Nuclear Fission

In our modern life, we have electricity all around us, providing our comforts.

There are many sources of this power, one of which is nuclear power.

At the heart of nuclear power is a process called "fission." But what is fission, and how does it work, and why?

The explanation is a bit difficult, but go and grab your white lab coats so we can look into it.

Introducing, the atom

The basic building block of all objects – the atom – is is viewed like our solar system with the middle being the nucleus. Here protons and neutrons live, while all around it tiny electrons buzz around like so many bees around a beehive.

But the weight of the atom is not divided evenly, the nucleus getting most of it.

How much? Take a typical swimming pool such as shown below.

It takes about 40 tons of water to fill it up, that's a lot of water! But only about 30 pounds, the weight of a typical two-year-old, is the weight of the electrons.

The rest is pure nucleus.

There is more to this story however than nuclear obesity. This also means that most of the atom's energy is with the nucleus, not the electrons. This is huge, considering that most of the energy we use relies on electron energy; from driving cars, to the food calories we eat, to a pleasant campfire.

So in the early 1900s, people realized that there is gobs more energy that can be tapped from the nucleus; nuclear energy.

But how to get at it?

A full house

The nucleus is not a quiet, peaceful place,

but a place of great activity. This is mostly because the protons hate to be so close to each other. They want to get away in the worst of ways. If successful, this would cause the nucleus to blow apart.

What's needed is a strong force to hold them together.

And so there is, and it's called the "strong force." This force is between neutrons and protons and holds the whole nucleus together. So, on the one hand, protons are trying to get away with all their might. On the other, neutrons are exerting their strong muscles to hold it all together.


And so it often happens that a nucleus has a hard time staying together; especially with a really large nucleus with lots of protons and neutrons. When this happens, pieces of it break away to form a new nucleus – and a new element – that is more stable. This is called radioactivity.

Breaking up the act

The strong force is like a cop, trying to keep the peace in the nucleus.

But also like a cop, it has only a limited reach.

This is an important point.

Take a huge nucleus, like that of Uranium metal – 92 protons and something in the ballpark of 150 neutrons. Because it's so big, it is hard to hold itself together.

So what if an outside neutron smacks into it like in a game of pool?

Think of the nucleus like a drop of water.

As you might expect, it would "splat" or spread out. As it does so, the force holding it together can only hold the ends together, not the middle.

The Uranium nucleus gets hit with a neutron. It then stretches out, but the strong force can only hold the ends together (shown as two circles at either end)

The two ends then get pulled apart like a cheesy pizza.

The atom flies apart in two pieces. The atom ends up splitting, this is "nuclear fission." (nuclear, because it is the nucleus that broke apart) But here's the punchline, in so doing, some of the atom becomes energy, lots of it! How much? Ships or power plants using nuclear energy go years without needing to replace the fuel.

The chain gang

But how to make it work?

There is one detail I failed to mention, and that when fission happens, two or three neutrons also get released. These can then cause more fissions.

This is called a "chain reaction," a common example of which is seen when dominoes are all lined up so that, when you knock one down, and they all end up knocked down one at a time.

So the splitting of one atom causes the rest to split.

Depiction of a chain reaction. A neutron (black line with a dot) hits an atom causing it to split into two smaller atoms (smaller circles) and neutrons that cause others to split.

This reaction can get out of control pretty quickly. This is useful for making a weapon, but not when it is to be used as an energy source. We need something to put the brakes on the reaction.

So in a nuclear reactor (the place where fission produces energy), there are what are called control rods. These gobble up neutrons and slow or even stop the reaction. In this way, the reaction can be slowed down or allowed to go faster.

Fission was the first practical use of the energy of the nucleus, and its discovery changed the world forever. In it is great power to destroy or to bless. I suppose the same can be said about humanity in general, along with the tools we make.

The choice, as always, is up to us.

Great books

Mr Tompkins Explores the Atom

Part of the Mr. Tompkins series written by the physicist George Gamov. The series is a fun way to explore topics in physics through the imagination of a mild-mannered bank clerk named Mr. Tompkins.

In most cases, the story follows a similar pattern. Mr. Tompkins listens to a series of physics lectures, but the lectures are too dry for him and he falls asleep. While asleep, he has dreams that put him into the topic at the moment. In this way, the content can be more easily visualized and understood.

I read this book to my kids, and they enjoyed the dream part, and it did help them understand the atom. In this book, the chapters that I think are most relevant to this topic are as follows:

The Making of the Atomic Bomb

A bit of heavy reading, but author Richard Rhodes carefully goes through the development of the understanding of the atom, nuclear energy, and then the atomic bomb itself. This book has a lot of good information.

On the web

Ulimate Chain Reaction created by 2014 mousetraps and 2015 Ping pong balls

This video shows a demonstration of chain reactions using mousetraps and ping pong balls. A single ball is launched into a collection of mousetraps, setting one off – launching another ball. And all it goes.

How nuclear fission works

A short video describes fission in a nuclear reactor.

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