Monday, 29 October 2012

Fusion fuels 4 - Electrolysis of heavy water

This is part of a series examining how the fuels for a fusion reactor are likely to be obtained.  In part 1, I described the Isotopes of hydrogen and named them.  In part 2, Mining deuterium, we saw how deuterium can be extracted from ordinary water, and brought up to a concentration of 20 to 25%, and in part 3 we saw how it can be 'vacuum distilled' to produce 'heavy water' with a purity of 99% deuterium (and a 1% impurity of protium).  This liquid now has to be turned into deuterium gas by a process called electrolysis.

In chemistry or physics lessons at school you might have electrolysed water.  You probably added a bit of salt to the water so that it conducts electricity, put two electrodes into the water, and connected one end of a battery to each electrode.  You would have seen bubbles rising from the two electrodes and you might remember that one electrode produces bubbles of hydrogen and the other produces bubbles of oxygen.  Collecting the bubbles in a test tube, you probably enjoyed igniting the gas and hearing a loud pop.

In doing that, you are releasing the chemical energy of the hydrogen as it recombines with oxygen and becomes water again.  Hydrogen contains quite a lot of chemical energy, and this is why it is being considered as an alternative, carbon-free, fuel for cars.  But hydrogen contains massively more energy than this if you can release its physical energy by fusing its atoms together to turn them into helium.  This is the power source of the sun and stars.  The heavy form of hydrogen, called deuterium, will be even better for that because it fuses more easily.  Stars contain almost no deuterium at all.  As soon as a deuterium atom is created it fuses with another almost immediately.  This is why we want it as a fuel for fusion reactors.

If we carry out this electrolysis process on the 'heavy water' that we have made by the processes of isotopic exchange and vacuum distillation, we find that something useful happens.  Remember that we want to get rid of the protium and keep the deuterium.

Fortunately, they do not electrolyse at the same rate as each other. 

Unfortunately, as with vacuum distillation, the protium is more easily produced than the deuterium!

But knowing this, you can remove the protium first to leave a higher concentration of deuterium in the water, and then choose when the deuterium concentration is high enough to start collecting it.

Concentrations of 99.9% can be reached by this method.  This is high enough for fusion reactors to use.

At last we have one half of the fuel we need for fusion.  The other half is potentially a little less complicated except for the problem that it is still a little more conceptual. 

Next time I will cover the production of tritium before returning to discuss the energy balance of the production of deuterium.  Is it really worth spending all this energy to make the fuel for fusion?  I hope to convince you that the answer is an emphatic YES!

Next time:  Fusion Fuels part 5 - Tritium

Other articles in this series:

Fusion Fuels: Part 1 - The isotopes of hydrogen
Fusion Fuels: Part 2 - 'Mining' deuterium.
Fusion Fuels part 3 - Making 'heavy water'

1 comment:

Andreas said...

I haven't really looked into it alot, but do you know how far he technology is developing to allow for efficient use of fusion reactors?
What problems there are, and how it might be possible to solve them?
For example, I am under the impression that due to the high energy cost of start up it could only reliably be done on a massive scale. Am I right in this assumption?