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

In summary, U-235 is better for fission than U-238 because U-235 can break apart evenly when struck by a neutron, while U-238 cannot sustain a chain reaction. Even numbered isotopes of U are not fissile and about 16% of thermal neutron absorptions of U-235 result in gamma emission rather than fission. Gamma rays do not typically lead to fission, but some research has shown the possibility of photofission of U and Th with high energy gamma rays, though the cross-sections are low. There is no fission process that results in the emission of a proton, but alpha radiation is similar as it emits a helium nucleus.
  • #1
cragar
2,552
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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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  • #2
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
 
  • #3
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?
 
  • #4
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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  • #5
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?
 
  • #7
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.
 

What is fission of Uranium?

Fission of Uranium is the process by which the nucleus of a Uranium atom splits into two smaller nuclei, releasing a large amount of energy.

What is the difference between U-235 and U-238?

U-235 and U-238 are two isotopes of Uranium, with U-235 having 235 nucleons (protons and neutrons) and U-238 having 238 nucleons. The main difference between them is that U-235 is fissile, meaning it can undergo nuclear fission, while U-238 is not fissile but can undergo nuclear fusion.

Why is U-235 considered better than U-238 for nuclear fission?

U-235 is considered better for nuclear fission because it has a higher probability of undergoing fission when struck by a slow-moving neutron. This makes it a more efficient source of energy compared to U-238.

How is U-235 used in nuclear power plants?

In nuclear power plants, U-235 is used in the process of nuclear fission to generate heat, which is then used to produce steam that drives turbines to generate electricity.

What are the potential risks associated with fission of U-235?

The main risks associated with fission of U-235 are the release of radiation and the production of nuclear waste. When U-235 undergoes fission, it releases radioactive particles that can be harmful to humans and the environment. The nuclear waste produced also poses a long-term storage and disposal challenge.

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