# Nuclear fission / fusion? **URL:** \ **Category:** Factual Questions\ **Tags:** science-math\ **Created:** [January 13, 2024, 5:06pm UTC](https://boards.straightdope.com/t/nuclear-fission-fusion/996046 "2024-01-13T17:06:15Z")\ **Posts on this page:** 6\ **Page:** 4
**Author:** ![Horatius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/horatius/32/3572_2.png) [@Horatius](https://boards.straightdope.com/u/Horatius)\ **Post date:** [February 16, 2024, 6:41pm UTC](https://boards.straightdope.com/t/nuclear-fission-fusion/996046/61 "2024-02-16T18:41:37Z")
> [@abcdefghij](#): > > The nanotechnologically processed structures are so tightly “packed” that they don’t _have_ holes! So I was idly wondering how dense this film would have to be to actually do what they say it does. I found a power point set for a lecture about neutrinos, in which they did a calculation of the mean free path of a neutrino in a given material. Using water as an example, they came up with this: λ is the mean free path σ is the cross section ρ is the density of targets (In water ρ~ 6 ×10^23cm−3) For 1 GeV neutrino interactions σ ~10^−38 and λ=1/ (10^−38 x (6×10^23)) ≈ 10^12 meters So we’d need something that is 10^12 times denser than water to reduce the mean free path to a meter. Although, we probably wouldn’t need such a short mean free path, as even a small percentage of the neutrino flux would provide a lot of energy. But even if we only sought a small fraction, say one-one millionth, that still leaves us with something far denser than water (million times, if my math is right, 10^6), and an interaction chamber a meter across. One cubic meter of water is 1000kg, so you’re looking at a device that masses 1000 million kg. Even if we reduce our expectations another couple of orders of magnitude, a 10cm cube would be a million kg. To get down to a mass of just 1kg, you’d have to reduce your 10cm cube to a 10x10 sheet only one micron thick. The density of that film would still be off the charts, and you’d reduce your capture of neutrinos by another factor of a million, if I haven’t lost an exponent somewhere. I think i have, actually, but let’s be generous. ---
**Author:** ![Frodo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/frodo/32/302_2.png) [@Frodo](https://boards.straightdope.com/u/Frodo)\ **Post date:** [February 16, 2024, 7:37pm UTC](https://boards.straightdope.com/t/nuclear-fission-fusion/996046/62 "2024-02-16T19:37:29Z")
Just ask _Qadgop\_the\_Mercotan_ for some neutronium and you’re set. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [February 18, 2024, 8:06am UTC](https://boards.straightdope.com/t/nuclear-fission-fusion/996046/63 "2024-02-18T08:06:12Z")
Alastair Reynolds wrote a trilogy of books set in a distant future where the planetary mass of the solar system has been broken down into thousands of small colonized spherical stations with gravity provided by tiny artificial black holes in their centers. The story takes place in a period of decline after several rises and falls of technological civilizations. Travel between the colonies is largely done by ships with enormous solar sails. One of the greatly prized lost-technology artifacts sometimes discovered is a thin (light) black fabric that is opaque to neutrinos and is extremely useful for neutrino-pushed solar sails. > **[Revenger (novel)](https://en.wikipedia.org/wiki/Revenger_(novel))** > > Revenger is a 2016 science fiction novel by British author Alastair Reynolds. It is unconnected to any of Reynolds's previous works, and is the first book in the Revenger Trilogy. A sequel, entitled Shadow Captain was published on 10 January 2019, and a third and final book in the trilogy, Bone Silence, was published in 2020. > Revenger won the 2017 Locus Award for Best Young Adult Book, and was a finalist for the 2018 Philip K. Dick Award. > Revenger has been labelled by some reviews as a young... ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [February 21, 2024, 9:07am UTC](https://boards.straightdope.com/t/nuclear-fission-fusion/996046/64 "2024-02-21T09:07:16Z")
In _Diaspora_, Greg Egan suggests an efficient neutrino detector using highly-excited nuclear isomers. To be honest, I’m not sure if this is technobabble or if it’s a hypothetical way to gain efficiency. The detector wouldn’t be _opaque_ the way the catchcloth was in _Revenger_, but it might at least enable better neutrino imaging (that could image the interior of an entire planet, for example). So far, I haven’t been able to find a paper suggesting the possibility (and my physics knowledge is too limited to evaluate it). On the other hand, Egan often works out his physics to incredible detail (including alternate physics), so I’m not inclined to wholly dismiss it. ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [March 4, 2024, 3:19am UTC](https://boards.straightdope.com/t/nuclear-fission-fusion/996046/65 "2024-03-04T03:19:16Z")
> [@xtenkfarpl](#): > > This is a bit of a hijack, but is there any possibility that a fusion reaction could be ignited without a fission starter? This is the goal of every **cold-fusion researcher** - the theory is that if you push large amounts of deuterium into some sort of matrix (palladium absorbs very large amounts of hydrogen), then the additional density would reduce the energy required to make nuclei fuse. Pons and Fleischmann claimed to have demonstrated thin in 1989, but their experimental results were not replicated and contained errors. It is considered a dead end. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 4, 2024, 9:58pm UTC](https://boards.straightdope.com/t/nuclear-fission-fusion/996046/66 "2024-03-04T21:58:38Z")
Cold fusion was one attempt to start fusion without a fission starter, but it’s not the only one, and there are other ways that work. None of them has yet been developed to produce a sustained reaction without a fission starter, though of course that’s a hot area of research. [Previous page](https://boards.straightdope.com/t/nuclear-fission-fusion/996046.md?page=3)