What actually happens to the U-238 part of fuel in a nuclear reactor?

[oblong-pea]

TL;DR

I know U-235 is the fissile part, but what happens to the U-238 parts during reaction and after?

So I'm aware that uranium 235 is the fissile isotope which is used in fuel for most reactors (about 3% of all uranium fuel for example), but what actually happens to the other 97% of the U-238 if it doesn't undergo fission?

I get some of it absorbs neutrons making it U-239? But I've also seen it can become Plutonium-239? But what actually happens to the rest, does it lay dormant and non-fissile? And how do they know when the fuel is depleted?

 

[anuttarasammyak]

Conversion to Plutonium and its reuse are attractive. Ref. https://en.m.wikipedia.org/wiki/Spent_nuclear_fuel

 

[snorkack]

I get some of it absorbs neutrons making it U-239? But I've also seen it can become Plutonium-239?

U-239 undergoes two beta decays, with halflife about 24 minutes to become Np-239 and then half-life about 28 hours to become Pu-239.

 

[mfb]

U-238 can fission when hit by a fast neutron. U-238 can't maintain a chain reaction, but it's a side reaction that contributes a few percent (typically) to the output of a nuclear reactor.

U-238 can absorb a neutron and become U-239 which quickly decays to Pu-239. That's a good reactor fuel, similar to U-235, and fission of that can be a significant contribution to the power in a reactor.

Edit: Typo

 

[Vanadium 50]

For commercial reactors, most of it stays where it is and what it is.

 

[Astronuc]

TL;DR Summary: I know U-235 is the fissile part, but what happens to the U-238 parts during reaction and after?

So I'm aware that uranium 235 is the fissile isotope which is used in fuel for most reactors (about 3% of all uranium fuel for example), but what actually happens to the other 97% of the U-238 if it doesn't undergo fission?

I get some of it absorbs neutrons making it U-239? But I've also seen it can become Plutonium-239? But what actually happens to the rest, does it lay dormant and non-fissile? And how do they know when the fuel is depleted?

Some ²³⁸U fissions from fast neutrons, which provide about 7-8% of all fissions, and some of ²³⁸U aborbs a neutron and become ²³⁹U, which undergoes successive beta decay to ²³⁹Np to ²³⁹Pu, and ²³⁹Pu is quite fissile, and provides a considerable fraction of fissions as ²³⁵U depletes. Modern power reactors use enrichments up 4.95% (less than 5% - some uncertainty). Typical burnups are up to 50 to 60 GWd/tU, or about 5 to 6% of initial metal atoms (consumed). At those burnups, most of the fission is coming from Pu.

What isn't fissioned or transmuted remains as is - ²³⁸U - which could be recycled, but the recycle U often contains ²³⁶U (converted from ²³⁵U), which is disadvantageous.

As V50 indicated, the spent fuel, which includes unused U, various transmuted products (transuranics), and the fission products, sits in spent fuel pools at the reactor site, where it cools. At some point, the older/aged spent fuel is transferred to dry (inert He) filled storage containers, which are stored at the reactor site. The US does not reprocess fuel, while some European and Asian nations do reprocess spent fuel and recycle the U and Pu, and perhaps some transuranics.

 

[Astronuc]

Conversion to Plutonium and its reuse are attractive. Ref. https://en.m.wikipedia.org/wiki/Spent_nuclear_fuel

Recycling of Pu (which includes ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴²Pu, ²⁴³Pu, and isotopes of Cm, Am, which have be separated) is problematic and not economically attractive, while uranium is plentiful. Mixed-oxide or MOX (U,Pu)O₂ must be handled (including manufacture and inspection) remotely because of the gamma and beta radiation. Remote handling greatly increases the cost, and in the past, MOX fuel was about 10x the cost of fresh U-based fuel.

Nations with limited supply of U or enriched U are motivated to recycle spent fuel.

 

[Vanadium 50]

You start with a rod of uranium. After a whole, it's uranium, plutonium, fission products (some of whic are gasses) and such, The density is lower, so the rod expands - called "swelling". As Astro points out, that's the end of the line for fuel rods.

In principle, one could separate the stuff you want from the stuff you don't. Such a process would also be very helpful in making bombs too, so the US at least doesn't do it. We burn up a little uranium and then dispose of the majority unburned fuel.

Well, we actually don't, but that's another thread,