Fission of Uranium: Why U-235 is Better than U-238

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    Fission Uranium
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Discussion Overview

The discussion centers on the differences between uranium isotopes U-235 and U-238 regarding their fission properties. Participants explore the mechanisms of fission, the role of neutron and gamma-ray interactions, and the implications for nuclear reactions. The conversation includes theoretical considerations and potential applications related to photofission.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that U-235 is better for fission because it produces U-236 upon neutron absorption, which can break apart evenly.
  • Another participant claims that even-numbered isotopes of uranium are not fissile and cannot sustain a chain reaction.
  • There is a question about the feasibility of using gamma rays to induce photofission in U-238, with a participant noting that gamma interactions typically result in neutron emission rather than fission.
  • A later reply mentions that not all neutron absorptions by U-235 lead to fission, with some resulting in decay processes instead.
  • Participants discuss the possibility of photofission in U and Th, noting that the cross-sections for such reactions are very low.
  • One participant inquires whether high-energy gamma rays can eject protons, to which another participant clarifies that no fission process results in proton emission, only alpha radiation.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms of fission and the potential for photofission, indicating that multiple competing perspectives remain. The discussion does not reach a consensus on the effectiveness of gamma-ray induced fission or the implications of neutron interactions.

Contextual Notes

Some claims rely on specific definitions and assumptions about nuclear interactions, and the discussion includes references to external sources that may not be universally accepted or verified.

cragar
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Why is U-235 better for fission than U-238, i was reading that when a neutron strikes U-235 than it create U-236 which has an even number of nucleons, so it can break apart evenly. When the neutron strikes the uranium it causes it to oscillate and then it breaks apart. Is this like have a compressed spring and then releasing the energy. Does the energy from the nuclear force get turned into kinetic energy of the fission fragments?
 
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cragar said:
Why is U-235 better for fission than U-238

Hi Cragar

Its because even numbered isotopes of U are not fissile, therefore cannot sustain a chain reaction

cheers
Dave
 
Ok thanks, is this because when a neutron strikes U-235 it becomes U-236 which has an even number of nucleons and then can break apart evenly.
but if a neutron strikes U-238 then it becomes U-239. What if instead of a neutron to induce fission we used a gamma ray to induce photofission, And we did this on U-238 would this work?
 
cragar said:
Ok thanks, is this because when a neutron strikes U-235 it becomes U-236 which has an even number of nucleons and then can break apart evenly.
but if a neutron strikes U-238 then it becomes U-239. What if instead of a neutron to induce fission we used a gamma ray to induce photofission, And we did this on U-238 would this work?
Firstly, not all absorptions of neutrons by U-235 lead to fission. About 16% of thermal neutron absorptions result in U-236 decaying by gamma emission, which then leads to either alpha decay to Th-232 or neutron absorption to U-237.

In a thermal reactor, some fissions are actually fast fissions of U-238, but more U-238 is converted to U-239, which undergoes beta-decay to Np-239, which undergoes beta decay to Pu-239, which is fissile.

Gamma interaction doesn't lead to fission, but simply one neutron is emitted (photo-neutron). However, gamma energies (including prompt gammas) are on the order to 1 to 2 MeV, and we don't typically see 5 to 7 MeV gamma rays in fission events.

http://web.ornl.gov/~webworks/cpr/pres/105860.pdf

https://www.oecd-nea.org/science/wpec/nemea7/docs/presentations/29_S4_Litaize.pdf
 
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Are you saying that when a high energy gamma ray hits it ejects one neutron, is it possible for it to eject a proton? thanks for posting that article i have read some of it, this is an interesting discussion. I have read articles that talk about photo fission of U-235, or is it when the neutron is ejected it makes it unstable and causes fission?
 
cragar said:
Are you saying that when a high energy gamma ray hits it ejects one neutron, is it possible for it to eject a proton? thanks for posting that article i have read some of it, this is an interesting discussion. I have read articles that talk about photo fission of U-235, or is it when the neutron is ejected it makes it unstable and causes fission?
No there isn't any fission process which results in the emission of a proton.
Closest thing to that is alpha radiation, the emission of 2 protons bundled with 2 neutrons - basically a helium nucleus.
 

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