everyday Life

Isotopes aren't just for nuclear power—they're everywhere:

Uranium: The Key Isotopes for Nuclear Power

For nuclear power, we care most about uranium (element 92). Uranium is found naturally in rocks and soil all over the world. When mined, natural uranium contains a mixture of isotopes:

The difference? Just 3 neutrons. But that tiny difference completely changes how they behave in a nuclear reactor. U-235 can sustain a chain reaction; U-238 cannot.

Where Is Uranium Found?

Uranium exists naturally in small amounts in most rocks, soil, and even seawater. However, it's only worth mining where it's concentrated enough:

Fun fact: There's about 4 billion tonnes of uranium dissolved in seawater—but at such low concentrations that extracting it isn't practical (yet).

So What Actually IS Radiation?

Here's the simple truth: radiation is just stuff flying out of atoms.

That's it. When an unstable atom transforms itself to become stable, it ejects particles or energy. Those things flying outward? That's radiation.

So radiation and decay are two sides of the same coin:

Why Do Some Atoms Do This?

Remember from the Isotopes section — some atoms have too many or too few neutrons. The nucleus is unbalanced and unstable. This imbalance creates internal stress, so the atom will spontaneously transform itself until it reaches a stable configuration.

Why is Alpha Radiation Dangerous Inside the Body?

Alpha particles seem harmless — paper stops them, your dead outer skin stops them. So why worry?

The danger is where the energy goes. An alpha particle is heavy and carries a lot of energy. Because it's big, it dumps ALL that energy in a very short distance — just a few cells wide.

This is why Polonium-210 (a powerful alpha emitter) was used to poison Alexander Litvinenko in 2006 — harmless to handle, deadly to ingest.

How Does Radiation Damage Cells?

Understanding how radiation damages living tissue helps explain why containment is so critical. It all comes down to DNA.

Half-Life: The Clock of Decay

Imagine you have a chunk of radioactive material — say, a billion atoms. Each atom is unstable and will eventually decay, shooting out radiation. But here's the strange part: you cannot predict when any specific atom will decay. It's genuinely random — like a lottery where each atom has a ticket, but you don't know when any particular ticket will be drawn.

However, with billions of atoms, statistics take over. We can predict with remarkable accuracy that in a certain time period — the half-life — exactly half of them will have decayed.

Why Such Different Half-Lives?

The half-life depends on how unstable the nucleus is. A highly unstable nucleus (badly unbalanced proton/neutron ratio) will decay quickly — it can't hold itself together. A nucleus that's only slightly unstable might hold on for billions of years before randomly falling apart.

🍌 Curiosity Corner: The Banana Equivalent Dose

Here's a fun fact: bananas are radioactive. They contain potassium-40, a naturally occurring radioactive isotope. Scientists sometimes use "banana equivalent dose" (BED) to help people understand radiation exposure in relatable terms.

What Actually Happens During Fission?

Remember: uranium-235 has 92 protons and 143 neutrons in its nucleus. When a slow neutron hits it, the nucleus becomes unstable and splits apart.

Here's the key insight: when the nucleus splits, the protons get divided between the two fragments. And as we learned earlier — the number of protons defines what element an atom is.

So fission doesn't just break an atom — it transforms uranium into completely different elements. Krypton (Kr) and Barium (Ba) in this example. These "fission products" are usually unstable isotopes that will continue to decay, releasing radiation — this is why spent fuel remains radioactive.

Where Does the Energy Come From?

Here's the remarkable part: if you add up the mass of the fission products and neutrons, it's slightly less than the original uranium atom. That missing mass has been converted directly into energy — Einstein's famous E=mc².

A single kilogram of uranium fuel contains as much energy as about 14,000 tonnes of coal.

Fissile Materials

Only certain isotopes can sustain a chain reaction. These are called fissile materials. What makes them special? Their nuclei are just unstable enough that when struck by a neutron, they readily split apart rather than simply absorbing it.

From Fission to Chain Reaction

We've learned that fissile atoms like U-235 don't just absorb neutrons — they split apart. But here's what makes nuclear power possible: when an atom splits, it also releases 2 or 3 new neutrons.

Why? Heavy atoms like uranium have more neutrons than lighter atoms need. When the nucleus splits into two smaller fragments, those fragments are "neutron-rich" — they have more neutrons than they can hold. The excess neutrons are ejected at high speed.

These released neutrons can then strike other U-235 atoms, causing them to split and release even more neutrons. This self-sustaining process is a chain reaction.

Why Can't Fast Neutrons Cause Fission?

When a neutron hits a U-235 nucleus, it doesn't just bounce off — it needs to be absorbed into the nucleus to trigger fission. But here's the problem:

Physicists measure this using capture cross-section — essentially the "target size" the nucleus presents to the neutron. For U-235:

Thermal neutrons are ~500× more likely to cause fission in U-235!