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

In summary, the U-238 isotope in nuclear fuel primarily serves as a fertile material rather than a fissile one. When exposed to neutron radiation in a reactor, U-238 can capture neutrons and undergo a series of transformations, eventually converting to plutonium-239 (Pu-239), which is fissile and can sustain a nuclear chain reaction. This process allows U-238 to contribute to the reactor's energy output, while also providing a source of plutonium for potential use in nuclear weapons or as fuel in fast reactors. Additionally, U-238's presence helps to absorb excess neutrons, thus moderating the fission process and enhancing reactor safety.
  • #1
oblong-pea
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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?
 
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  • #3
oblong-pea said:
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.
 
  • #4
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
 
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  • #5
For commercial reactors, most of it stays where it is and what it is.
 
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  • #6
oblong-pea said:
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.
 
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  • #7
anuttarasammyak said:
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.
 
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  • #8
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,
 

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

What is U-238 and how is it different from U-235 in nuclear reactors?

U-238, or Uranium-238, is an isotope of uranium that is more abundant than U-235. Unlike U-235, U-238 is not fissile, meaning it cannot sustain a nuclear chain reaction by itself. However, it can capture neutrons and transform into other isotopes, such as Plutonium-239, which is fissile and can contribute to the reactor's energy output.

Does U-238 contribute to the energy production in a nuclear reactor?

Indirectly, yes. While U-238 itself is not fissile, it can capture neutrons and eventually transform into Plutonium-239 through a series of nuclear reactions. Plutonium-239 is fissile and can undergo fission to produce energy, thus contributing to the overall energy output of the reactor.

What happens to U-238 when it captures a neutron?

When U-238 captures a neutron, it becomes Uranium-239 (U-239). U-239 is unstable and undergoes beta decay to form Neptunium-239 (Np-239), which further undergoes beta decay to become Plutonium-239 (Pu-239). Pu-239 is a fissile material that can contribute to the reactor's energy production.

What percentage of U-238 is converted into Plutonium-239 in a typical reactor cycle?

In a typical reactor cycle, only a small percentage of U-238 is converted into Plutonium-239. The exact percentage can vary depending on the reactor type and operating conditions, but it is generally around 1-2%. This conversion process is known as breeding.

What happens to the remaining U-238 after a reactor cycle?

The remaining U-238 that does not capture neutrons or transform into other isotopes remains mostly unchanged. It is still present in the spent nuclear fuel and can be considered for reprocessing or disposal. In some advanced reactor designs, this remaining U-238 can be used in future cycles to breed more fissile material.

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