Pique the Geek 20100328: Nuclear Fusion: Hell on Earth

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There was no Pique the Geek last week because I was preparing for Youngest Son to visit.  We had a great time last week, cooking, eating, throwing darts, and rebonding.  For those of you interested in what we ate, I hosted What’s for Dinner last evening, here.

It is not either possible nor feasible to attempt the fusion that Sol does here on earth because of the impossibility of gathering enough mass to make a very slow reaction work (remember, fusing two protons to a deuteron requires the involvement of the weak nuclear force, and that is a very slow process), nor the temperatures required to make that happen.

Please see the two previous installments of this series here and here, to make things more clear.

As I have maintained since the beginning of this series, the most energy releasing (exothermic) process is fusing four protons (hydrogen nuclei) into one helium-4 nucleus (aka alpha particle), because the mass defect is huge for that process.  Please look at the previous essays for the logic.

However, unless you are a star with enormous amounts of mass, this will not work.  I posit that it will never be possible to fuse four protons into a helium nucleus on earth without the expenditure of many more orders of magnitude of energy to get the process to work than could be realized by it, because of the slow nature of the reaction and the lack of density and temperature required to make it so.

But we have succeeded in fusing isotopes of hydrogen into helium, with hellish consequences.  The way that we do it now is to use a trick.  Remember, the weak nuclear interaction is required to “flip” a quark in a proton to make it into a neutron, so we just bypass that extremely slow process by starting with neutrons from the beginning.  There are several ways to do that, but the only one that is really useful is to take deuterium (a stable, somewhat rare, form of hydrogen that has both one proton and one neutron in its nucleus) and fuse it with either another deuterium nucleus or with a form of hydrogen that has two neutrons and one proton (tritium) in its nucleus.

Fusing two deuterium nuclei into a helium-4 nucleus would be the best way, but for both theoretical and practical reasons this is difficult.  Since deuterium is a gas, under normal circumstances its density is roughly 1/1000 as solid or liquid matter, and as we have discussed before, getting enough density is a real requirement.  Thus, we have found a way to fuse deuterium with tritium, and this is the trick.  Even if we were using more dense forms of deuterium, the activation energy required to get the reaction to go is very high.

Tritium hardly exists at all in nature.  What little does exist is mainly from cosmic ray bombardment of lithium-6 nuclei, causing the Li-6 to form helium-4, and tritium.  This reaction is rare in nature because of the low flux of neutrons and the relative scarcity of lithium.  Tritium is very much like ordinary hydrogen, except it is radioactive with a half life of around 12.33 years, yielding helium-3, an electron (a beta particle), and an electron’s antineutrino (a massless particle that is required for the conservation of angular momentum).  This process releases around 19 MeV (million electron volts), a good chunk of energy.  Lithium-6 can also react with a neutron to release helium-4 and tritium, with a large release of energy.

Lithium-7 can also produce tritium with high energy neutron bombardment, but the process requires more energy than it releases.  Those products are helium-4, tritium, and another neutron.  The process requires the addition of around 2.5 MeV, so sucks up lots of energy.

This is leading to the thermonuclear device, also called the hydrogen bomb.  Here is the trick:  instead of deuterium and tritium in their native states, we take advantage of the fact that lithium and hydrogen form, after some chemical manipulations, a well behaved solid called lithium hydride.  This is a natural product (in the sense that it is available for sale, sorry, an inside chemist joke) that is stable, solid, and, for such light elements, dense, certainly much more dense than gaseous deuterium and tritium.  Now, if we use only lithium-6 for the lithium part, and deuterium for the hydrogen part, we are just about there.

Here is how the “hydrogen” bomb works.  A fission (plutonium-239 or uranium-235) device is fitted with a mass of lithium-6 deuteride such that the fission part in the core that is compressed when the conventional explosives are detonated.  A plutonium bomb in its most basic form is simply a large, hollow sphere of plutonium-239 covered with chemical explosives and surrounded by a reflector of high density metal, typically depleted uranium.  When the chemical explosives are detonated, the hollow sphere collapses into a dense object, going critical and exploding.  If you pack the center with lithium deuteride, the neutron flux from the exploding plutonium device compresses the LiD into a very small, very hot volume, whilst simultaneously converting the lithium-6 to tritium.  The result is that the plutonium explosion provides both the high energy for activation and the containment for tritium and deuterium to fuse, causing a huge explosion.  Remember, we do not have to flip a quark since we already have plenty of neutrons, so this reaction is extraordinarily fast.  That is thermonuclear Holocaust, my friends.

Thermonuclear devices are really much more complicated than that, but this essentially the way they work.  Now, it is possible to use the neutrons generated from the fusion to cause the uranium-238 to undergo fission, and in the largest devices that is the case.  In theory there is no limit as to how large such a device can be made in terms of explosive power.  The Soviets built one that released over 50 megatons of TNT explosive power, and it actually caused a new radiation belt to form in space.  Compared to Gadget (from the first nuclear test, Trinity) at 18 kilotons, it was thousands of times larger.

A very little known fact about the thermonuclear device is that a very large majority of the energy produced comes from fission of uranium or plutonium, and actually quite a small amount from fusion of deuterium and tritium.  This has to do with the fact that the density of lithium deuteride is very low, compared to plutonium, so you have to make a fairly large device to get enough fusion material in place.  It is much more practical to use a big tertiary target of the cheap U-238 to provide most of the energy of the device by fission, triggered by the fast neutrons from the fusion reaction.  Unlike U-235, U-238 does not undergo fission spontaneously because it decays so slowly (half life of around 4.5 billion of years), compared to U-235 (half life of around 700 million years) that it does not release enough neutrons to cause spontaneous fission with the exception of having many tons in one compact mass.  Thus, U-235 and plutonium-239 are called fissile materials, because if enough of either are brought together (the critical mass), they spontaneously fission, whislt U-238 is fissionable, meaning that if enough neutrons can be provided to it, it will fission.

This actually happened naturally on earth.  The Oklo Phenomenon, a natural nuclear reactor, was discovered in Africa in 1972.  It turns out that a huge natural deposit of uranium was large enough to fission on its own, but it must be pointed out that this was not pure U-238, having the natural abundance of U-235 in is as well, supplying enough spare neutrons to start the reaction.  My professor, Dr. Paul Kuroda of the University of Arkansas actually predicted in 1956 that such a deposit was a possibility.  I think the Professor Kuroda should have been awarded the Nobel Prize for this brilliant piece of theoretical physics.

If by some magickal reason the protium in Sol were to be changed to deuterium or to deuterium and tritium at this instant, in around eight minutes (the time it take light to get here from there), we would not even not realize that we had died.  Our entire solar system, out to the heliopause and further, would be incinerated since the sun would instantly convert essentially all of its mass to energy.  Good thing that we need the quark flip in our protium rich sun to regulate the rate of energy output, and there is no chance of protium instantly changing to other hydrogen isotopes.

We on earth are constructing nuclear reactors to try to fuse deuterium with tritium, in the hope of essentially limitless energy.  We have lots of deuterium, a fraction of one per cent, available from seawater.  We also have lots of lithium, so we can make tritium for the reaction.  The deuterium/deuterium reaction has a very high activation barrier, and is not feasible, at least for now.  (Some of the very early thermonuclear devices were based on deuterium, but in the early sixties LiD was settled on).  I might add that the deuterium/tritium reaction is at least for now out of our reach, for several reasons.

There currently are two best candidates for how we might get there.  One is magnetic containment of a plasma (a gaseous phase of matter wherin nuclei that have been stripped of one or more electrons so that they can be contained using electric or magnetic fields, or both).  Such devices are called tokamaks (from the Russian, since the former Soviet Union pioneered the effort).  However, the low density of the plasma is the really difficult bit to overcome.  A plasma is usually even less dense than a gas, because of the energy pumped into it to make it, well, a plasma.  By the way, you see plasmas every day.  Neon signs are plasmas, at relatively low temperatures and pressures, contained in glass envelopes.  So are our compact fluorescent bulbs.  Another problem with plasmas is that the plasma particles, stripped of their electrons, repel each other electromagnetically, since they are now positive in charge.

But a plasma that can be made to fuse has to be many orders of magnitude hotter than a compact fluorescent bulb, and this becomes difficult.  The fundamental problem is making a homogeneous magnetic container with no weak spots.  Someone who is fluent in this field once described it to me as like taking a balloon filled with a fluid hotter than would burn through it and trying to keep it perfectly spherical using your fingers.  There are always imperfections in the containment, and at fusion temperatures, a finger will slip.  To date, and to my knowledge, no tokamak has come to the breakeven point (the point wherein energy input it balanced with energy output).  Unless we develop the technology to produce better confinement, I do not see any future at all for the tokamak If I am incorrect, please let me know.

As our understanding of electromagnetics increases, and our technology advances with better materials and controls, this approach might be viable.  For you Star Trek fans, remember that Voyager was always looking for deuterium for her fusion reactors.  Even though Roddenberry did not create that one directly, he has rarely been wrong.  How many reading have telephones that opens, just like Kirk’s communicator?

The other viable idea is to take deuterium and tritium and encapsulate the mixture into a pellet of glass, gold, or some other material.  Then that pellet is, extremely carefully, put into the focal point of many high energy lasers.  On the signal, the lasers fire, imploding the pellet, increasing the density by many orders of magnitude, and heating it tremendously.

This actually works, and we have, insofar as I know, gone barely beyond breakeven.  However, the energy that we get out is not all that useful, and we have done it only on tiny bits of deuterium and tritium.  In a theoretical manner it is important, but you will not be turning on your lights with it in our lifetimes.  One problem is the stray neutron, one for every reaction.  In a commercial plant, that is a problem because neutron irradiation causes many materials not only to become brittle, but also to become quite “hot” radioactively.  We have materials to deal with this, but it is not a given that the intense neutron flux can be dealt with well at levels useful for commercial power production.

One idea is to use those excess neutrons to make more tritium from Li-7 in a vessel surrounding the fusion reactor, and even more from Li-6 in even another surrounding vessel.  That is well and good, but we have to get fusion going from more than a few atoms at a time to investigate those ideas.

Some of this may sound like science fiction, but remember this:  Art Clarke did the math and, in one his books, speculated on a geosynchronous orbit in which satellites could be placed for communication.  I speculate that many of you reading are getting this signal, directly or indirectly, from a satellite in the Clarke belt.  Anyone with satellite TeeVee is watching from there directly.  So who knows?

There are a couple of other ideas how to make fusion work in a controlled manner.  One has to do with using modern technology not to compress a pellet with a laser nor squeeze plasma in a tomamak, but rather to cause intense heating on a microscopic basis on a solution containing deuterium and tritium.  I have not researched this technique in detail, but do recall that intense sonic energy is the vehicle to produce localized, intense matter compression and local heating.  This might well be the way to do it, because with the proper feedback mechanisms, the input triggers could be modulated to provide a proper rate of energy release.  Still, something would have to be done with the stray neutrons, since they are very destructive.  If we could find a way to use deuterium only, that neutron problem would vanish.

No discussion of fusion on earth would be complete without mentioning cold fusion.  When I was a graduate student, this was all the rage.  Two folks from out west claimed that they had fused deuterium and tritium in what is essentially a beaker with not too complex devices.  I will not name their names, since they have been shamed enough.

Simply stated, cold fusion not only does not work, but it is impossible on very simple physical grounds.  It is impossible to make any two nuclei approach each other closely enough for the attractive strong nuclear interaction to overcome the repulsive electromagnetic interaction at temperatures beyond Hellish.  The guys that reported positive results were drunk with false positive results, and others, using identical reactors but better detection instrumentation, quickly debunked their results.  Cold fusion can not exist, unless one finds a way to cancel electromagnetism, or figures out how to manipulate gravity.  Good luck with those two.

In summary, I have these conclusions:

1)  Nuclear fusion on earth is possible, but the ways that we have done it are more Hellish that earthlike, viz, the thermonuclear device.  The physics work.

2)  Our attempts at controlled fusion have not worked out very well so far.  That is fine, since we are infants in a really big universe of experiments yet to be conceived, let alone completed.

3)  Failure should make us try harder, not to give up.

4)  Visionaries should give hints to scientists (as Clarke did).

5)  Science is one hell of a lot of FUN!

Well, you have done it again!  You have wasted many moles of photons (actually, einsteins, when photons are concerned) reading this stupid drivel.  I am glad that you made it to the end, if you did.  I am looking for new topics, and any suggestions are welcome since we have completed this series on fusion.

Even though Sarah Palin drops her plans for a $1 million per episode reality show when she hears me say it, I always learn much more than I could possibly hope to teach when I write this series.  Remember, nothing scientific or technical is off topic here.  I especially love to hear your corrections (them the most, because I never want to mislead), questions, ideas, and other thoughts.  Obviously, tips and recommendations are welcome as well.

Warmest regards,

Doc

Crossposted at Daily Kos