Aneutronic Fission

[Moontanman]

Are there any reactions that allow aneutronic fission? I remember vaguely reading about the possibility in Omni magazine many years ago but I forget the details.

[nghosh@india]

i just search the wikipidea about aneutronic fission.and i think you should do it too

[swansont]

i just search the wikipidea about aneutronic fission.and i think you should do it too

 

If you bothered to post this you could have provided a link. (I can find one for aneutronic fusion, but not fission)

[Moontanman]

I did google it, not much to go on for sure...

[Enthalpy]

Never heard of. Elements heavy enough contain a higher proportion of neutrons than fission products, and some excess neutrons use to fly away early: within some ps of the fission for most neutrons, about 1s later for 1-2% of them. The remaining neutron-proton unbalance in fission products evens out through beta minus radioactivity.

 

Worse: fission neutrons are needed to sustain a chain reaction, provided this is the intent. Without the chain reaction, for instance through proton bombardment; fission needs more input energy to proceed than is extracted from. Some reactors (like Rubbia's design) claim to work from a proton accelerator, but still rely on neutron emission for 99% of all fissions, protons making only 1% (which makes stability not so obvious...). Even then, the necessary accelerator's power has never been achieved up to now.

[Moontanman]

I do remember some bits of the article, if my memory serves me they suggested a boron target for accelerated protons. They claimed a net energy gain from this.

[swansont]

I do remember some bits of the article, if my memory serves me they suggested a boron target for accelerated protons. They claimed a net energy gain from this.

 

That makes sense, as the stable isotope N/Z ratio increases once you get past Z=20, so you'd expect neutrons liberated from higher-Z reactions. Below that, N/Z is ~1.

 

A proton bombarding B-11 could eject an alpha, leaving you with Be-8, which almost immediately splits into two more alphas. I get 8.7 MeV liberated by the reaction

[Moontanman]

That makes sense, as the stable isotope N/Z ratio increases once you get past Z=20, so you'd expect neutrons liberated from higher-Z reactions. Below that, N/Z is ~1.

 

A proton bombarding B-11 could eject an alpha, leaving you with Be-8, which almost immediately splits into two more alphas. I get 8.7 MeV liberated by the reaction

 

 

At the time the article claimed it was the new energy source that would revolutionize our society, allow reactors of about a cubic meter whose only waste was helium. I've noticed it didn't come to pass, any reasons why ?

[swansont]

At the time the article claimed it was the new energy source that would revolutionize our society, allow reactors of about a cubic meter whose only waste was helium. I've noticed it didn't come to pass, any reasons why ?

 

Not sure. You still have to capture the energy and convert it to a usable form, and there are issues of how accessible B-11 is and whether you need isotopically pure samples.

 

There's also the issue of having to run a proton accelerator. I recall from my time at TRIUMF that a 1 microamp current of protons was the limit, before they modified the cyclotron (I think now they do 10 or perhaps 100 microamps; it's possible that a linear accelerator with protons can get higher values. I'm not sure). Let's assume we have 100% efficiency in reactions. Each amp is 6.25 x 10^18 reactions per second, releasing 8.7 MeV each. So, 8.7 MW of energy generated per amp of proton beam current. You'd have to find a way to generate ~ milliamp, I would guess, because you need to convert your thermal production to electricity and also have the overhead of running your accelerator. You'd get some fraction of the proton KE back, of course, but we're probably talking an MeV or maybe several (i.e. MW per amp of current) to give to the protons in the beam to initiate the reaction.

 

I don't think there's a tremendous amount of energy left over to make it economical (though you do have the bonus of making Helium, which is becoming scarce) and you need a decent accelerator current to make it work.

[Moontanman]

I can't remember how they proposed extracting usable energy from this "reactor" the memory is dim now but for some reason the sterling cycle engine connects in my mind. it was proposed that every home would have on of these under it generating power. Omni magazine didn't last very long and i have given this some thought from time to time. thanks for the info.

I found this interesting tidbit on wiki

 

 

https://en.wikipedia.org/wiki/Boron

 

¹¹B is also a candidate as a fuel for aneutronic fusion. When struck by a proton with energy of about 500 keV, it produces three alpha particles and 8.7 MeV of energy. Most other fusion reactions involving hydrogen and helium produce penetrating neutron radiation, which weakens reactor structures and induces long term radioactivity thereby endangering operating personnel. Whereas, the alpha particles from ¹¹B fusion can be turned directly into electric power, and all radiation stops as soon as the reactor is turned off.^[54]

It has a name.. Migma Reactor...

 

What it is seems to be a bit more than confusing.

 

http://www.rexresearch.com/maglich/maglich.htm

 

some of the papers are pay per view type stuff i can't access...

For some reason all of the reactions are called fusion instead of fission even if the end result is fission of a larger nucleus.

 

http://en.wikipedia.org/wiki/Aneutronic_fusion

[swansont]

Interesting. I had not considered direct conversion to electricity — the alphas will ionize atoms as the pass through a material, so there's potential for a significant multiplication of current. And only 0.5 MeV to initiate it is much simpler to do.

[imatfaal]

Crystalline Boron is expensive USD5 per gram but not bad - uranium is about $0.13 per gram - but the expense of the raw material is dwarfed by the clean up and waste storage costs. But that would be a mix of 10B and 11B - whether the 18-20% of 10B would screw up the reaction who knows?

[Enthalpy]

Proton-Boron is a well-studied reaction, generally given as an example of aneutronic fusion - but it produces three alphas so one could call it fission as well... I'd say: more energy comes from the strong force than the electric repulsion, hence fusion.

 

This reaction is a remote hope to produce heat without the neutrons that make the surrounding materials radioactive. Though, I know no single reaction that relies solely on accelerated particles and has a chance to release net energy. This reaction is considered in plasmas instead, with Z-striction leaving a tiny hope, laser not being considered a candidate in a planned timeframe, magnetized target being about exluded and tokamaks hopeless.

 

If any fusion released net energy; the conversion to electricity would be a lesser difficulty... Vapour turbines work! Direct electric conversion to several kV has been demonstrated with alpha and beta radioactive sources, but is less efficient and convenient even than thermoelectric elements - which should be replaced by a thermal engine anyway, a turbine rather than a Stirling. Charge carriers multiplication has been demonstrated and even used also: a radioactive material coats a photocell, where silicon takes 3.5eV to make a carrier pair harvested at 0.45V - the serious limit is the deterioration of the Solar cell, whose semiconductor properties are more fragile than a metal's mechanical performances.

[swansont]

Crystalline Boron is expensive USD5 per gram but not bad - uranium is about $0.13 per gram - but the expense of the raw material is dwarfed by the clean up and waste storage costs. But that would be a mix of 10B and 11B - whether the 18-20% of 10B would screw up the reaction who knows?

 

Not just clean up and waste storage costs, but also the containment/shielding of the reactor itself, most of which is rendered moot.

 

The presence of B-10 means you'd get some C-11 if it captured the proton. C-11 electron captures to B-11, so it's radioactive and you'd still need some shielding, but the half-life is ~20 min, and, bonus, you get more B-11. If it ejected an alpha, you'd have Be-7, which electron captures to Li-7, with a half-life of 53 days. Neither of those seem like a deal-breaker. Ejecting a neutron seems much less likely to me, since the isotopes under discussion are already neutron-deficient.