Reply to post: Re: "The fission process turns two forms of hydrogen"

Hotter than the Sun: JET – Earth’s biggest fusion reactor, in Culham

bombastic bob Silver badge
Boffin

Re: "The fission process turns two forms of hydrogen"

"The neutrons pass out of the magnetic fields because they are neutral, and strike atoms in the lining of the torus. These then undergo radioactive funtimes and release heat that's used to generate electricity."

nuclear reactions are fun in a lot of ways. but they still obey the laws of physics, like conservation of momentum.

When you add up the total momentum of the fission or fusion products, it will match what it was before the reaction. In the case of fission, the uranium is sitting still, so the split atom products will buzz off in opposite directions [this heats the fuel material]. You also get neutrons, and a LOT of gamma.

In the case of fusion, it's a bit different. the products fuse together to produce one "thing" plus a neutron. The velocity of the neutron may be very high compared to the original products, but it can't be TOO high, because [after all] you MUST conserve momentum.

So where does the energy end up? gamma. Gamma has no actual mass, and so its momentum is based on the "planck's constant stuff" and you can have more than one and they can all buzz off in different directions as long as TOTAL momentum is the same as it was when you started out. [in theory the neutron could go back the other way at extreme speed and the helium go in the same direction, and ALL of the energy could be in neutron + helium, with total momentum conserved, but this is much less likely than the ejection of gammas from the fused helium, which may actually NOT produce a neutron immediately].

And so, most of the energy is gamma. Trapping gamma is easy. You just need enough mass to slow ti down and heat up in the process.

Slowing down neutrons is a bit harder. you need something to 'scatter' it, something that weighs about as much as a neutron. like hydrogen. in water.

You could try to ABSORB neutrons, but that tends to deplete the material that does it. Boron 10 has a high affinity for absorbing neutrons. It becomes Boron 11 and then doesn't do squat for neutrons after that. boron is cheap, however, and might still be useful if you replace it often [or somehow just pump it through the system].

In any case, all of that energy becomes a) fusion products [He5, He4, Li5, etc.], b) neutrons [probably moving very very fast], and c) gamma radiation [most of it]

So you just need to collect the energy from the gamma. Easiest method: cooling system, using water. If the water blanket is thick enough, about 3 feet per "tenth thickness" as I recall, you'll get the energy from the neutrons as well. you just need to make sure the temperature is high enough so that you can boil water [direct boiling is possible, but probably a bad idea, because of neutron activation] and make steam and drive generator turbines with it.

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