arclight Profile picture
22 Jul, 26 tweets, 5 min read
Curious what non-nuclear people think that hole in the middle of the pellet is for...
It's time to divulge my answer, fully expecting the fuels people to correct me if I stray too far.

The central hole serves a few related purposes, the main one being to reduce the maximum temperature of the fuel pellet.
Fission is strongly dependent on material density and geometry. Ignoring heat for the moment, the fission rate at the center of the pellet is highest along the centerline; neutrons born near the surface are more likely to `leak` out of the pellet and not cause another fission
So just from a neutron diffusion and nuclear reaction point of view, the centerline is where most of the action is taking place. But fission doesn't just give off 2-3 neutrons and an atomic nucleus broken into (usually) 2 parts.
Energy is released, partly as radiation, but mostly as recoil energy in the fission fragments; they bounce off other atoms and what started as the velocity of two particles gets transferred to the neighborhood. We see that as heat.
Short story: fission rate directly translates to thermal power. Heat is transferred from the surface of the pellet to coolant, etc. so it's not surprising the centerline has the highest temperature.
Even if fission rate was uniform throughout the pellet, the center would still have the highest temperature just because of heat flows. What we'd like to do is flatten out the temperature profile across the pellet.
If we flatten out the temperature profile, that means the outside surface would have to be hotter and the peak center temperature would be lower. If we raise the outer surface temperature, we increase the rate of heat transfer out of the pellet. Usually.
We set a maximum temperature limit on the fuel for various reasons: safety, engineering limits, economics, etc. Also, remember these pellets aren't pure uranium metal; they're a ceramic. Compared to metal, they're fairly brittle.
So with large temperature differences across the pellet, the ceramic can crack. And it does. Sustained fission in the center of the pellet also causes cracking from fission fragment knocking all the atoms around.
If your pellet didn't start it's life with a hole down the centerline, it has one by the time it's retired. You get a hole whether you want one or not.
As the fission fragments accumulate in the pellet (one reason ceramic is used), they tend to absorb neutrons and make the fuel less -- fission-y? It's easier to get a chain reaction going in fresh fuel than fuel that's been in a reactor 3 years.
The concentration of fissionable uranium in the pellet goes down and the concentration of neutron-absorbing fission fragments goes up. This is dependent on power (fission rate) so the center of the fuel 'burns' faster than the surface. This is basic accounting.
The burn-in process for new fuel can make reactor control more difficult and lead to higher peak temperatures. There are various ways people have come up to reduce the effects of this initial burn-in process but they tend to suck up neutrons which impairs efficiency
From a power person viewpoint, you want all the neutrons to cause fissions: fissions -> heat -> steam -> mechanical power in the form of a spinning turbine, electric generator, submarine driveshaft, etc.
If the pellet is eventually going to get a hole down the center anyway and fresh cylindrical pellets make the fuel inconveniently perky and peak temperature in the pellet is reduced (flatter profile), why not just put a hole in the middle of the pellet to begin with?
Fission fuel behavior is a really amazing and complex topic and I've barely scratched the surface. Some of the fission fragments are gas (xenon, krypton, helium, hydrogen, etc.) and can accumulate in the cracks and pressurize which further stresses the fuel.
Another weird thing is the direction of the voids and cracks that form. Usually you get cracks from the center towards the surface but lens-shaped (lenticular) voids also form like little magnifying glasses pointed at the center of the pellet.
These slowly move toward the surface almost like tiny lens-shaped bubbles trying to get to the outer surface of the fuel. Those also fill with gas and pressurize. You don't normally think of ceramic being so active inside :)
Also, h/t to @syndroma3 for posting the photo that set me off down this tangent.
FWIW, the codes the NRC developed back in the late 70s for modeling fuel behavior are still actively developed. They started with FORTRAN IV (F66) and have migrated to at least F90. Never underestimate the longevity of scientific code...
For those thinking of serious answers, an important fact I completely forgot to mention is that these pellets are stacked in a metal tube which is then welded shut. Ideally the pellet never touches water. #InAPerfectWorld
Also, there are a few minor inaccuracies regarding about the fission rate (it's higher near the edge of the pellet than the center); credit for corrections is given at the end of the thread #ThisIsWhyWeDoSoMuchIndependentPeerReview
Note that this is _not_ correct. The fission rate is highest near the surface of the pellet, both because of low-energy neutrons reflecting back from bouncing off the coolant and because of "self-shielding". Absorption inside the pellet "shields" the center from neutrons.
The upshot is that the profile of the fission rate across the pellet depends on a lot of factors. Heat transfer is slightly more straightforward; the coldest part of the pellet is the surface so the temperature peaks closer to the centerline because heat moves from hot to cold.
More Nerd Info: If anything in my explanation conflicts with this PDF, trust the PDF over what I've written. It's got some good pictures but it's really not a book you want to read, even if you are a math/science/engineering nerd.

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22 Jul
I love this from the preface to McCracken's "A Guide to FORTRAN IV Programming"
I'm not sure which is funnier; computer programming courses being worth no more than 1 credit, or Fortran programming "is not very difficult to learn".
That's from 2nd Edition (1972); I have 2st Edition on order (covers FORTRAN II; circa 1962-3)
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21 Jul
My goal today was to work on my code, not complain about Python. It's just incredibly frustrating trying to Do The Right Thing as a professional developer and have a language/ecosystem fight you so vigorously.
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I'm coming to the conclusion that Python really doesn't want you to develop packages and modules. No guidance on setting up project directories, etc., no info on running local dev code vs installed code. This ecosystem is a goddamn nightmare.
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It's like we're paying for them to ignore us and feign amnesia
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