Neutrons from unwanted side reactions?

[Lars]

Some People have claimed the p-B11 reaction would not be truly aneutronic, since there are unwanted side reactions that produce neutrons.

This Wikipedia article states:

Detailed calculations show that at least 0.1% of the reactions in a thermal p–11B plasma would produce neutrons, and the energy of these neutrons would account for less than 0.2% of the total energy released.

At first glimpse this would mean a 1GW reactor to produce 2MW of neutrons - quite much.
But this calculation was made for a thermal plasma. For a nonthermal monoenergetic ion distribution like in a polywell the neutron production should be several orders of magnitudine lower. Since there are nearly no ions with the optimal energy for these side reactions.

Wikipedia states two side reactions that involve the reactants and products of the primary fusion reaction.

First:

These neutrons come primarily from the reaction

11B + α → 14N + n + 157 keV

... A clever magnetic confinement scheme could in principle suppress the first reaction by extracting the alphas as soon as they are created, but then their energy would not be available to keep the plasma hot.

Well, in a polywell the alphas do leave the plasma as soon as they are created! Making the plasma hot by thermalising them would be unwanted!
And even if some alphas collide with B11s their energy will be much to high to have a significant cross section for fusion. Only by thermalising they would go through the right energy for the reaction.

So this reaction will not happen for lack of alphas with energy low enough.

Second:

Another significant source of neutrons is the reaction

11B + p → 11C + n - 2.8 MeV

... The second reaction could in principle be suppressed relative to the desired fusion by removing the high energy tail of the ion distribution, but this would probably be prohibited by the power required to prevent the distribution from thermalizing.

In a polywell the ion distribution is monoenergetic, so there is no high energy tail of the ion distribution. So this reaction will not happen either. This time for lack of ions with enough energy to do it.



Another neutron source claimed are deuterons from isotopical unclean hydrogen.

Finally, isotopically pure fuel will have to be used and the influx of impurities into the plasma will have to be controlled to prevent neutron-producing side reactions like these:

11B + d → 12C + n + 13.7 MeV
d + d → 3He + n + 3.27 MeV

But at first natural hydrogen has a very low deuterium content to begin with and again the energy distribution in a polywell optimised for p-B11 will be wrong for these reactions.

So p-B11 per se might not be called truly aneutronic. But p-B11 in a polywell would come much nearer to this ideal.


Much more then by neutrons I would be concerned by gammas from this reaction:

In addition to neutrons, large quantities of hard X-rays will be produced by bremsstrahlung, and 4, 12, and 16 MeV gamma rays will be produced by the fusion reaction

11B + p → 12C + γ + 16.0 MeV

with a branching probability relative to the primary fusion reaction of about 10−4.

But here even the wall of the vacuum chamber should absorb a significant part of these gammas.

[MSimon]

Lars,

Excellent bit of work!

I think the H used would be the "waste" from Deuterium production so that the concentration of D would be lower than natural. Which would help suppress that chain even more. Also it further lowers D-D side reactions.

Nothing a mfg likes more than to be able to sell his waste.

[drmike]

I'm not too worried about the - 2.8 MeV reaction, no protons or borons will be going that fast. Agreed that isotopic purity will help and gammas will be part of the thermal source, so they won't hurt either.

I think knowing the cross sections as a function of energy will help finish this off. As long as we can stay away from the peak cross sections, we can reduce side channels even more.

[Tom Ligon]

Thanks for finding that! I found an article, possibly that one, over a year ago, that detailed the neutron-producing reactions. I also concluded several of the major contributors simply did not apply to the Polywell, for about the reasons you state. Unfortunately, I lost the bookmark to the article, and was unable to find it in subsequent web searches.

A couple of the reactions I recall presumed a thermalized environment, including an upscattering of the reactants to achive the required energy, and it looks like you found the same thing.

Alphas hitting the walls hard would cause a host of reactions, but we'll want to pull that energy out in the direct conversion scheme before the walls are hit. The magrids will still contribute.

Dr. Bussard concluded the bremsstrahlung emissions would discourage dallying around an unshielded reactor. Provided he was right about reducing it to a point that allows net power and then some, this is a problem we can live with, using distance and shielding. It does not convert the reactor guts to radioactive materials.

[JohnP]

Lars, great analysis, very interesting.
I have a question though (for anyone) - even if you have only a few hundred watts of neutrons to deal with, won't that still affect reactor material lifetime, shielding considerations, etc?
Or will the other, larger stresses on an operating reactor make the neutrons a nonissue in practical terms? Also, would a smaller neutron output still be a hazard for space travel use?

[MSimon]

John,

It is a matter of total dose.

Take MgB superconductors made with natural B. They have a total dose tolerance of about 1E18 n/sq cm before they lose their superconductivity. So the lower the rate the longer the lifetime.

[Tom Ligon]

John P,

I would only worry about reactor lifetime.

Figure the estimates are a tokamak running DT would have a life expectancy of two years, due to neutron damage. That's unacceptable according to EPRI.

Here is a really rough guestimate, which must be applied with caution as the power density on a tokamak would be much lower than a BFR, and we do have to worry about magrid superconductor lifetime. Drop to 10% of the expected tokamak neutron flux, and you're probably in the acceptable range. Drop to 1% and neutron flux is probably not the determining factor in reactor life.

Neutrons are almost trivially easy to shield if you can stand any bulk. Boron 10 (the throw-away 20% of natural boron left over from refining B11 fuel) is an ideal neutron poison. Mix with a little water or other moderator.

As with any reactor, distance is the best shielding. Most nuclear spacecraft designs put the crew compartment as far from the reactor as possible, typically on a long mast, and put fuel and cargo in between for good measure. Deep space craft can easily be made this way. Landers and Dr. Bussard's fanciful aerospace shuttle would be the more interesting packaging problems. In any case, the high overall performance of the system allows more shielding to be carried, if needed.

[Torulf2]

There may be alpha problems with C*12. For the DPF Lerner said it must be closed 9 hours from turned of for this carbon to decay. May the production of this carbon also be lesser in the polywell?

[tonybarry]

Hello Torulf,
Carbon 12 is the stable stuff. Maybe you meant another isotope?

Link

Regards,
Tony Barry

[TallDave]

Thanks for the post Lars, I had never considered the fact that these were calculated for thermal distributions.

Can'r wait till we get a machine built that's powerful enough to start playing with energies this high.

[Keegan]

A most excellent post Lars !

I had always feared that neutrons from side reactions could be problematic. After trawling through reaction cross sections, it seems that either materials resist alpha or neutron bombardment but rarely both.

Its fantastic news that p + B11 can now be confirmed Aneutronic in Polywell machines. This is a fact that Dr Bussard most likely discovered, one we are only now rediscovering ourselves.

I believe the first wall problem is largely solved. We only have to consider alpha particles and gammas.

So far my favourite is Diamond coating.