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Physics, Astronomy & Cosmology / Re: Why are some isotopes fissionable?
« on: 13/11/2023 00:53:14 »
Hi.
I'm not really an expert on Nuclear reactions. I've just been reading some lecture notes and a textbook on it recently (they weren't my lecture notes, one of the family just kindly made them available to me).
U236* should be so short lived, you won't find any significant amount of it. As fast as you may produce it, it will decay and usually by fission. So you'll just find the later decay products. You ( @paul cotter ) mentioned it should have a half life of 10^-12 seconds. I previously mentioned it may not even be a real particle that exists, it's a useful model and/or may be something like U236 with a deformed nucleus etc. There should be very little you can do to increase the proportion of U236*, the main problem would be keeping it alive (undecayed).
U236* is what you shoud be producing (as the first step) from U235 and a thermal neutron. You shouldn't produce U236 (the ordinary ground state). However, there probably are some routes where the excited state U236* can lose its surplus energy (say by emitting a gamma ray) and become U236 (the ground state). There are probably also some chains (of neutron absorption and/or decay events) that would also lead to the production of U235. For example, we can already use neutron bombardment to change one element into another, it's a bit random and the yield isn't great - you produce all sorts of stuff and only a small portion was what you wanted - but it happens.
Nothing goes absolutely as it should in the textbooks inside a nuclear reactor. Sometimes a contaminant will be present, that might absorb some neutrons and form something unexpected. There are various things that could happen, I'll avoid presenting examples but I'm sure you can imagine the situation. We know that you end up with traces of all sorts of stuff being left in the reaction vessel and spent fuel rods when reactors are finally decommissioned.
More significantly, you never really start with just 100% U235. Natural Uranium is a mixture of isotopes and it's difficult to enrich the proportion of the (desirable for reactors) U235. Getting to 5% U235 is very good and is considered a high quality fuel. So, although on paper we can write down what should happen to U235 with a thermal neutron, in practice you never really had pure U235 to start with, there was quite a lot of other Uranium isotopes in it already.
Best Wishes.
I have another question for you: u235 on exposure to a thermal neutron what ratio of u236/ u236* would one expect and are there conditions that affect this ratio?
I'm not really an expert on Nuclear reactions. I've just been reading some lecture notes and a textbook on it recently (they weren't my lecture notes, one of the family just kindly made them available to me).
U236* should be so short lived, you won't find any significant amount of it. As fast as you may produce it, it will decay and usually by fission. So you'll just find the later decay products. You ( @paul cotter ) mentioned it should have a half life of 10^-12 seconds. I previously mentioned it may not even be a real particle that exists, it's a useful model and/or may be something like U236 with a deformed nucleus etc. There should be very little you can do to increase the proportion of U236*, the main problem would be keeping it alive (undecayed).
U236* is what you shoud be producing (as the first step) from U235 and a thermal neutron. You shouldn't produce U236 (the ordinary ground state). However, there probably are some routes where the excited state U236* can lose its surplus energy (say by emitting a gamma ray) and become U236 (the ground state). There are probably also some chains (of neutron absorption and/or decay events) that would also lead to the production of U235. For example, we can already use neutron bombardment to change one element into another, it's a bit random and the yield isn't great - you produce all sorts of stuff and only a small portion was what you wanted - but it happens.
Nothing goes absolutely as it should in the textbooks inside a nuclear reactor. Sometimes a contaminant will be present, that might absorb some neutrons and form something unexpected. There are various things that could happen, I'll avoid presenting examples but I'm sure you can imagine the situation. We know that you end up with traces of all sorts of stuff being left in the reaction vessel and spent fuel rods when reactors are finally decommissioned.
More significantly, you never really start with just 100% U235. Natural Uranium is a mixture of isotopes and it's difficult to enrich the proportion of the (desirable for reactors) U235. Getting to 5% U235 is very good and is considered a high quality fuel. So, although on paper we can write down what should happen to U235 with a thermal neutron, in practice you never really had pure U235 to start with, there was quite a lot of other Uranium isotopes in it already.
Best Wishes.