B Nuclear Fission & Creation of Plutonium

[JLynch]

Just joined the forum after youtubes algorithm suggested a story documenting the ‘Chicago Pile’. I ended up watching a bunch of other power plant videos becoming more confused with each one.

I apologize up front as I know nothing about the field of nuclear physics and not even sure if I’ll even word my questions properly.

So when a neutron collides with and splits a uranium atom, two new atoms are left behind. I understand that the individual size of the new atoms are smaller then the uranium atom they came from, but when you take into account the combined size of 2 new electron clouds, wouldn’t there sum be larger then that of the single electron cloud of the original uranium atom?

I ask because if this were true it would seem like the uranium fuel would continually grow in size as the chain reaction continued.

 

[Vanadium 50]

This can happen. It's called "swelling".

 

[Baluncore]

I ask because if this were true it would seem like the uranium fuel would continually grow in size as the chain reaction continued.

You are correct.
If you start with a compact crystal, then as the nuclear fission reaction proceeds, the compact crystaline order will be lost and the volume must increase.

Swelling and distortion of fuel rods may not be important in experimental reactors, but it becomes very important in higher power reactors, and critical in reactor failures.

The number of products and the ionic radius of all the products is important.
https://en.wikipedia.org/wiki/Ionic_radius

 

[phyzguy]

As stated above, what you say is true, but you may not realize that the two new atoms (fission products) are flung apart at high speed, so when they come to rest in the fuel they are far apart from each other and from the original site of the fission.

 

[Vanadium 50]

That is true. But the fission fragments have to end up somewhere. And as a general rule two atoms are bigger than one, so the atoms that were there need to move out of the way. So the fuel swells (and cracks and other bad things).

 

[phyzguy]

That is true. But the fission fragments have to end up somewhere. And as a general rule two atoms are bigger than one, so the atoms that were there need to move out of the way. So the fuel swells (and cracks and other bad things).

Agreed. I just wanted to make sure the OP understood that the two atoms end up far apart.

 

[JLynch]

Thanks for the all the replies and information about fuel rod swelling, much appreciated.

My initial thoughts from the Chicago Pile documentary and learning about the creation of Plutonium was that this atomic ‘growth’ must be occurring at an exponential and unsustainable rate, which had me very confused. Have to admit still a bit puzzled that it’s not more of a problem.

With the new understanding that swelling does occur I’m now wondering about the distribution of energy being released from within a reactor. Again not quite sure if I’m wording this correctly here. So while there’s energy being released from the actual splitting of the uranium atom is there not also a considerable amount of energy or heat being generated as the fuel itself grows in volume?

I would think the newly created Plutonium atoms squeezing there way into existence, forcing there way through the crystalline structure of the fuel has to be contributing to the overall output of the reactor.

 

[russ_watters]

So while there’s energy being released from the actual splitting of the uranium atom is there not also a considerable amount of energy or heat being generated as the fuel itself grows in volume?

I would think the newly created Plutonium atoms squeezing there way into existence, forcing there way through the crystalline structure of the fuel has to be contributing to the overall output of the reactor.

Mechanical engineer here, not a physicist; I would think that would absorb energy, not release it. And even then it would be miniscule compared to the total energy released in the reaction. Maybe not even enough to bother calculating.

 

[snorkack]

So when a neutron collides with and splits a uranium atom, two new atoms are left behind. I understand that the individual size of the new atoms are smaller then the uranium atom they came from, but when you take into account the combined size of 2 new electron clouds, wouldn’t there sum be larger then that of the single electron cloud of the original uranium atom?

I ask because if this were true it would seem like the uranium fuel would continually grow in size as the chain reaction continued.

A lot of the fission fragments are not even individually smaller than U, due to lanthanide contraction:
https://www.schoolmykids.com/learn/interactive-periodic-table/molar-volume-of-all-the-elements
Ta to Au are not formed by fission. Only Nb to Ag are smaller (elements 41 to 47) but of their conjugates, 45 to 51, only 45 to 47 are in that range, and it is a small yield region anyway.

Breakdown of crystal structure does not necessarily cause expansion. Pu actually shrinks on melting. Nor does solid damage necessarily accumulate and cause storage of energy. Some solids accumulate Wigner energy, others do not. (Fluids do not have Wigner energy as such, but they may accumulate metastable reaction products, and energy in them.)
The energy stored in Wigner energy is smaller than the energy immediately dissipated as heat, but it is relevant and worth calculating because energy stored up as Wigner energy, elastic stress energy and chemical energy has ways to do things that heat immediately available for conduction does not.

Fission tracks in solids are limited to a few μm. Fast neutrons deposit their energy over multiple cm. Prompt gammas even farther than neutrons, but not much farther. I do not remember beta ranges so out of hand.