Pesky Neutrons

[Data Rider]

Hi,

 

When I compare the reactions of fission to fusion,

I notice that fusion produces more than ten times as

many neutrons per unit energy.

 

The neutrons in a fission reactor are absorbed by the

shielding mass. The problem is that they are absorbed

into the atoms of the shielding. The isotopes and even

elements change under neutron bombardment, so that

the shielding itself becomes too radioactive to work

near. After 50 years they shut down the reactors and

abandon them.

 

If there are ten times as many neutrons produced by

fusion, doesn't that mean a fusion reactor will burn-up

its shielding in only a few years?

 

Data Rider

http://data-rider.org

[miknsk]

AFAIK, it is believed that the main task is to run fusion reactor, and that the other problems will be solved later.

[Klaynos]

One of the jobs of ITER is to determine good materials to build the reactors out of IIRC.

[Mr Skeptic]

If you use lithium walls, you can get the neutrons to cause it to decay to get tritium, which is the hydrogen ion in shortest supply that might be used in fusion reactors. So the stray neutron can be used to make more fuel. There are different fusion reactions that are possible, and some IIRC don't release neutrons. However, before worrying about any of that too much, we need working fusion reactors.

[Data Rider]

If you use lithium walls, you can get the neutrons to cause it to decay to get tritium, which is the hydrogen ion in shortest supply that might be used in fusion reactors. So the stray neutron can be used to make more fuel. There are different fusion reactions that are possible, and some IIRC don't release neutrons. However, before worrying about any of that too much, we need working fusion reactors.

 

Hi,

 

There are no fusion reaction that don't produce neutrons.

Technically some don't, but they're ones that happen at a

couple magnitudes greater pressure or temperature as deuterium,

and they also produce large amounts of easy fusioning isotopes

that produce neutrons. If you can define the fuel for a

neutron-free fusion reactor, I'd like to see it.

 

Fusion neutrons really are pesky. There doesn't seem to be a

way to avoid them.

 

There's not a lot of lithium in the world and it's very expensive, so

it better be good at catching the neutrons. It takes yards of concrete

to catch them all. But there's a bigger problem: the magnets will be

exposed to the full neutron flux regardless of external shielding. With

neutrons transmuting elements in the magnets, their chemical bonds

will no longer fit them and disappear: concrete turns to powder under

neutron bombardment. No only do we need super magnets, we need

ones that continue to function as they disintigrate. Talk about a

challenge!

 

Data Rider

http://data-rider.org

[miknsk]

One of the jobs of ITER is to determine good materials to build the reactors out of IIRC.

 

The question is whether such materials can be found...

[Jacques]

From wiki :Aneutronic

Aneutronic fusion is any form of fusion power where no more than 1% of the total energy released is carried by neutrons.

There are a few fusion reactions that have no neutrons as products on any of their branches. Those with the largest cross sections are these:

D + 3He → 4He (3.6 MeV) + p (14.7 MeV)

D + 6Li → 2 4He + 22.4 MeV

p + 6Li → 4He (1.7 MeV) + 3He (2.3 MeV)

3He + 6Li → 2 4He + p + 16.9 MeV

3He + 3He → 4He + 2 p

p + 7Li → 2 4He + 17.2 MeV

p + 11B → 3 4He + 8.7 MeV

[John Cuthber]

"There's not a lot of lithium in the world "

estimates vary but it's about an ounce to the ton on average.

 

"But there's a bigger problem: the magnets will be

exposed to the full neutron flux regardless of external shielding"

What magnets?

Anyway, the most the neutrons could do is transmute a mole or two of shielding for each mole of fuel fused.

Not that big a problem.

[insane_alien]

the reactor walls would absorb the neutrons. and there can be internal sheilding as well.

[Jacques]

I found that paper very interesting. It give an overview of the problems with fusion

The Trouble With Fusion

[Data Rider]

There are a few fusion reactions that have no neutrons as products on any of their branches. Those with the largest cross sections are these:

D + 3He → 4He (3.6 MeV) + p (14.7 MeV)

D + 6Li → 2 4He + 22.4 MeV

p + 6Li → 4He (1.7 MeV) + 3He (2.3 MeV)

3He + 6Li → 2 4He + p + 16.9 MeV

3He + 3He → 4He + 2 p

p + 7Li → 2 4He + 17.2 MeV

p + 11B → 3 4He + 8.7 MeV

 

That's a sophestry.

 

D+3He produces no neutrons, but how do you stop D+D, a more reactive chain, from producing netrons in the same plasma? In seeking neutron free fusion we must rule-out any fuel containing Deuterium.

 

3He+3He produces 4He + protons. Protons fuse to make Deuterium, which fuse to make neutrons. In that way, any fuel with 3He produces neutrons.

 

6Li is burned with protons or 3He, both of which produce neutrons as described above.

 

7Li fuses with protons, that fuse to produce neutrons.

 

11B also fuses with protons, producing neutrons when the protons fuse.

 

While the quoted reactions produce no neutrons themselves, each one happens in a fuel mixture that results in neutrons from side reactions, as shown. That was my point.

 

"There's not a lot of lithium in the world "

estimates vary but it's about an ounce to the ton on average.

 

Pardon me. I confused its high cost with scarcity. Looking into it, I see that lithium supply will not be a problem.