Tuesday, 16 August 2011

Helium-3. The precious little sister of helium-4.

As I mentioned the other day in a post A skeptic's guide to helium (which became much too long), helium comes in two forms, namely helium-3 and helium-4, the latter being the common one.  These two forms are called isotopes.  Many elements have two or more isotopes, and although the word isotope is commonly used to describe something radioactive, that is not what it actually means at all.  Both types of helium are completely stable and neither is radioactive.

As an aside, carbon is an element that has several different isotopes.  Anyone who has an interest in history or archeology, and most other people in the western world, will have heard of radio-carbon dating.  In this case the carbon has a stable isotope (carbon-12) and other unstable isotopes which decay to the stable form.  I won't go into the details but just mention it so that you realise you have heard of the concept.

Helium-3 naturally occurs at about 1 part per million in normal sources of helium, but it is not commercially viable to separate it out.  Instead, it is usually man-made as a by-product in nuclear reactors.  Early in the cold war the strategic value of this special gas was realised because helium-3 plays an important role in the workings of some nuclear weapons.  Fortunately it has some peaceful uses too.  As an example, the two forms of helium can be used together in a machine called a 'dilution refrigerator' to produce ultra low temperatures (a few milli-kelvin) for various research purposes.  I think it has also recently found a new use in the newer scanners used in airports.

When I started working with the low temperature refrigerators which rely on it as their coolant, this rare form of helium gas used to cost about £500 per litre of gas (at normal atmospheric pressure).  Gradually through the 1990s the price dropped, and it is not a coincidence that this happened after the end of the cold-war.  Both super-powers had decommissioned a LOT of warheads.  They were keen to sell off their stockpiles for peaceful purposes and pocket some cash.  Prices bottomed out at about US $100 per litre.  A bargain!

Things were all well and good until about 2 years ago when reserves suddenly dried up.  It became virtually impossible to buy helium-3 at any price, and this makes it exceptionally difficult for the physics community.  Not only do the ultra-low temperature physicists need it to produce the temperatures that they require, but the ultra-high temperature physicists use some of it in nuclear fusion experiments too.

However, you look at it, helium-3 still has a strategic value today - albeit a peaceful value this time.

1 comment:

Dobbin said...

If media reports are to be believed, the Chinese are planning a mission to the Moon to extract helium 3. Mind you the same reports usually say they intend to use it for fusion energy, but I understand it's no good as a fusion fuel (I'm sure you'll correct me if I'm wrong Plasma Engineer)?