Fusion: it’s really hard

CrossFire Nuclear Fusion Reactor
CrossFire Nuclear Fusion Reactor (Photo credit: Wikipedia)

 

I write exclusively fantasy but my son writes science fiction so we talk about a lot of concepts suitable to that genre. One thing that popped up recently relates to fusion. Fusion reactors and weapons are a long-time staple of science fiction. And someday, we may actually figure out how to make one work but there is a reason why for any point in the last 50 years, a workable fusion reactor has always been “30 years away”: it’s a really tough problem. Although these days, there’s some hope it’s only 10 years away (see some links below).

 

Two facts help to put this into perspective. First, fusion releases a lot of energy, right? We’re all familiar with the enornmous blast of a fusion bomb with the largest ever, the Tsar Bomba coming in around 50 megatons (and supposedly capable of 100 megatons: the Soviets scaled it back so their drop plane had a chance of escaping the blast.) An uncontrolled fusion reaction can release a lot of energy (though keep in mind thermonuclear bombs are typically a fission-fusion-fission reaction). However, more sustainable fusion reactions proceed at quite leisurely rates: in the core of our sun, with all the vast energy it produces, a cubic centimeter produces a whopping 276 microWatts. It so hot because it is so big. There’s a lot of these paltry, energy producing cubic centimeters in the solar core.

 

Nuclear weapon test Mike (yield 10.4 Mt) on En...
Nuclear weapon test Mike (yield 10.4 Mt) on Enewetak Atoll. The test was part of the Operation Ivy. Mike was the first hydrogen bomb ever tested, an experimental device not appropriate for use as a weapon. (Photo credit: Wikipedia)

For comparison, that’s about a quarter of what you, a mammal, produces in a cubic centimeter of your body. It’s on par with what a reptile produces. That energy rate must have been what the makers of the movie, The Matrix, must have been thinking of when they made the machines want humans as energy sources. That rationale, by the way, is one of the most bone-headed premises in the history of science fiction. Yes, humans produce energy at that rate. Simply burning the food would produce the same energy (and both require the same amount of oxygen). But ultimately food is just repackaged solar energy. Find some way to convert the solar energy more directly to machine power. Adding humans into the loop makes no sense whatsoever. (And yes, I recall the Matrix Earth was all cloudy… where does the food for the humans come from?) Think about it: the sun with all its mass and its very dense core (~160 times water at 15M degrees Kelvin) manages less energy that you produce in an equivalent volume of your body. Go cheeseburger-reactors!

 

The Sun is a natural fusion reactor.
The Sun is a natural fusion reactor. (Photo credit: Wikipedia)

Stellar fusion is more efficient in higher mass stars where other fusion reactions are available but in the end, even a very high mass star isn’t making that much energy in a cubic centimeter. Whatever processes we use for a fusion reactor (and options are nicely described in the Wikipedia articles linked above), it would have to be a lot more energy dense than stellar reactions.The other thing to keep in mind is that if we can jack up the fusion reaction rate to something useful for power production, we will be making a vast amount of radiation by-products. Many years ago on a (unclassified) tour of the fusion reaction experiments at Lawrence Livermore National Labs, the presenter mention that at the time, estimates suggested that every single atom in the inner chamber of a fusion reactor wall would be displace six times each year. That’s really not a good thing for metallurgy: even if you can cool the reactor core walls, it’s going to get pounded to dust. Interestingly, the suggestion, at least at the time, was to use a blanket of liquid lithium as the inner “wall”: it carries heat and as a  bonus, the high energy neutrons reacting with the lithium atoms would produce deuterium, i.e., fuel for the reactor. As with so much in the fusion business, for that actually to be viable, much would have to be worked out and I have no idea of the deuterium production rate turned out to be useful. Still, interesting thoughts.

Where does that leave fusion reaction for science fiction stories? Well, they are an accepted staple (along with anti-matter reactors) so they can be used as-is with no explanation in most stories. Perhaps in a near-term science fiction story some nod we have to be made to how these worked but the Wikipedia articles and other resources have plenty to work from there. But as with the problem of how to get a dragon to fly to fly, these challenges can offer interesting facts to reference for verisimilitude or even make an interesting premise for a story. I don’t call these issues out in order to say you can’t use fusion in your stories, I call them out because details like this could make for interesting elements in a story. (And because it is fun to slam The Matrix for using humans as batteries. Geeze that was stupid. Yeah, I know they had a better rationale and they decided to dumb it down. They succeeded.)

There are potentially similar problems with fusion weapons, especially man portable ones, but there things aren’t quite as bad as they seem: for something man-portable, the energy release is relatively close to the man using the weapon. Therefore, you don’t need a vast amount of energy released and therefore you aren’t necessarily generating a lot of non-thermal radiation by-products (not really sure how much, you’d have to do the math).  But in the end, there are probably a lot better ways to release tactically useful energy than causing a fusion reaction in proximity to the soldiers using the weapon, who presumably want to survive the discharge of the weapon.

Happy fusing!

 

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4 thoughts on “Fusion: it’s really hard

  1. Pingback: Fusion Breakthrough | David's Commonplace Book

  2. Pingback: One giant leap for mankind: £13bn Iter project makes breakthrough in the quest for nuclear fusion, a solution to climate change and an age of clean, cheap energy | Cbcburke9's Blog

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