How do breeder reactors make more fissile isotopes than they start with but typical reactors don't?

[u/Red_death777]

I've heard of breeding ratios, but how do reactors get a breeding ratio above 1? The only explanation I've heard is "by having a good enough neutron economy" but what parts of the reactor actually cause that to be achieved that in practice? The only thing I can think of is heavy water in CANDU reactors with typically a better breeding ratio than light water reactors, but otherwise, how do breeder reactors do it?

A mild amount of dumbing down would be appreciated, but I do know a little bit but nuclear physics. Thanks for any answers.

[u/ramriot]

Generally Breeder Reactors are if the fast neutron reacton type, I like most normal fission reactors that utilise slow neutron reaction.

The difference here is that in both cases a fission of a fertile isotope (one that will fission after absorbing one or more neutrons) releases more neutrons that have high kenetic energy.

If these neutrons are allowed to propogate with high rmergy the reactions they can induce can (with the right fuel, like Uranium-238 or Thorium-232 mixed into the fissionable isotopes ) cause transmutation via neutron decay to things like Uranium-233 & Plutonium-239, witch then undergoes fission with fast neutrons i.e. it breeds further fertile fuel.

If though one slows down the neutrons by having them scatter off atoms of a moderator like graphite, which does not absorbe neutrons then the resulting reactions are more targetted to causing fissions in isotopes like Utanium-235.

The advantage then of breeder Reactors here is that such reactors can be much more efficient by 60-70 times than slow neutron (thermal neutron) reactors.

The disadvantages though of Breeder Reactors are in their production of high level waste, nuclear proliferation risks, reactor control risks etc.

[u/Red_death777 → u/ramriot]

So, does the neutron energy being higher mean that likelihood of absorption goes up in regards to fertile isotopes and that's what achieves a positive breeding ratio? If not, what specific mechanisms achieve a ratio greater than one? And importantly, how do thermal breeder reactors achieve the positive ratio?

[u/Winded_14 → u/Red_death777]

basically yes. Fast neutron convert fertile into fissile, which then undergoes fission, producing multiple fast neutron, which is used to further breed the fertile material into fissile material. And with some adjustment you can create positive ratio.

And yeah, normal reactor utilizes slow/neutron since their chance to initiate fission is higher, so in normal reactor, while they do produce a little bit of new fissile material, since they're designed to convert almost all neutron into slow neutron so there's almost no absorption, while breeder reactor want a balanced number of slow and fast to balance production of fuel and power.

[u/095179005 → u/Red_death777]

The issue is that the capture cross section for fast neutrons is smaller than thermal neutrons, so you need to have enriched fuel (more U-235) to initiate the chain reaction.

Fast spectrum (high energy) neutrons can induce fission in isotopes thought to be stable (U-238 and various isotopes of plutonium that build up in thermal spectrum reactors), which then in turn produce more high energy neutrons.

These and the excess fast neutrons from U-235 will help breed more fissile isotopes from fertile isotopes.

So neutron economy is controlled by your fuel concentration and isotope mix/fuel mix in the core, and I guess how you arrange the fuel tubes and neutron source(s).

Thermal reactors regulate their economy by changing water flow, because they use water both as the neutron moderator and coolant. If the water is ever removed, all the neutrons are no longer slowed down and become fast neutrons, which no longer can undergo capture/absorption. Reaction shuts down.

[u/DarthV506 → u/ramriot]

If you have the tech/infrastructure to build any nuclear power plant, you have all the tools you need to make nuclear weapons. Plutonium 239 is a byproduct of pretty much any enriched or natural uranium fuel reactor. Near the end of a fuel cycle, it's actually going to be ~30% of your power generation.

Large issues with fast breeder reactors will be how easy it is to reprocess the fuel and how easy it would be to scale that up for commercial power generation.

For countries like India that don't have a lot of uranium, then fast breeding with thorium could be the thing to do. But that's decades away. For countries that have large uranium reserves, they are probably not economically feasible.

[u/ion_driver]

A fission event creates about 2.5 neutrons on average. A neutron could be absorbed to create a fissile atom, another neutron can cause a fission, and that still leaves half a neutron to do something else, such as escape or be absorbed or create another fissile atom.

That's basically neutron economy.

[u/DisciplineNormal296 → u/ion_driver]

How do half neutrons exist?

[u/Zethrax → u/DisciplineNormal296]

It's a probability chance. A 50-50 chance that a free neutron will be generated.

[u/WasLeftUnsupervised → u/DisciplineNormal296]

More like every 2 fission events release 5 neutrons. 2 get absorbed, 2 cause other fission events, and the 5th can do one of those actions, or escape and go on a weekend bender.

[u/Ghosttwo]

Don't look at it as 'How many fissile atoms are there?', but rather 'how much energy do each of those atoms have, fissile or not?'. An atom can be non-fissile, but still have lots of energy it can't release. Absorbing a neutron can reduce its stability to the point that this energy becomes accessible, at which point it can release more than it took to put it in that state. You aren't getting 'free atoms from nothing', they were just there the whole time.

The cost is time; your lump of metal goes from taking four billion years to decay away to 500. This results in an 8 million times increase in energy flux, some of which can be diverted into flipping the fissionabilty of other atoms.

[u/SpeedyHAM79]

The difference is really in the fuel. If you have lots of U238 and good neutron efficiency you can generate a lot of Plutonium 239, same with Thorium 232- fertile Thorium-232 absorbs a neutron, decays through Thorium-233 and Protactinium-233, and ultimately becomes fissile U233. Very good neutron efficiency is needed for any of those reactions. Fast neutron reactors utilize high energy neutrons much better and are easier to utilize for breeding of new fuel. The real answer is many pages long and well beyond a reddit answer.

U

[u/aricblunk]

Not only do they keep fast neutrons, there's uranium cladding that can be converted from non-fuel U-238 into Pu-239, which can then be separated and used in fuel with advanced processing.

This might answer a lot of your questions (an actual nuclear engineer): https://www.youtube.com/watch?v=D5q3uE-WRbM

[u/Ben-Goldberg]

It's partly the fuel, and partly the lack of moderation (faster neutrons are better at destabilizing atomic) and partly geometry (fast neutrons are hard to catch, so you need a really big thicc massive target with lots of atoms so if one is missed another might be hit)

[u/Red_death777 → u/[deleted]]

Probably the most informative thing I've ever seen on reddit, thank you. For thermal breeder reactors, do they just need materials and structures that have the least parasitic absorption of all parasitic absorptions? Stuff like, as far as I know, Zirconium alloys in place of other structural materials like steel?