Following on from my previous post on splitting the atom (nuclear fission) I feel it is fitting to look into nuclear fusion.Today we are going to be discovering what fusion is, and, similarly to fission, finding out if it can be applied to every element or only specific ones; how fusion happens; and why fusion is important to us.
With that being said, lets dive in. As the last post mentioned, nuclear fission is all about splitting an elements atom into two smaller elements, whereas nuclear fusion is about merging (or fusing, if you like) two smaller elements together into one heavier atom. Upon this process occurring, a “tremendous amount of energy is released” [1]. Sound familiar? Fusion is similar to fission in the sense that a lot of energy is given off when the process occurs. I feel that we are going to see a lot of similarities today; fusion and fission are the opposites of each other when it comes to playing with atoms.
Lets look at the similarities now, which should hopefully help in the understanding of fusion. If you remember from the fission post, pretty much any elements atom can be split into smaller elements. If you don’t remember this (or weren’t aware of this until just now) then I encourage you to head over and give it a read. I hope you will find it thrilling. Anyway, one similarity is that with fusion any element can be fused to create another heavier element* (there is a slight caveat with this, see below). And it is similar in the fact that the two smaller atoms fuse and produce a single heavier element. And yet another similarity, is that the mass of the heavy element is not exactly double the mass of the two smaller elements, which is why energy is created from the process. That is a lot of similarities, and maybe a little bit confusing so lets have a quick review;
- Fusion – Any element can be fused* (see below for a caveat to this)
- Fission – Any element can be split
- Fusion – Two smaller elements fuse and produce one heavier element
- Fission – One bigger element splits into two smaller elements
- Fusion – The mass of the new heavier element is not exactly double the mass of the two smaller elements, thus energy is created
- Fission – The mass of the new smaller elements is not exactly half the mass of the original heavier element, thus energy is created.
*NB: Elements heavier than Iron do not fuse naturally. This is important for fusion in stars, which we will cover below.
We know that both fusion and fission release a lot of energy, but just how much, compared to conventional chemical reactions? Well the following chart shows a comparison between the amount of energy created by conventional chemical reactions; nuclear fission; and nuclear fusion.
The amount of energy (J/Kg) created by the chemical reaction is 33,000,000. Whereas the amount of energy created by nuclear fusion is 340,000,000,000,000. That is over 10.3 million times more energy!
I think that has covered what fusion is, and how it is similar to fission. However how the two are achieved is rather different. Where fission requires the atoms to be bombarded with neutrons, fusion requires extremely hot temperatures and extremely high pressures, such as those found in our sun. Now the sun is rather hot, too hot to be handled with conventional oven gloves, for example. Nasa places the temperature of the sun’s core around 15,000,000 degrees Celsius [3]. At these temperatures “the hydrogen is no longer a gas but a plasma, an extremely high-energy state of matter where electrons are stripped from their atoms.” [2]
There are multiple ways in which this heating process can be achieved, and each sound rather complex with names such as “Deuterium-Tritium fusion”, and “Proton-proton fusion” [2]. Delving into these processes isn’t completely under the scope of this post, so I will just direct you to read about them on [2]. The core part to take away, is that the elements are heated to such a temperature, under such a great pressure, that the electromagnetic repulsion which normally keeps the elements repelling each other is overcome; they no longer repel each other, and subsequently fuse together.
Interestingly, fusion is a process that occurs naturally in our very own sun. And every other star in the known universe. As was mentioned above, elements heavier than Iron do not fuse naturally; so when all of the lighter elements (fuel) are burnt up, the star attempts to burn the next lightest element which happens to be iron. However as we have learnt, Iron doesn’t fuse naturally, thus the star collapses [5]. This can be a little confusing as to why this is the case, and is something I will talk about when we look at the lifecycle of stars in a future blog post.
The sun’s temperate allows for fusion to take place, and the energy that is given off is what warms the earth and provides the right conditions for life. That is a pretty big answer for why fusion is important to us. Without the process of fusion, none of humanity would exist. I wouldn’t be sat here writing this post for you to read. And you wouldn’t be sat there in some far flung country (relative to me, anyway) reading about nuclear fusion. Aside from the naturally occurring fusion, there is effort underway to harness the power of fusion for the production of electricity, another similarity to fission. There are multiple labourites that are working towards sustainable nuclear fusion, although it hasn’t yet been harnessed to the extent of nuclear fission.
Unfortunately there is one example where fusion has been created by humans, the hydrogen bomb [4]. Now you may be asking as to how this was achieved, if the temperatures required are so vast. Well this is where splitting the atom makes another appearance; detonating an atomic bomb produces enough energy for fusion to occur, which has been harnessed in creating another bomb. A bomb within a bomb, A devastating cocktail of splitting and fusing elements.
But I digress; today we have seen the similarities between fission and fusion. We have discovered what fusion is; the process of fusing two elements together to create one heavier element, thus creating energy. We have discovered the how fusion takes place; that it occurs naturally, and also that it requires mass amounts of heat to reproduce here on earth. And we have looked at why fusion matters; the energy can be harnessed to create electricity (and also to create weapons).
I hope you have found this post interesting, and that you have learnt something new. If you have any comments or criticisms, please leave a reply below.
Edit: Sept 29th 2015; Thanks to Jay R, in the comments, for highlighting some errors in my original post.
[1] ITER – What is Fusion?
[2] LiveScience – What is Fusion?
[3] Nasa – The Sun’s Layers and Temperatures
[4] AtomicArchive – The Hydrogen Bomb
[5] Ask a Physicist / Ask a Mathmatician – Why does iron kill stars?