As I said, most people aren't accustomed to thinking of helium as a non-renewable resource.  But it is.  The underlying issue isn't tricky or complicated; it's just not the way we're used to thinking about things.  We're used to thinking in terms of nonrenewability when it comes to fossil fuels like petroleum.  After all, petroleum comes from dead dinosaurs, or something, and they haven't been around for a while.  So it's not surprising that oil is a limited resource.  (Until we finally start cloning dinosaurs.) 

But what about helium? Isn't it the second-most common element in the entire universe?  Yes.  Yes it is.  It's extremely common in the universe at large.  But here on Earth?  Not so much. [Addendum: According to Physics Today, June 2007, helium is ironically one of the most abundant elements on Earth.  This in an article about helium shortages.]  In fact, the very name "helium" reflects the fact that this element was first identified in the sun, spectroscopically, before it was identified on Earth.

But in order to explain that, maybe it would be worthwhile to start at the beginning.

What Is Helium?  Why Is It Special?

Helium is a chemical element.  It is element number 2, hydrogen's aide-de-camp and the majordomo of the Periodic Table of Elements.  But it's a puckish and contradictory thing, simultaneously disdainful of chemistry and yet the bedrock of nuclear physics.

From its perch high atop the rightmost column of the Periodic Table, helium waves across the canyon of p-, d-, and f-orbitals to its gregarious cousin, hydrogen.  Yet, in the realm of chemistry (which mountain rests upon the bedrock of the Periodic Table), helium is icy and aloof.  Helium is a noble gas, which means it doesn't willingly form chemical bonds with other elements. It is the Greta Garbo of the table; it wants to be alone.  It takes considerable effort and extreme conditions under laboratory settings to make a helium submit to molecular humiliation.

But delve deeper, and we find that helium is the Samwise Gamgee of nuclear physics.  It is stable, unperturbable, steadfast.  The helium-4 nucleus is a happy quartet, like two couples on a double date: two protons, two neutrons, and no fifth wheel.  (There's also the much rarer helium-3, where one proton and neutron try for some private time but can't quite escape one lonely, clueless, hanger-on proton.)   This is one of the most stable multi-particle configurations around.  In fact, it has the highest binding energy per nucleon of the smallest nuclei at the top of the periodic table.  

And that means the helium nucleus plays a special role in nuclear physics.   This configuration is so important, in fact, that it has a special nickname: alpha.  When one talks of alpha particles (as we will, below) one is really talking about naked helium atoms, nuclei bereft of their electrons.  Sometimes, under the proper circumstances, heavier atomic nuclei can be imagined (roughly) as loose bags of alphas rattling together.  Alphas even play an important role in radioactive decay.  More on that below.

So Where Does Earth's Helium Come From?  Why Can't We Make More?

Helium's elemental nature means it isn't a chemical compound that can be created through rearranging molecules via chemical reactions, like water or carbon dioxide or DNA.  Helium is a particular species of atom, and thus can only be altered via nuclear reactions.

Contrary to one possible misconception, helium is not a significant component of our atmosphere.  In fact, helium accounts for less than one thousandth of one percent of the Earth's atmosphere.  More on that below, but for now the important point is that atmospheric helium is far too rare and dilute to make large-scale extraction economically feasible.  (Although that might change in coming decades, if the price of helium undergoes an exponential increase.)

There are only three processes for building a helium atom from scratch.  (Technically, I'm talking about making an "alpha particle" in the parlance of nuclear physics, explained above.  Turns out, though, that electrons are plentiful enough.  If you make a bare alpha, sooner or later it'll pick up a couple of electrons to keep it company.  The resulting threesome is a helium atom.) 

Method 1: Create the Universe

The Big Bang made a lot of helium.  The vast majority if of helium in the universe, in fact, is leftover from the Big Bang. 

When done right, this approach yields a lot of helium.  Sadly, however, it is impractical for most purposes.

Method 2: Nuclear Fusion on a More Modest Scale

The sun burns hydrogen gas into helium, via nuclear reactions deep in the core.  The sun's core, in fact, is presently the only place in the neighborhood where this process is happening to an appreciable extent. 

Earthbound efforts to create sustainable fusion reactions for power generation have made much progress in the past few decades, but present experiments don't produce helium in anything more than trivial amounts. If and when the National Ignition Facility achieves ignition, a single ignition shot will produce helium-4 in paltry microgram quantities.  (But helium production isn't the point of NIF.)  We're talking about making a few tens of micrograms of helium per week, at best.   Yields from JET won't be orders of magnitude higher.

According the the ITER website, the kind of magnetic-confinement fusion reactor envisioned for the second half of this century would burn approximately 250 kg of hydrogen per year, total, split between the isotopes deuterium and tritium. 

125 kg of deuterium (D) is roughly 62,500 moles; 125 kg of tritium (T) is about 41,625 moles.  So, at 100% efficiency (HA!), that hypothetical reactor could create 41,625 moles of helium-4, or approximately 166 kg in one year.  (Each D+T fusion reaction creates one atom of helium-4.)

So a second- or third-generation fusion reactor, so advanced it probably won't exist for at least another forty years, would make far less than 200 kg of helium per year.  Right now, in 2011, the Large Hadron Collider uses 96 metric tons of liquid helium in its magnets.  The hypothetical successor to ITER would have to run for over 500 years slake the LHC's thirst just once.   Or 500+ such reactors, spread around the world, would have to work full bore for an entire year just to satisfy the demand for this one scientific application of helium. (And again, that's assuming 100% efficiency in the helium production.  Which is thermodynamically impossible.  The real efficiencies would be much lower than this.)  And the LHC is a drop in the bucket compared to the total usage of helium around the world.

So, while earthbound nuclear fusion might oneday produce energy for our grandchildren, it won't be replenishing their helium stocks. 

Method 3:  Radioactive Decay

When an atomic nucleus undergoes radioactive decay, there are several nuclear reactions by which that decay can happen.  One of the most common is known as alpha decay; it's no coincidence that the helium nucleus goes by a similar nickname.  Alpha decay is the process by which a nucleus attempts to make itself more stable by breaking off and ejecting a piece of itself.  The ejected piece is an alpha particle: the heart of a helium atom.  (The heart of a helium-4 atom, to be more precise.  Helium-3, to which I referred earlier, is also the byproduct of radioactive decay.  But in this case, the progenitor element is tritium (an exotic cousin of regular hydrogen), which has a half life of about 12 years.  So ³He is even rarer than ⁴He.)

If you take a piece of polonium (or any good alpha emitter) and seal it into an evacuated glass tube, something very cool happens.  Wait long enough, and you'll find your vacuum tube is now filled with helium gas.  You can prove it by running an electric discharge through the gas; a spectroscopic analysis of the emission will indicate the presence of pure helium.   In the process of turning itself into lead, through a kind of reverse nuclear alchemy, polonium produces helium gas.   So do all radioactive isotopes that decay via the alpha-emission process. 

When the process happens inside a rock, rather than inside an evacuated glass tube, the resulting helium can get trapped inside microbubbles and fissures and even within the atomic lattice of the surrounding rock itself.  And it can sit there for a long time.  A very long time. Billions of years, even.

The Earth's crust contained a higher concentration of radioisotopes billions of years ago than it does now.  (The concentration declined as the radioisotopes underwent decay and slowly achieved nuclear stability.)  Some of those radioisotopes underwent alpha decay over many eons, injecting helium into the surrounding minerals while turning themselves into stable elements.  But once stability was reached, the helium production stopped.  The end result being a slow buildup of helium within the Earth's crust over geological timescales.

And that's the source of all the helium we use today.

The Earth's helium reserves were created by a very slow process of geology and nuclear physics working in concert for millions of years.  There is no feasible technological solution for artificially replenishing the Earth's stock of helium.

Okay, Fine, We Can't Make Helium.  How Hard Can It Be To Recycle?  Why Can't We Keep Reusing It, Like Aluminum and Glass?

Most large-scale users of helium do just that.  They take pains to try to recover, retrap, and recycle escaping helium atoms.  But this can never be done with 100% efficiency; it's thermodynamically impossible.  Leaks will always happen.  And that's in systems engineered with a thought toward helium conservation.  There are many, many other places where helium is willingly squandered every day.  Every lost balloon is more significant than we might realize.

The issue boils down to what happens to helium when it leaks into the atmosphere.

As I said earlier, it may be a commonly held misconception that helium is extracted from the atmosphere, like oxygen and nitrogen.  After all, they're all just gases, right?  And the atmosphere is full of all sorts of gases.  Isn't it?

Well, kind of.  But there are limits on what gases that atmosphere can and cannot contain.  I'm not talking about biology or geology or meteorology here.  The simple fact is that atmospheric helium is not gravitationally bound to the Earth.  

Strange but true. 

Individual gas molecules and atoms in the atmosphere are constantly moving very quickly.  Much more quickly than wind and weather.  Far faster, even, than the winds in an F5 tornado.  We don't notice because they're constantly bumping into each other and other obstacles.  So the bulk speeds we notice, in the form of flapping flags and convective currents, are much lower.  But on a microscopic scale, the individual atoms and molecules zip around as though some demented genius decided to swap in Formula One race cars for the balky bumper cars on a circus midway.

It turns out that the average speed of an atom (say, helium) or molecule (say, nitrogen, or N₂) is inversely related to its mass.  (Technically, for those with a fetish for thermodynamics, the RMS thermal velocity is inversely proportional to the square root of the mass.)  Heavier stuff moves more slowly, and lighter stuff moves more quickly.  Any given population of atoms or molecules will have a distribution of velocities, owing to collisions and whatnot.  So it's not true that all atoms of a given species move with the same speed.  But helium, it so happens, is so light that atoms on the faster end of the distribution exceed the Earth's escape velocity.  Meaning that if you dump a bunch of helium into the atmosphere (whether it be via zeppelin crash, a burst party balloon, or catastrophic failure of your experiment's superconducting magnets), some of that helium will immediately start leaking out of the atmosphere.  And as the quicker helium atoms go away forever to the Gray Havens, random collisions will eventually kick some of the their slower and dimwitted cousins into that special regime.  And thus those atoms, too, will eventually escape.

Statistically speaking, a small population of helium atoms are destined to stay behind.  But only a very, very small population.  (Remember that helium accounts for a minuscule fraction of the Earth's atmosphere.)

Any given atom of atmospheric helium near the surface of the Earth must run a zigzag relay race while it diffuses through the atmosphere.  But it's almost a statistical certainty that atom will slowly work its way up, to where the atmosphere is thinner and the distances between collisions are greater.  Eventually, it'll undergo one final ricochet that knocks it free and clear of the atmosphere.  And the Earth isn't strong enough to pull it back.

And, because helium doesn't form compounds with other elements, there's no chemical process that will absorb or trap that helium.  It can't be recycled.  The nitrogen cycle has no analogue for helium.

In other words, once an atom of helium leaks into the atmosphere, it's only a matter of time (in a statistical sense) before that atom will leave the earth and bake off into space.   It is lost forever.

And there you have it!  To summarize this post, then:

1) Virtually all of the helium on Earth was built up over millions or billions of years.

2) We can't make more helium.

3) Generally speaking, once helium gas enters the atmosphere, it's gone forever.

These points suggest that if we want to ensure a good store of helium for future generations (or even for ourselves, a few decades from now), careful and intelligent stewardship of the Earth's reserves is critical.   Unfortunately, that isn't happening.

In my next post, I'll try to make sense of the world's helium reserves, and try to understand (in the fashion of a non-economist layman) current insults to the helium market.