Nuclear Fuel Rods & Fission

[Jimmy87]

Hi,

I haven't studied much nuclear physics and would be very grateful if someone could answer a few basic questions I had about fission in the fuel rods on how the water is actually heated in the reactor core:

1) I don't know if I have this correct but is it that the way the water is heated in a reactor is that the kinetic energy of the neutrons released from a fission event are transferred to the water molecules (obviously the ones that don't go on to cause more fissions). Is this mainly how the water is heated or are there other energy transfers e.g. energy transferred by gamma/beta particles that also come out of the fission events?

2) Wiki says that the fuel is surrounded by zirconium cladding. I take it that this means if the above is correct then the neutrons will pass through the cladding without being absorbed (otherwise they couldn't interact with the water molecules)? Or does the water heat up because the fission particles collide with the cladding making the rods very hot so the water is heated like a filament in a kettle type effect?

Thanks for any insight offered!

 

[.Scott]

Some heating is as you described. But the reactor fuel itself gets very hot - from neutrons, gamma, beta, and direct impulse to the decay products. The fuel will then heat the cladding and water.

 

[Astronuc]

The fission reactions occur in the fuel, and that is where the bulk of the thermal energy is deposited. That thermal energy is conducted from the fuel, through the cladding into the coolant. In LWRs, the cladding is a Zr-alloy. Some fraction of the heat (~2.3 to 3.5%) is generated by gamma radiation interacting with the cladding, coolant and structural alloys in the core.

 

[rpp]

Most of the energy (~82%) from a fission becomes kinetic energy of the fission products, which causes the fuel to heat up. The heat in the fuel is conducted through the fuel, through the clad, and into the coolant.
Fission product decay produces an additional amount of energy (~7.3%) that is deposited directly into the fuel.

There is also additional energy produced from prompt gamma rays (~3.4%), neutron slowing down (~2.4%), and capture gamma rays (~4.9%). This energy may be deposited in other locations. The neutron slowing down energy is deposited directly in the coolant. The energy from prompt gamma rays and capture gamma rays may be deposited in the fuel or other structural material.

The total amount of power from a reactor is limited by the amount of energy that can be safely removed from the fuel.

 

[geoelectronics]

Most of the energy (~82%) from a fission becomes kinetic energy of the fission products, which causes the fuel to heat up. The heat in the fuel is conducted through the fuel, through the clad, and into the coolant.
Fission product decay produces an additional amount of energy (~7.3%) that is deposited directly into the fuel.

There is also additional energy produced from prompt gamma rays (~3.4%), neutron slowing down (~2.4%), and capture gamma rays (~4.9%). This energy may be deposited in other locations. The neutron slowing down energy is deposited directly in the coolant. The energy from prompt gamma rays and capture gamma rays may be deposited in the fuel or other structural material.

The total amount of power from a reactor is limited by the amount of energy that can be safely removed from the fuel.

Would you please comment on the root source of the energy released by fission? Is it mass deficit converting to energy? Does it follow E=mc^2?

thanks. Geo

 

[PeterDonis]

the root source of the energy released by fission?

It's the release of energy due to an exothermic reaction. In this case the reaction is nuclear instead of chemical, but the principle is the same.

Is it mass deficit converting to energy?

It's additional binding energy being released; the fission products are more tightly bound, so the fission process releases energy.

Does it follow E=mc^2?

$E = mc^2$ is just a unit conversion equation between mass units and energy units.

 

[geoelectronics]

It's the release of energy due to an exothermic reaction. In this case the reaction is nuclear instead of chemical, but the principle is the same.



It's additional binding energy being released; the fission products are more tightly bound, so the fission process releases energy.



$E = mc^2$ is just a unit conversion equation between mass units and energy units.

Thank you.
So the tightly bound, massive nucleus undergoes fission (either induced or spontaneous SF), the nucleons are separated, thus weighing more than the did when bound (correct?) and the difference in that weight is the source of energy? So in E=mc^2 in the frame of fission, the m=mass which really just means mass-energy equivalence, is this correct?

Geo

 

[PeterDonis]

So the tightly bound, massive nucleus undergoes fission (either induced or spontaneous SF), the nucleons are separated, thus weighing more than the did when bound (correct?) and the difference in that weight is the source of energy?

No, you have it backwards. The weight of the fission products is less than the weight of the original nucleus. The fission products are more tightly bound. That's why fission releases energy. If the fission products weighed more, fission would require an input of energy; it would not release energy.

 

[geoelectronics]

No, you have it backwards. The weight of the fission products is less than the weight of the original nucleus. The fission products are more tightly bound. That's why fission releases energy. If the fission products weighed more, fission would require an input of energy; it would not release energy.

That makes sense. Thank you. If the nucleons were separated totally and not bound in another, lighter set of nuclei would those free nucleons (protons, neutrons) weigh more or less than the original massive nucleus?

Thanks
Geo

 

[PeterDonis]

If the nucleons were separated totally and not bound in another, lighter set of nuclei would those free nucleons (protons, neutrons) weigh more or less than the original massive nucleus?

More. That's what binding energy means.

 

[geoelectronics]

More. That's what binding energy means.

Got it.
Thanks. I was truly thinking backwards, because of the missing negative sign.
Geo
-30-