Nuclear Fusion: The Science Behind Turning Water into Electricity

11 minute read

Updated on Wed Mar 03 2021

Have you ever wondered why the sun shines? Could we use the same mechanism to produce energy here on Earth? The answer to both these questions is nuclear fusion!

Earthly admiring the sun

What is nuclear fusion?

In previous chapters, we discussed nuclear fission - splitting a big atom into smaller ones and releasing energy. Nuclear fusion is the opposite. It takes two smaller atoms and joins them into one.

Why fusion would be a great source of energy

Which of these do you think are benefits of nuclear fusion? Pick 4:

If splitting atoms releases energy, how can joining atoms also release energy?

Splitting atoms (fission) uses heavy elements (elements that have lots of protons and neutrons) while joining atoms (fusion) uses very light elements. Why? Because, for elements lighter than iron (26 protons), fusion releases energy, and fission consumes energy. For elements heavier than iron, this is the other way around.

Fusion / Fission Potential

So, for big atoms like Uranium, we get energy from a split but require lots of energy to join the atoms together. By contrast, joining lighter elements actually gives us energy!

From the graph above, can you guess which element we’ll be using as an input?

How does fusion work?

You could try to fuse any two elements, but we will talk about the one specific fusion reaction that would be used to generate electricity.

We’ll refer to elements by the number of protons and neutrons they have: “1p2n” is a nucleus with 1 proton and 2 neutrons. We can forget about the electrons for now - you’ll see why later.

Joining 1p1n with 1p2n gives you what?

1p1n + 1p2n = 2p3n. BUT: 2p3n is an unstable isotope, meaning it naturally breaks apart quickly and turns from 2p3n into 2p2n plus an extra neutron and energy.


What are these elements? 1p0n is Hydrogen “H”. 1p1n and 1p2n, as used for fusion, are isotopes of hydrogen called Deuterium and Tritium (they have the same number of protons, but different numbers of neutrons):

Hydrogen Isotopes

What about the result of this reaction? It’d be bad if 2p2n was something dangerous, like nuclear waste.

What element is 2p2n?

Luckily, helium is not dangerous - it's what's used to fill balloons. Now that we know the elements involved, let’s look at the reaction again:

Full Deuterium-Tritium fusion reaction

Why does this release energy?

As weird as it sounds, Helium plus one neutron has less mass than Deuterium plus Tritium. Yes, really! By recombining the same number of protons and neutrons into a different configuration, the whole nucleus loses mass.

The lost mass is converted to energy, which is released in the form of heat and electromagnetic radiation. Einstein’s famous equation E = mc² is at work here: we are converting mass into energy.

So why aren’t we using fusion to make energy?

In the sun, fusion occurs because of the intense pressure in the Sun’s core caused by gravity. It gets as hot as 15,000,000°C!

The good news is we can create fusion on Earth. The bad news: it’s really difficult.

To date, all fusion reactors use more energy than they produce. This is, of course, a problem. A power plant that uses more power than it produces is useless.

The ratio of input to output energy is often called “Q”. Nuclear fusion has a long history, but, so far, we haven’t managed to get Q to 1:

Fusion Reactors performance

As you can see from this graph, we got so close to Q=1, but then improvements stopped happening - why? In the next chapter, we’ll explore what the graph above means, discover what prevented further progress (so far), and discuss recent work that aims to get to Q=10 and higher.

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