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Boeing Patents an Engine Run By Laser-Generated Fusion Explosions
MarkWhittington writes: Boeing has had a patent approved for an aircraft engine that uses laser-generated nuclear fusion as a power source, according to a story in Business Insider. The idea is already generating a great deal of controversy, according to the website Counter Punch. The patent has generated fears of what might happen if an aircraft containing radioactive material as fuel were to crash, spreading such fuel across the crash site.
U238 will happily absorb neutrons (which are produced by the fusion) and become U239. U239 will happily absorb beta radiation (also production by the fusion) and become Np239. Np239 will also happily absorb beta radiation and become Pu239. Pu239 is nasty stuff that you don't want to get anywhere near you.
This is in fact exactly the reaction used in the production of Pu239 for nuclear weapons.
we don't currently have fusion working
We don't currently have economically viable, contained fusion reactors working.
It's worse - as is noted below, it's not actually a fusion engine proposal, but rather a hybrid fission/fusion proposal. It's not a new concept, but the key is, a lot of (read: "most of") the power is to come from fission.
I really can't think that Boeing would be so daft as to think that anyone would ever use this on Earth. Surely the point of the patent is to use it for exploration of other planets. Right? I hope so...
Note that it's not 100% necessary for neutron bombardment to create radioactive material. One of the nice things about incident induced radioactivity is that it's avoidable and/or manageable... albeit with tradeoffs that usually mean that accepting some radioactivity is the best option. You could for example have enough of a neutron absorbing material to fully consume the neutrons - for example, boron, which breaks down via the huge cross section B10(n,alpha)7Li reaction. 7Li neutron capture produces 8Li, which quickly decades into 8Be (releasing a ton of energy), which virtually instantly breaks down into two alphas. B10 neutron capture (much rarer than (n, alpha) yields B11, which is stable. B11 neutron capture produces B12, which very rapidly breaks down into C12 (stable). C12 neutron capture is rare and turns into stable C13. The only way you get to anything that's radioactive that doesn't instantly break down is to get a rare neutron capture of C13 after going through all of those previous steps, some of which are rare. And that "radioactive isotope" is only C14, which is a naturally occurring radioisotope we're evolved to live with, with a not very powerful decay. And if you isolate it (which isn't anywhere on the difficulty scale of, say, removing actinides from nuclear waste), it's highly valuable.
Another good example would be to make your structure out of beryllium. Beryllium is a superb metal in almost every respect and would be widely used in the world if not only for two niggling details: its dust is highly toxic and it's very expensive. But things do get built out of it (and it's not hazardous when there's no dust). 9Be capture produces 10Be, which is radioactive, but with a half life of 1.5 million years, the radiation level is extremely small, you'd need a lot of it to present a hazard. Which would never happen; 10Be has a reasonably high neutron capture cross section, becoming 11Be, which breaks down into 11B, which we've already covered above.
You can also get additional reactions to the above cases with fast neutrons, but they generally only improve the situation.
Pretty much anything out of light elements poses little to no hazard from induced radioactivity. You start to get a bit once you get to aluminum, but not much - aluminum has to go through an awful lot of captures to turn into a silicon or phosphorus isotope with a relevant half life, the amount transmuted is pretty irrelevant in most situations. It's only if you need higher strength or heat tolerance than aluminum (or better, lithium-aluminum) can give you that you start getting into problems - titanium, iron, iron alloying agents, other common structural metals, they all have significant issues with induced radioactivity. But even with them, it's still nothing on the scale of, say, waste fuel rods.
The human body can be drained of blood in 8.6 seconds given adequate vacuuming systems.
All current fusion reactor designs rely on deuterium and tritium. Tritium is _quite_ radioactive, with a half-life of 12 years. There is also very little of it. The world supply is on the order of 20 kilograms, and it's all accumulated from fission reactors. quoting Wikipedia, "Commercial demand for tritium is 400 grams per year and the cost is approximately US $30,000 per gram." Tritium cannot be reasonably refined: all tritium on earth in quantities large enough to refine is from fission reactors. Growing commercial production could improve the price tremendously, but it's source remains dangerous and expensive and inefficient to produce tritium.
Deuterium is stable, and available, but also quite expensive at $1000/kg. for deuterium oxide. With an atomic weight of 2, with two oxygen atoms of atomic weight of 16, the deuterium is only 2 / 34 of the mass. So the cost for pure deuterium itself is roughly $17000/kg, or about $17/gram. It's refinable from water, but the dollar cost reflects the energy costs of refining it.
The only large scale source of either isotope that would not be prohibitively expensive or rely on quite large scale fission generators is the solar wind. But much like large scale fission generators to create tritium it's senseless in terms of energy production. If you're bothering to build the plant for tritum, why not simply harvest the energy of the plant itself? A solar sail in orbit gathers roughly 2 kilowatts/square meter, and a roughly square kilometer mirror is quite feasible. That would be roughly 2 Terawatts of power. One could theoretically harvest deuterium and tritium from it, but with such a large power source, there seems to be no need to harvest it for fuel production for a much less efficient and quite radioactive system.