Thursday, February 24, 2005

Fusion, will it ever happen

Someone brought up some alternative energy articles at the nation of riflemen forum, and it broguht me over to SDB's archive at uss clueless which got me to thinking again about fusion, and more specifically how terrestrial fusion isnt going to be a viable solution for a hell of a long itme if ever.

Anyway I wrote this a while back, and I'm updating it here because it's something I want to talk about with "my audience".

I said above, IF fusion is ever going to be viable, meaning that I think there are some reasons that's going to be tough. There's a few BIG issues here on the fusion topic:

More in the extended entry...

1. Touch off point/break even point.

This is the amount of energy and reactive mass (which are ultimately the same thing but that's another topic) required to produce a self sustaining reaction that outputs more energy then it sucks in. Basically how do we get the damned thing primed. Thus far we have been mostly unsuccessful in reaching the breakeven point. The few times it was MAYBE achieved it didn't last long and it was uncontrolled which brings up point two.

2. Controllability

We have no idea how to control a self sustaining fusion reaction, or if it is even possible to control. the best ideas so far involve massive torroidal field generators which control plasma flow. Small problem, what happens when the energy of the fusion reaction vastly exceeds the energy of the fields controlling that reaction? Oh and assuming we contain the reaction how do we throttle it without dropping below the touchoff point? Because the natural tendency of the reaction is to grow til the point where it is either fuel exhausted, too unstable to continue, or otherwise self limiting for various reasons.

According to everything we know (which I'll admit isn't a hell of a lot) these self limiting points are far greater than we can currently handle, or even have any concept of how we might handle them in the future.

If you don't believe me think about this. The largest fusion reactions we as humans are able to produce are in the gigaton range, the largest we can control are in the several molecule range (yes I know there's no basis for dimensional analysis here because the units are incompatible). These gigaton reactions are not inherently self limiting in the pure sense, though because of the methods used to initiate the fusion as well as the materials used in the devices and produced during the reaction (primarily tritium and helium which tend to absorb neutron flux) they actually are.

5. Neutron flux and hard alpha

Guess what folks, fusion reactions aren't 'clean' in that they do produce massive amounts of radiation that is harmful to carbon based life forms.Primarily these are in the form of neutron flux and alpha particle radiation.

Neutron flux is one of the primary sources of background radiation in the universe, all that nice radio noise, microwave radiation through space etc... But that's at light-years distance. At anything less than half an AU it starts getting more dangerous.

Hard alpha is the emission of high energy alpha particles. These nasty little buggers can at most cause the disintegration of your molecular structure (not atomic structure, molecular structure) and at the least cause genetic defects in a few cells. It's kind of like shooting marbles with your molecules, cept the relatively large molecules that make up much of our bodies are like 1" aggies and the little alpha particle is a BB some asshole just shot at them.

Yes, all of these are shieldable... the massive magnetic fields used to control the reaction, and the multiple layers of heavy shielding in the reactors... Though they wear out eventually, and if the magnetic fields were to collapse while the fusion reaction was going on...

.. That would be bad...

6. Fuel

So far the best success we've had with fusion comes from using hydrogen isotopes (some blend of tritium and deuterium) as the reactive mass. There's three problems with this. First, too little tritium and deuterium and the reaction starves out. Second, too much and the reaction absorbs itself because tritium and deuterium absorb the neutron flux that is generated by and sustains the reaction. Third, tritium is literally the most expensive commercially available substance on the planet. The amount of tritium in a high quality watch is far less than a milligram and yet costs in the neighbourhood of $10. By comparison a gram of .999 fine commodity gold is also about $10. Doing the math out that means tritium is at least a thousand times more expensive than gold.

Also we still haven't figured out a way to produce tritium on a large scale that doesn't involve nuclear fission reactors, and there is no way to store it for long periods of time because tritium has this irritating tendency to decay into other substances (deuterium, helium, and hydrogen).

7. Usability

Okay so lets assume we have a controlled self sustaining reaction that doesn't explode massively, instantaneously burn all matter on the planet, or emit so much hard alpha and neutron flux that we all dissolve into flaming little puddles of semi organic goo that glow like light sticks. Let us further assume that we have figured out how to fuel these reactions without bankrupting national economies.

Big assumptions those.

But let's say we do get past these issues, and I am sure that eventually we will if we research enough, what do we do with this fusion reaction?

The instinctive gut response is "use the energy". Ok, how? The most widespread way we as a species have come up with to put energy to use is electricity. Alright so we turn it into electricity.


In the past three hundred years we have come up with precisely four ways for generating practical amounts of electricity (and a couple of interesting but impractical things too, but I won't get into them here): Interesting chemical reactions (this includes solar), smashing crystals, rubbing dissimilar materials together, and moving magnets near each other.

How is it that we will use the fusion reaction to do one of these things?

Okay how do we use the energy form a fission reaction to generate electricity? Well primarily we use the waste heat of the reaction to boil water, which then builds into high pressure vapor, which can be forced through a turbine.

That process will use what, a millionth of a percent of the energy released in the fusion reaction, a billionth? And of course the rest will be waste.

That much waste heat will be at minimum interesting to deal with.

Oh if only there were direct conversion. Of course then we wouldnt need fusuion in the first place, or rather we wouldnt need terrestrial fusion, because all of our energy needs would be supplied by direct conversion of sunlight (instead of the now 10% or so maximum conversion efficiency we have with photovoltaic cells).