Monday, 8 October 2012

Fusion Fuels: Part 1 - The isotopes of hydrogen

Believe it or not, I am sometimes asked a question that most people would never have thought of:

"Where do you get your deuterium from?

The reason that I am asked this unusual question is that my 'day-job' is in the field of nuclear fusion research - fusion being the safe form of nuclear - the one that can't run away.

You might ask "What is deuterium?"  So, let's start from the basics and then cover the rest of the process in a series of other posts. 

The simplest type of atom on the periodic table is normally known to you and me as 'hydrogen'.  Its nucleus contains one proton and nothing else.  A single electron accompanies that simplest-possible nucleus to make a hydrogen atom.

We know hydrogen in its gaseous form as the light gas that filled the magnificent but ill-fated German airship Hindenberg, which crashed in flames in the 1930s.  We also find hydrogen in water, in natural gas (methane) and in all organic molecules that go to make up life.  Hydrogen is the fuel of the whole universe.  Stars 'burn' it in their nuclear fires to make heavier elements.  It might even become a conventional fuel that will be burnt to generate energy in the future, but only if a few of its inconvenient features can be overcome by good engineering.  (As a gas, if it is mixed with air it tends to be extremely explosive.  It is also very good at escaping from containers used to store it. Clearly these are potential inconveniences.)

This common form of hydrogen is sometimes known to scientists as 'protium'.  This term is used to distinguish it unambiguously from its heavier sister, 'deuterium'.  Deuterium atoms contain a single proton, but their nuclei also contain a neutron, making them roughly twice as heavy as protium.  Add a single electron and you get a stable atom called deuterium.  When two different types of atom vary only in the number of neutrons, we call them 'isotopes'.  That doesn't mean that they are radioactive, even though science fiction and the popular press tend to use the term for something that is.  In fact both protium and deuterium are stable in nature, and both can correctly be called 'hydrogen' because they are both hydrogen isotopes.

isotopes of hydrogen - protium, deuterium and tritium
Hydrogen (protium), deuterium and tritium

There is a third isotope of hydrogen which contains two neutrons.  It is still quite stable, but slowly over time it decays, turning itself into the lighter isotope of helium, known as helium 3, and emitting a beta particle.  If you start off with 100 tritium atoms and simply wait for about 12 years . . . half of them will have turned into helium 3.  After a further 12 years half of the remainder will have decayed . . . and so on.  Therefore you don't find much tritium in nature, even though small amounts of it are created by natural processes in the upper atmosphere.

Fusion of protium powers the sun as I mentioned earlier, because the sun produces conditions of high pressure and reasonably high temperature that are suitable to fuse ordinary hydrogen. But if we want fusion to be the power source of the future on earth (where we can't easily develop very high pressures, and don't have anything like as much volume as the sun) we would need much higher temperatures than the sun produces.  The sun gets away with being relatively cold because it only burns its hydrogen very slowly.  A cubic metre of the sun produces only about as much energy as a cubic metre of a compost heap (on average).  We should be quite happy that the sun works so slowly, because otherwise it could have burned up all its protium a long time ago, and life on earth would have ceased to exist before humans evolved.

In order to get an efficient enough reaction in fusion reactors on earth - to develop a 'sun on the earth' - we know from experiments that it will be easier to fuse the two heavy isotopes of hydrogen; namely deuterium and tritium, instead of protium.  We can use these fuels to produce fusion even now, with 'small' machines built in the 1980s and 1990s, but they are only big enough to produce tens of megawatts.  It works, but not yet well enough.  However, a large international technical project called ITER aims to achieve a power of around 500MW in the mid 2020s, to demonstrate fusion on the scale needed for a power station.

So - where do we get deuterium?

Read the rest of the series: 
Fusion Fuels: Part 2 - 'Mining' deuterium.
Fusion Fuels part 3 - Making 'heavy water'
Fusion fuels 4 - Electrolysis of heavy water
Fusion Fuels part 5 - Tritium

Small note:  The glib answer to the original question is "Out of a high pressure gas bottle delivered to us by our supplier".  That would be accurate and true, but not altogether in keeping with the spirit of the question.


John Chapman said...
This comment has been removed by the author.
John Chapman said...

OK as an ex-chemistry teacher I'm quite familiar with normal hydrogen and just how easily it diffuses through apparently solid objects such as steel cylinders. For that reason the cylinders are always stored in an external store. There's a classic experiment where a rubber tube is filled with hydrogen and clamped at each end. A day later it will be flattened because the hydrogen diffuses out through the rubber faster than air can diffuse in.

I have no experience with working with deuterium though. Does it behave in exactly the same way because the size of the hydrogen molecule is about the same? Or does it diffuse more slowly due to it's larger nucleus?

Are you working with enriched hydrogen, deuterium hydride, deuterium molecules or lithium deuteride?

Derby Sceptic said...

So - mid 2020s for ITER. That sounds promising but is there any timescale yet for commercial power stations using the technology?

Our present government, and their predecessors, are closing power stations down to meet EU targets without any viable replacements - windmills are just not working.

The government and their successors need to get their fingers out and fix this shortfall.

Plasma Engineer said...

Sorry John - I meant to reply the other day. You are quite right about hydrogen's ability to escape through permeable materials, and the other isotopes of hydrogen are equally mobile. Stainless steel is often used to keep hydrogen in, just as it is used in ultra high vacuum systems to keep hydrogen out. The form of hydrogen needed to fuel fusion reactors is gaseous (and yes, molecular). In some cases tritium is stored on a hydride bed, but it is not really going to be relevant to this series about deuterium.

Plasma Engineer said...

@DS - yes. I agree.