Liquid Lithium to Contain Fusion Reactors

nigelc writes: "ABCNews.com reports on Liquid Metal walls for a fusion reactor, and how it may solve some of the temperature problems. Probably only of scientific interest to most of us, unless you're into some serious overclocking.""

Comments

Flow problem

[Mr.Intel]

The article mentions that controlling Lithium flow is the main problem of this design:

"...scientists use a magnetic field to control the high-energy plasma inside a fusion reactor. And since liquid lithium is a metal, it's tugged around and made erratic by the magnetic field.

So they basicaly have introduced a new problem by trying to solve the containment issues with a liquid metal. My understanding of previous reactor designs was that the magnetic field was so strong that the plasma never touched the walls of the containment unit and thus did not create as much of the thermal problems (neutron bombardment is another matter). So by introducing liguid Lithium into the reactor core as a sheilding agent, they solve containment but the same magnetic field now causes flow problems with the Li.

No being a fusion technology specialist, I wonder how feasible it would be to refine and control the magnetic fields they are using. Create one for the plasma and another for the Li. Would the fields interfere with each other? If so, could they dampen the plasma field to give stability to the Li? What kind of power generation is required for two fields of similar strength?

And think again, without paranoia

[Spamalamadingdong]

Logically, a layperson would consider a liquid metal to be a very dangerous material to have around...

Like solder? It can burn you, you know. But so can a great many solid materials and vapors (even water). You do have the toxicity of lead-based solders, but they're toxic even when they're in solid form.

Lithium is gonna come in contact with water somehow, by accident (or design) and make hydrogen gas [gcsechemistry.co.uk] which is not only explosive, but turns into radioactive tritium when bombarded by the neutrons put out by ANY reactor - fission or fusion.

The lithium will breed tritium under neutron bombardment whether water is involved or not; the production of hydrogen is a chemical reaction caused by the decomposition of water, the production of tritium is a nuclear reaction caused by the neutron-induced fission of lithium-6 into helium-4 and hydrogen-3.

Lithium is a lot less active (and thus corrosive) than sodium, but it's not suitable as a coolant for fission reactors because it has this pesky tendency to capture neutrons. In a fusion reactor which needs tritium anyway, this is an advantage.

Playing around with explosive hydrogen gas near a reactor is often done deliberately and may be a hidden agenda here.

Just FYI, people play around with "explosive hydrogen gas" for lots of reasons in lots of places. You'll find people playing with hydrogen in every plant which manufactures vegetable shortening from oil, because hydrogenating the oil is part of the process to allow it to solidify at room temperature. Ditto every plant which manufactures nitrogen fertilizers (which starts with fixation via the Haber process, N2 + 3 H2 -> 2 NH3).

A little more information and a little less paranoia would serve you well.

Re:And think again, without paranoia

[cybrpnk]

Well, I agree I oversimplified some things to make for a short and understandable post, but I assure you that I've got the "little more information" at hand as well. Everything I said was a verifiable fact, although I agree that my introduction of the tritium topic in the post by referring to neutron irradiation of hydrogen gas instead of fission of lithium was somewhat misleading, tho true...

I know a little about the nuclear uses of lithium. In Oak Ridge I worked on the team that documented the pollution that occurred in the program there in the 1950s when Lithium 6 was separated from Lithium 7 and purified to use as fusion fuel in hydrogen bombs. There was a step where the Lithium 6 was dissolved in liquid mercury to purify it and later recovered by reacting it with nitric acid. Over 7 million pounds of mercury were dumped into Bear Creek, then the Clinch River, then the Tennessee River, then the Ohio River and ultimately the Mississippi by lithium processing in Oak Ridge. My point remains, and it's not paranoia, that there are unintended consequences that come with the use of major technologies like this. The public would do well to keep this in mind...if only they were smart enought to, as you correctly allude to in your other post here.

I've got a permanent burn mark above my right knee where I dropped a blob of solder on bare skin and I'm still using it routinely at work and at home in building my Beowulf cluster, etc, so I assure you I'm not afraid of technology for technology's sake. The fact remains that a reactive liquid alkali metal at 600 degrees C is inherently much more dangerous than non-reactive tin and lead at 250 degrees C. Fusion powerplants of the future will almost certainly use a lithium-water heat exchanger as I stated (and you did not deny) and it's almost a certainty that THESE THINGS LEAK at times and such a leak will probably cause LOTS of hydrogen gas to form. Hydrogen gas explosions were serious steps in both Three Mile Island and Chernobyl accidents and have the potential to be one in a lithium cooled fusion reactor accident as well. Such a reactor probably wouldn't contaminate miles of countryside but the loss of the investment in the reactor would be devastating, as well as public confidence in the supposed "safety" of fusion reactors over fission ones. Such a public relations disaster doesn't occur at vegetable shortening of fertilizer plants even when they have explosions there (and they do) because those are "low-tech" kinds of plants in the public eye and they are more conditioned to think of them as "risky". Things like reactors and space shuttles are held, perhaps unfairly, to a higher standard.

Again, you post proves my point when you note correctly that sodium doesn't absorb neutrons like lithium does. It's easier in explaining it to the public to say sodium is "more corrosive" which is true. But the very fact a fusion reactor group would pick lithium instead of sodium for the coolant shows that somewhere along the line, neutron absorption and tritium production is important...the "hidden agenda" that is neither paranoia on my part or even hinted at in the ABC article.

Re:And think again, without paranoia

[ilbrec]

Your obsession with hydrogen is misguided anyway. The reaction is

alkali metal + H2O --> alkali oxide + H2 + heat

Well, you are wrong on this as well. The proper reaction is something like this:

alkali metal (M) + 2 H2O --> alkali hydroxide (MOH) + H2 + heat

This isn't the first concept to use lithium thusly

[Spamalamadingdong]

One of the first concepts for the use of lithium as a renewable "first wall" involved chambers which contained a vortex flow more or less like a toilet bowl. Pellets of D-T would be dropped into the center of the vortex and triggered with laser pulses; the flowing lithium would absorb the energy of the resulting micro-explosion and renew itself in time for the next pulse. The lithium would also carry away the heat. This concept was discarded when it was found that laser fusion required implosion of material to far higher densities than could be obtained with a single incoming beam, and the necessary symmetric system of incoming beams prohibited the use of a lithium vortex.

This concept appears to use lithium as the "first wall". I'm not sure exactly what the first wall has to absorb, besides the heat conducted to it from plasma leakage (plasmas do leak, they are subject to all kinds of instabilities) and soft X-rays. I do know that if you had a symmetric torus you could rotate the magnets to "pump" the liquid metal along the wall using eddy-currents, but I expect that this would be far more expensive and difficult than what they're planning. You can't have two different magnetic fields; you have one field, which is the sum of all the fields induced by all the current-carrying elements in the reactor, plus whatever the Earth decides to give you (which is probably not significant on this scale). You could have a multi-pole magnetic field at the surface which would fall off rapidly toward the center (not unlike the focussing magnets used in a synchrotron) but I'm not sure what effect this would have on either the metal wall or the plasma (and unlike synchrotrons, I don't know any tokamak experts I could ask).

Re:Why Li and Na?

[cybrpnk]

A pound of lithium or sodium will absorb a LOT more heat than a pound of solder, so by using reactive alkali metals you have a lot less stuff to pump around than if you used something else as a coolant. Pumping a lot less stuff means a simpler system, and that's always good.

Also, the key reason (which isn't even mentioned in the ABC article) for using lithium is that when lithium is hit by the fusion reactor's neutrons, it will change into radioactive tritium gas, which is a rare fuel the fusion reactor needs to keep going. So you start the reactor with a little bit of tritium you got someplace else, then use the reactor's own neutrons to convert common metal (lithium) into the rare fuel you need to keep it going. No other metal - sodium, lead or tin included - will do this, only lithium.

You are the smartest person in this whole thread, pal - instead of showing your ingnorance by stubbornly defending a particular position with facts (me included), you openly admit the things you don't know and ask intelligent questions to make yourself more knowledgable. Do not ever lose this open mind you have - it is much more rare than tritium gas.