Monday, 22 October 2012

Fusion Fuels 3 - Making '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%.

In order to get the water to higher concentrations, a process called 'vacuum distillation' is often adopted.  That is the subject of this post.

Water that contains almost all deuterium and almost no protium is often called 'heavy water'.  You might remember that the 1965 movie "Heroes of Telemark" was a dramatisation of the true story of the destruction of a German heavy water plant in occupied Norway during the second world war.  This was necessary because heavy water is useful in some of the techniques used to produce enriched uranium for a nuclear weapon.  (As it turned out, German technology was not going in quite the right direction, but that was not known at the time.)

However, heavy water is also a source of the deuterium that is needed for the entirely peaceful and environmentally friendly fusion reactors of the near future, and it is only for this reason that I care about it enough to write this series.

In the last instalment, we reached the point where we have water with 20 to 25% deuterium atoms and the remainder still containing protium.  You might remember that I explained in part 1 that some lakes around the world (which have no rivers flowing out of them) have slightly higher concentrations of deuterium than 'ordinary' water.  This is because water molecules containing deuterium evaporate slightly less easily than water molecules containing protium.

Now you might be able to imagine a method of using this process on an industrial scale.  If you study the physics of the evaporation of water, you find that you can choose the right conditions of temperature and pressure where the H2O evaporates preferentially and leaves the HDO and D2O behind.  In fact it turns out that the best conditions for this are at a temperature slightly above the freezing point of water, and in rather a good vacuum. 

In an ideal world you would like to be able to evaporate the product that you want to keep in preference to the waste product.  After all, the whisky industry in Scotland makes its living by doing exactly this 'distillation'.  They warm a dilute mixture of alcohol in water and the alcohol evaporates preferentially.  The liquid that is distilled contains more alcohol and less water.  They usually repeat this distillation a few times in order to concentrate the alcohol further, taking the product from the first distillation and putting it through the process again and again to increase the purity. 

Unfortunately, in making heavy water the opposite is true.  The part that evaporates first is the part that you want to 'throw away' although in practise it will still contain much more deuterium than most of the water in the world.  It will not be discarded but returned to an earlier stage in the process.

The water that is left behind will be a bit more concentrated than it was before the H2O was distilled away, and the last water to evaporate will be the most concentrated in deuterium.  Of course the whole process has no clear cut off points where all the H has been removed, leaving all the D behind.  But an iterative approach can yield higher and higher concentrations of deuterium and, in practise, a concentration of 99% can be achieved.  This might be good enough for most applications of heavy water. 

As a fusion fuel, a slightly better concentration might be preferred.

Fortunately the next stage in the process helps further.  It is in this stage that the heavy water is converted into deuterium gas, by a process known as electrolysis.  This gas is one half of the fuel we need for fusion.

More next time: Fusion fuels 4 - Electrolysis of heavy water

Other articles in this series:
Fusion Fuels: Part 1 - The isotopes of hydrogen
Fusion Fuels: Part 2 - 'Mining' deuterium.
Fusion Fuels part 5 - Tritium

1 comment:

SOLOMON SAMI AZAR said...

CAN SOMEONE PLEASE TEACH ME A :GARAGE SHOP: WAY TO MAKE THIS HEAVY WATER. here is a copy of email i just sent to Alistair Miller, atomic energy of canada.:Hello Alistair,

My name is sam azar, researching methods to use heavy water as the fuel itself for fusion, not just moderator in fission. What an opening line. This is now the 7th year after hand building a Tesla coil at approximately 850,000 ac volts,little over 100 mhz( standard discharge oscillations of Tesla coils at high voltage range), needless to say a Dangerous experiment trying to prove my idea.

I only had several ounces of heavy water procured from United nuclear at 100 dollars, very expensive, and i diluted it into ultra pure reagent grade water, extremely pure and used primary in medical uses. I need such to establish plasma arc within water bath vessel. experiment worked with stable discharge(videos on youtube under noblefuse), but not enough measurable excess enegery as I never possed enough fuel itself D20, i desire a 100 percent bath medium for my Q o efficiency is at such a low rat now with simple tesla coil, no external means for added magnetic pinching, multiple ways to Tune the oscillation for within each "strike" of arc discharge exists quantum possibility for magnetic pinching towards helium production and some energy hopefully ofcource.

Thats my opening statement Sir, after reading your wonderful article on the subject during a time for me to conclude these experiments with test vessel filled with heavy water, I just emailed United nuclear for a price on five gallons, an email before i picked up your letter, Two days i posted another request for heavy water via ELANCE, an online board of eager workers in any profession to bid on any highly technival trades, there is a geentlemen from pakistan which states he could advise on a personal level how to make it "garage shop" style. I made a tunable tesla coil and electricity is my knowledge, i will have to become a chemist i presume to make this fuel.

Can you please advise me how i can make heavy water to attempt this energy crisis, i have a chapter for you in the manufactures guide I hope :) Thank you for listening

sincerely
sam azar
610-533-7078