Fast Neutron Reactors and Breeder Reactors

[ajbwarrrior]

just curious, is Americium 241 a fissionable isotope. if it is, is it possible for it to undergo a controlled chain reaction from bombardment of neutrons.

 

[Khashishi]

All the heavy elements are fissionable, but some are much easier to fission than others. With a strong neutron source, we can fully utilize nuclear fuel.

There are some ideas for a fission-fusion hybrid reactor http://en.wikipedia.org/wiki/Nuclear_fusion-fission_hybrid
where fusion is used as a source of neutrons. The nuclear waste is a lot cleaner than from a light water reactor because essentially all of the transuranics can be destroyed in the reactor by neutron bombardment, which are responsible for the very long lived radioactivity.

Fast neutron reactors are also capable of fissioning Am 241, but fast neutron reactors are potentially a lot more dangerous than light water reactors, and therefore politically unsuitable.

 

[ajbwarrrior]

All the heavy elements are fissionable, but some are much easier to fission than others. With a strong neutron source, we can fully utilize nuclear fuel.

There are some ideas for a fission-fusion hybrid reactor http://en.wikipedia.org/wiki/Nuclear_fusion-fission_hybrid
where fusion is used as a source of neutrons. The nuclear waste is a lot cleaner than from a light water reactor because essentially all of the transuranics can be destroyed in the reactor by neutron bombardment, which are responsible for the very long lived radioactivity.

Fast neutron reactors are also capable of fissioning Am 241, but fast neutron reactors are potentially a lot more dangerous than light water reactors, and therefore politically unsuitable.

thanks,

In a reactor that uses U 235 as the fuel, from my understanding, neutrons need to be slowed down so they can be absorbed by the U 235. what was the original source of the fast neutrons/how were they produced.

are fast neutron reactor also known as breeder reactor or is that different.

 

[snorkack]

thanks,

In a reactor that uses U 235 as the fuel, from my understanding, neutrons need to be slowed down so they can be absorbed by the U 235.

No. In a reactor that uses U isotope mixture. Very different thing!

what was the original source of the fast neutrons/how were they produced.

In fission events.

are fast neutron reactor also known as breeder reactor or is that different.

Different concepts - though related.

All heavy elements can be fissed - by fast neutrons. Only some of the isotopes can be fissed by slow neutrons.

When a neutron approaches a heavy element nucleus, there are 3 possible outcomes:
1) it simply bounces off elastically
2) it is captured by the nucleus which emits γ
3) it causes fission and usually releases several neutrons, which are initially fast.

Some isotopes - the ones with even number of neutrons and protons - only undergo 1) and 2) if the approaching neutron is slow. They start to undergo 3) if neutron has sufficient energy.

Some of the neutrons released in fission are always fast enough to cause fission. But even when the neutrons are fast enough to cause fission, many of them are still simply captured.

Uranium 238 can only be fissioned by fast neutrons. And even fast neutrons are most of time simply captured.

If uranium 238 is bombed with large amount of fast neutrons, like from a fusion explosion, the neutrons cause fission, and initiate a chain reaction which releases a significant amount of energy. But it is still a convergent chain reaction. If, say, each fission event releases average of 2,5 neutrons, of which 30 % cause new fission and 70 % are captured, then the multiplication factor 0,75 means that each neutron reaching uranium 238 causes 3 induced fissions and 5,5 neutron capture events, right?

Now, it is known that uranium 238 and thorium 232 cannot be made to undergo divergent chain reaction even with fast neutrons. If the neutrons are slowed, however, they undergo no fission at all, and all neutrons are captured.

Plutonium 238 cannot undergo fission with slow neutrons. But with fast neutrons, the fission fraction is bigger than that of uranium 238 - so the multiplication factor is over 1, and plutonium 238 can explode. So can plutonium 240 and plutonium 242.

Uranium 235 can undergo fission with slow neutrons. It can also have neutron capture and turn into uranium 236.

If you have pure uranium 235, then the fraction of neutrons wasted on uranium 236 is small enough that the neutron multiplication factor is always over 1. It can explode as a bomb, or just be kept critical in a reactor.

If you have uranium isotope mixture then the neutrons can be either wasted by capture in uranium 238 or propagate chain by uranium 235 fission. It turns out that uranium 238 has low cross-section for slowed down neutrons. So that is why the neutrons need to be slowed - if slowed then the neutrons can ignore uranium 238 neutrons and cause uranium 235 chain reaction even in natural uranium (0,7 %), but if not slowed, then the uranium needs to be enriched to at least 15 % or so.

Now, how about breeding?

When uranium 238 absorbs the neutron, it is not completely wasted. Uranium 239 rapidly turns into plutonium 239, and plutonium 239, like uranium 235, can be fissioned by slow neutrons.

But note that breeding needs more neutrons.

If you fiss uranium 235, you need just 1 neutron to fiss it. Since the fission releases average 2,5, you can waste 60 % of the neutrons - you have to waste them - and the 40 % continue the chain reaction.

If you breed, however, you need 2 neutrons. 1 neutron to breed Pu-239, and then the second to actually fiss the Pu-239. So you can only waste 20 % of the neutrons.

And that is hard. Small losses, like Pu-239 capturing neutrons and becoming Pu-240, can make the breeding unworkable because not enough is bred to replace the consumed fissile isotopes.

The point of fast breeders seems to be that with Pu-239, the losses are too big with slow neutrons - here, leaving the neutrons fast actually decreases the amount of wasted neutrons.

Hope someone can comment more.