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Boron - Neutron Fuel

Posted: Wed May 29, 2019 11:19 pm
by IsaacBean
My experience with nuclear fusion is minimal though I have done some research on it and I know this idea is technically neutron capture, not fusion buuuuuttt....

I was wondering if neutron absorption of boron would be a feasible way to produce energy. I couldn't find anything relating to this topic, so I figured this forum would be a good place to start.

One of the main issues with generating net-positive energy in a fusion reaction, as I understand it, is the necessary energy to force the two positively charged nuclei together. While the boron-proton reaction yields about 14 MeV of energy, a boron neutron reaction yields about 2.5 MeV along with Lithium and an alpha, and occurs with thermal neutrons.

The neutron source could be a large amount of americium-beryllium or even a deuterium fusor if that does not take too much energy. This source could be surrounded by borated-water that could thermalize the neutrons and also absorb the energy from the boron-neutron reactions.

Any insight into how bogus or practical this idea is would be greatly appreciated.

Re: Boron - Neutron Fuel

Posted: Thu May 30, 2019 3:59 am
by Rich Feldman
OK, so you start with a bunch of neutrons at 2 or 3 MeV from a fusor or alpha-Be reaction.
Moderate them down to thermal energy. Almost all of the initial neutron energy goes into heating the moderator.

Some fraction of the initial free neutrons will be absorbed by the moderator, some will escape, and some fraction will be captured by your boron, followed by a 2.5 MeV decay (the number you gave). I would think that at best, the heat added by the boron step is less than the heat without it. No great "energy multiplication".

Remember that the neutron power from a fusor, or a radioisotope source that isn't incredibly hazardous, is less than a microwatt.

Re: Boron - Neutron Fuel

Posted: Thu May 30, 2019 4:29 am
by IsaacBean
I was thinking it would have to be an incredibly hazardous amount of radioisotope to get a usable amount of energy...but then I suppose the power level would not be controllable through chain-reaction, and you may as well be dealing with fusion at that point.

Thanks for the insight Rich.

Re: Boron - Neutron Fuel

Posted: Thu May 30, 2019 7:58 am
by Richard Hull
No matter the idea, fusion reactions have all been figured out and mused over for years. All have been net energy SINKS, in the sense that all required input energy to work the reaction will return far less, and in some cases, virtually infinitely less energy than was applied to make the reaction go.

Fusion is the energy of the future..... and it always will be.

Richard Hull

Re: Boron - Neutron Fuel

Posted: Thu May 30, 2019 9:47 am
by ian_krase
Ehhhn, not quite all of them - and we have practical applications ready to go, as far as space travel is concerned, for people who aren't cowards.

Re: Boron - Neutron Fuel

Posted: Thu May 30, 2019 9:14 pm
by Richard Hull
In space, fusion reactions low energy returns are just great, of course. In space, you want propulsion and the sky is the limit on fusion losses if it gets you moving. A fission power source on board driving a fusion propulsion system might be viable. Out to the orbit of Jupiter a solar power source might be possible. It would depend on the mission goals, of course. Deep space, fission power would be demanded.

Net power gain fusion reactions that are put forward for commercial purposes on earth, (electrical power), are another matter.

Richard Hull

Re: Boron - Neutron Fuel

Posted: Fri May 31, 2019 2:11 am
by Rich Feldman
I think Ian was talking about atomic bomb impulse rockets. Wasn't there a Project Orion about them?

Putt-putt boats, but efficient putter technology is quite mature. IIRC, specific energy of nuclear explosives (whole device) leveled off at about a million times greater than that of chemical explosives.

Re: Boron - Neutron Fuel

Posted: Fri May 31, 2019 5:01 pm
by Dennis P Brown
Nuclear fission drives via explosions is nether doable nor mature technology but most of all, simply not needed; first and foremost, the misconception that this is an overwhelmingly superior drive compared to chemical rockets is one issue.

Relative to that too simplistic belief one must start by understanding how the drive works (most people somehow miss this rather important point for reasons that are not clear to me) - A small atomic bomb explosion in vacuum actually creates little drive force directly but requires radiation induced erosion via vaporization of a "pusher" to get its maximum performance. Ignoring the terrible shock wave that results, the size and weight (mass) of this pusher will have to be very large indeed to enable the rest of the rocket to survive both the explosion and erosion damages - of course, I'm ignoring the serious issue of radiation induced damage for long term drives that are envisioned; the bottom-line is that this large mass decreases some of the advantage of using "pusher" fission for driving a real payload.

Further, the minimum size of an atomic bomb is not all that small (one must include the chemical driver.) So this extra parasitic weight further reduces drive performance far more than just looking at the mass of the fissionable material. As such, the shear mass of the required atomic bombs for a given fuel to payload ratio isn't anywhere near the level a simple comparison between chemical rockets and fission rockets would appear if one simplistically calculates the efficiency by just using the difference in energy conversion ratio's for the chemical fuel vs. atomic fuel.

Then the extremely important issue of availability of the required "minimum" sized atomic fuel (U233) - that uranium isotope (not natural) is staggeringly expensive to acquire (ignoring the extreme danger of using that highly fissionable material).

Using U-235 (and that is also very expensive to make in near pure form) is better but requires significantly greater over-all mass for the bombs and hence total fuel mass. These issues don't even approach the staggering engineering issues caused by the radiation, shock waves or how to get bombs into the reaction chamber on a regular bases without fail.

And for what reason? Faster travel to exactly where in the solar system and for what purpose? The finest available mineral resources are here on Earth and vastly cheaper to obtain and this will most certainly be true for many, many years still - energy production in a safe manner is our only critical limiting factor over the next hundred years. For living space? LOL. I won't waste space here on that extremely stupid idea. Then exactly why develop this extremely difficult concept? Maybe to enable robotic interstellar travel? LOL. Again, waste of time to discus the many reasons that idea is laughable.

The only reason this type of drive hasn't been developed is because its a terrible idea.

Re: Boron - Neutron Fuel

Posted: Tue Jan 07, 2020 12:57 pm
by russssellcrow
Looks like the Russians have a breakthrough in nuclear powered flight, albeit a messy one. "Avangard" is one of the weapons that Putin touts as being game changing. My understanding is that they have two systems. One launches on a chemical rocket to space, then skips off the atmosphere, dodging and weaving to avoid interception, before it plunges to the target. The other is boosted to the edge of the atmosphere, where it scoops the thin air into its nuclear core, and cruises around the Earth for weeks at hypersonic speed. Either pose an escalation threat to the Arms Race. The idea of a nuke core heater engine, spewing out radioactives for weeks of testing, seems really nasty.