Fusion tritium problem solution?

[artis]

There are many articles like this one
https://www.science.org/content/article/fusion-power-may-run-fuel-even-gets-started
Where scientists talk about the problem of obtaining tritium even if the fusion engineering itself finally crosses the "net gain" margin for electricity production.

Here is a bit of a "Mr. Obvious" question from me. I do recall reading that the reason the US first "solid state" H bomb - Castle Bravo experiment went largely overboard in terms of yield was because the lithium deuteride second stage fuel contained both Li6 as well as Li7 and Li7 also fission just like Li6 and release either tritium, alpha +n or 2 alphas, n and about 15MeV of energy, In other words both stable Li isotopes fission in exothermic manner and release alphas and KE to the surroundings in which they are in.

Now the tokamak will use a blanket which will also have a neutron multiplier (Beryllium) and Lithium in it. But this process is lossy because the lithium is surrounded by other structural materials and some neutrons will eventually get lost.

So instead, why not put the Li in a gaseous or aerosol form within the fuel (D-T) itself and then during fusion it would release the needed tritium + KE right into the fuel mix.
Now in a tokamak it might not work because the plasma is very low density and the cross section for the neutrons to strike the Li particles would be small, but how about in inertial confinement schemes?



PS. One crazy idea that comes to mind is making the tokamak blanket out of lithium, the toroidal magnets are already cryogenically cooled to maintain their superconductivity, why not have a torus inner wall that is made of a Li lining that is cooled. The surface Li would be bombarded with neutrons continually and Li fission products released into the reaction chamber directly instead of indirectly?


So how far off are these ideas?

 

[Arjan82]

It looks like someone already stole your idea, from the article you cited:

"
Most fusion scientists shrug off the problem, arguing that future reactors can breed the tritium they need. The high-energy neutrons released in fusion reactions can split lithium into helium and tritium if the reactor wall is lined with the metal. Despite demand for it in electric car batteries, lithium is relatively plentiful.
"

 

[artis]

@Arjan82 , well I have to inform you that the fission of Lithium is not "my idea", nor is my idea to line the reactor namely tokamak walls with lithium. They do that but the lithium is behind a "first wall" of radiation absorbing/enduring materials.

 

[Astronuc]

So instead, why not put the Li in a gaseous or aerosol form within the fuel (D-T) itself and then during fusion it would release the needed tritium + KE right into the fuel mix.

Well, firstly we envision fusion reactors using plasmas, because fusion requires high temperatures.

Secondly, at such high temperatures as the atomic number (Z) increases (H,D,T: Z=1, ³He, ⁴He: Z=1, ⁶Li,⁷Li Z=3, . . .), the recombination and bremsstrahlung radiation losses increase. One has to look at the temperatures for each possible reaction, as well as the whole set in order to determine the optimal temperature for the plasma.

Thirdly, to maintain a neutral plasma, the number of electrons must match the total nuclear charge, and so Li (Z=3) has three electrons, and so their contribution to the electron pressure ~ nkT is three times that for the same nuclei density as D or T with Z=1. Electron pressure doesn't help.

 

[BillOnne]

Astronuc did answer your question, but here is a restatement of the problem in simpler terms.

Putting stuff in the plasma will have a strong tendency to mess up the fusion process. You want only the things that are reacting, especially you want as little as possible of isotopes with higher number of protons. The more protons in an isotope, the more effect they have on the plasma. Bigger mass things are moving slower at the same temperature. So putting Li6 or Li7 in a plasma with T and D will be a big problem. It will strongly tend to kill off the plasma.

As well, the neutrons from the easiest reaction are quite high energy, typically 10 MeV or so. They won't pay much attention to the plasma, just breezing out. So there won't tend to be a lot of T produced from Li in the plasma, even if you can keep the plasma hot enough to react.

That is a non-trivial part of why fusion is a hard problem. If you do get it working, you get this flux of high energy neutrons. Their effect on the walls of the system are annoying, since over time they tend to break down any structure. So first you have to deal with the plasma doing stuff to the wall, and spalling stuff off the wall to poison the plasma. Then you have to deal with the neutrons coming out and activating and crumbling the walls.

So the blankets of absorbing material are the idea. They solve the neutron problem, at least part way. And they provide you with your fuel. And they stay out of your plasma.

But give Astronuc the "like" because he said all of that.

 

[berkeman]

Welcome to PF.

You want only the things that are reacting, especially you want as little as possible of isotopes with higher number of protons. The more protons in an isotope, the more effect they have on the plasma.

You mean different numbers of Neutrons in the isotopes, right?

 

[artis]

Welcome to PF.


You mean different numbers of Neutrons in the isotopes, right?

I think yes. Because both Deuterium and Tritium are Hydrogen isotopes and by the definition of an isotope that is an element with the same number of protons but different number of neutrons.

I think he meant that one doesn't want different elements in the plasma, particularly ones that are with higher Z therefore heavier.


PS. @Astronuc now that this point was made I just realized, is it not the case that even in a purely D-T plasma the D temp aka KE is higher than that of T ? Because T is one neutron heavier than D isn't it ?

 

[artis]

So the blankets of absorbing material are the idea. They solve the neutron problem, at least part way. And they provide you with your fuel. And they stay out of your plasma.

That is true.
If one could get the temp high enough for long enough like in a H bomb then it doesn't matter anymore I think, there the Li simply undergoes fission and adds more tritium to the mix +KE of it's leftovers.

I think I have seen fusion schemes where the first wall is a slowly flowing thick molten Lithium or some mix thereby even though it doesn't directly participate within the fusion reaction, the fast neutrons hit it and T is produced within the Li but since T is gaseous it escapes and mixes in directly into the reaction chamber.

I would think that irrespective of the fusion scheme used one would wish to have the T extraction and production as close as possible to the actual plasma therefore minimizing leakage and loss.

 

[Astronuc]

PS. @Astronuc now that this point was made I just realized, is it not the case that even in a purely D-T plasma the D temp aka KE is higher than that of T ? Because T is one neutron heavier than D isn't it ?

Ion (nuclei = fully ionized) 'temperature' is a common parameter, which implies that the various ion species have some average/mean temperature. At the 'same' temperature, the heavier ions would have a lower 'speed', so the speed (magnitude of velocity) distribution would be different.

 

[Astronuc]

Welcome to PF.


You mean different numbers of Neutrons in the isotopes, right?

With respect to the statement, "The more protons in an isotope, the more effect they have on the plasma," it would be more appropriate to state, "The more protons in a nucleus (or nuclide)". 'Isotope' does refer to nuclei of an element (determined by the number, Z, of protons in the nucleus) with different numbers of neutrons. However, it is the nuclear charge that influences the interaction with the electrons, primarily through bremsstrahlung emission, as well as recombination.

Neutrons produced in fusion have very little interaction with the plasma, since the plasma has very low density. We typically consider plasmas with density of about 10¹⁴ nuclei/cm³, which is much less than an atomic density of an order of 10²² atoms/cm³ in solids.