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

[cragar]

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?

 

[davenn]

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

 

[cragar]

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?

 

[Astronuc]

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

 

[cragar]

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?

 

[Astronuc]

It seems photofission of U and Th is possible, but the cross-sections are very low.

http://journals.aps.org/pr/abstract/10.1103/PhysRev.59.57 (Fission of uranium and thorium has been observed to be produced by irradiation with γ -rays from fluorine bombarded with protons.)

http://journals.aps.org/pr/abstract/10.1103/PhysRev.95.1260

 

[rootone]

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.