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echawkes t1_j4s3z3l wrote

U-235 has 143 neutrons. When you strike it with one more neutron, the total is 144.
Fission usually splits one atom into two atoms, with some left over neutrons. There are a number of different pairs of atoms that are possible outcomes. Sometimes, the pairs of atoms produced add up to 141 neutrons, with three free neutrons left over, and sometimes they add up to 142 neutrons, with only two free neutrons left over.

For low-energy neutrons striking U-235, the chance of fission is about 6 times as high as absorbing the neutron and becoming U-236.


UpperCardiologist523 t1_j4sgdet wrote

Does the level of energy the neutron hit with, decide what new atoms and therefore how many neutrons are left over? If not, what does? Or is it random?

Oh, and in your last paragraph. Does this mean that when they enrich uranium/plutonium, reactors are run on lower energy?

Sorry if I'm way off here. I'm a TV repair man, but curious about this.


echawkes t1_j4snn8g wrote

>Does the level of energy the neutron hit with, decide what new atoms and therefore how many neutrons are left over?

The kinetic energy of the incoming neutron can affect which pairs of atoms are produced. Higher energies generally result in more neutrons being released, which means that different fission products must result.

>Does this mean that when they enrich uranium/plutonium, reactors are run on lower energy?

Enrichment isn't done in a nuclear reactor. Typically, something like a centrifuge is used to separate the isotopes of uranium. U-235 is a little lighter than U-238, so the centrifuge can be used to get two output streams in which one has more U-235 than the input (higher enrichment) and the other has less U-235 (lower enrichment).

I think you might be asking about something like a breeder reactor, which can produce Pu-239 from U-238, or U-233 from Th-232. There have been very few of these, because some of the neutrons are used up in transmuting one element into another (by absorption without fission), which is a technical challenge. The usual technique is to use a fast reactor (high energy neutrons), so that there are more neutrons produced per fission.

There haven't been many breeder reactors because they are more complicated and expensive to build and operate than normal reactors. Uranium is pretty common and not that expensive, so we usually just mine it and use that. The uranium in power reactors is usually enriched a little. Natural uranium is 0.7% of the uranium you would find in the ground, and it is usually (but not always) enriched as high as 5% in nuclear power plants.


UpperCardiologist523 t1_j4sx3h2 wrote

Wow, thanks for a great answer. I've always thought (probably because i misunderstood or remember wrong) while watching videos about Thorium-Salt reactors, how they were better than the breeder reactors we've currently on, and that current reactors were breeders, because of how inexspensive they were to build. I better go back and watch those videos one more time.

I knew about Hanford, which is a breeder.

Anyways. Thanks a lot for answers.


zowie54 t1_j4t26ae wrote

Think about it like breaking a cookie. It will break slightly differently each time you break it, and produces two major pieces (not necessarily equally sized), and some crumbs. While it is easy to break a cookie, exactly how it will break will be determined by lots of variables, so many, that measuring the statistical frequency is usually how outcomes are predicted. Some reactions require certain minimum energy thresholds to be overcome, and the energy of an incoming particle can determine how likely certain types of decay are. That being said, a neutron is actually absorbed by the 235, which becomes unstable and breaks apart. U-235 fission produces an average of 2.41 neutrons per fission, the neutrons being analogous with a seed or something in a cookie that cannot break apart easily, and so either is in one half or the other, or in neither as a crumb.