How can quantum mechanics contribute in a Nuclear reactor

[madhisoka]

Hi, does treating neutrons as waves make any difference when it comes to micro cross sections ? , does quantum mechanics help more than classical mechanics in nuclear reactor physics ?

Noticing that I am a nuclear engineering student

 

[hilbert2]

Isn't nuclear fission based on the quantum tunneling of neutrons out of the binding strong force field of a heavy nucleus? Not sure about cross sections/scattering.

 

[PeterDonis]

Isn't nuclear fission based on the quantum tunneling of neutrons out of the binding strong force field of a heavy nucleus?

No; fission isn't the emission of single neutrons, it's the splitting of a nucleus struck by a neutron into two fragments, each of roughly half the size of the original nucleus. (For example, the main fission reaction of U-235 when hit with a neutron is into Ba-139 and Kr-94, plus 3 neutrons.) The best model I'm aware of for fission is the liquid drop model of the nucleus, which basically says that the incoming neutron perturbs the shape of the drop from a sphere into two smaller spheres joined by a narrow "neck": the Coulomb repulsion between the two halves then causes fission. See, for example, here:

http://www.nuclear-power.net/nuclear-power/fission/

This model is basically classical, although there are some underlying quantum effects taken into account.

 

[PeterDonis]

does treating neutrons as waves make any difference when it comes to micro cross sections ? , does quantum mechanics help more than classical mechanics in nuclear reactor physics ?

See the link I gave in my previous post, and in particular the explanation of the various terms in the semi-empirical mass formula. Some of the terms take quantum effects (such as the Pauli exclusion principle) into account.

 

[PeterDonis]

No

One caveat to this: some very heavy nuclei can spontaneously fission, which can be thought of as a quantum tunneling process-but what tunnels out is not a single neutron but a smaller nucleus.

 

[hilbert2]

For some reason I had an idea of a nuclear chain reaction beginning with a heavy nucleus losing a neutron by tunneling, then breaking up due to the electrostatic repulsion between the protons and releasing more neutrons in the process. Where does the first neutron that starts the reaction come from, anyway? Is there a significant background radiation neutron flux on earth?

 

[PeterDonis]

Where does the first neutron that starts the reaction come from, anyway?

See this thread in the Nuclear Engineering forum on PF (a good resource in general on this subject, btw):

https://www.physicsforums.com/threads/nuclear-reactors-how-you-start-the-reaction.481702/

Is there a significant background radiation neutron flux on earth?

There is some from cosmic rays. I'm not aware of any other natural neutron sources.

 

[madhisoka]

See this thread in the Nuclear Engineering forum on PF (a good resource in general on this subject, btw):

https://www.physicsforums.com/threads/nuclear-reactors-how-you-start-the-reaction.481702/
There is some from cosmic rays. I'm not aware of any other natural neutron sources.

Thank you for the info I fully understand the fission process I took an advanced nuclear reactor theory course , I guess u didnt get my question I meant that how good can quantum mechanics be with measuring neutron flux or cross section ? is using shrodinger equation other than using diffusion equation or transport equation of neutrons can make any difference .

 

[PeterDonis]

how good can quantum mechanics be with measuring neutron flux or cross section ? is using shrodinger equation other than using diffusion equation or transport equation of neutrons can make any difference

Do you mean, can we use quantum mechanical models to make more accurate predictions of neutron flux or cross sections in nuclear reactors? I'm not aware of any results that show this. That might be partly because of the limits in accuracy of our measurements, and partly because any quantum effects that aren't already incorporated into the models are too small to matter.

 

[madhisoka]

Do you mean, can we use quantum mechanical models to make more accurate predictions of neutron flux or cross sections in nuclear reactors? I'm not aware of any results that show this. That might be partly because of the limits in accuracy of our measurements, and partly because any quantum effects that aren't already incorporated into the models are too small to matter.

yeah that was I meant, any idea ? like using shrodinger equation instead of the transport

 

[PeterDonis]

any idea

As I said, I'm not aware of any results that show any improvement in accuracy using quantum models instead of the current ones.