saluksic t1_jc2m1ci wrote
Reply to comment by PHATsakk43 in Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama
So a regular reactor with uranium enriched beyond natural levels can’t make plutonium?
Hiddencamper t1_jc2nwl9 wrote
All uranium based reactors produce plutonium.
It’s a feature! We use U-238 as the filler material in the fuel, knowing we will get some breeding and use that plutonium to extend the fuel cycle.
When you pull fuel out of a LWR after three cycles, it’s running on about as much Pu-239 as it is U-235.
We have to account for that in fuel cycle analysis, hot excess reactivity / shutdown margin, and the Beta factor (fast/thermal fission ratio). It also can impact moderator temperature coefficient and cause it to shift to zero or even slightly positive.
Gunnarz699 t1_jc2nula wrote
It can and does, but makes less of it.
It would be fine in normal times, but the Soviets were stockpiling warheads in the tens of thousands.
PHATsakk43 t1_jc31lte wrote
While you’ve got lots of answers, one that’s been left out is the slower production of “even isotope” plutonium.
Only about 2/3 of U-235 neutron absorption creates fission. The other 1/3 simply does nothing besides creating U-236. Ultimately, U-236 through a series of decay and more adsorption reactions becomes plutonium 238 (or 240) which can’t be separated from the Pu-239 that is used in nuclear weapons (basically, we can’t “enrich” plutonium like we can uranium.)
So, for a given amount of Pu-239 produced, the RBMK with lower initial enrichment has a “cleaner” material.
This is a gross simplification and there are other things that can affect this, but it’s part of the equation.
UnamedStreamNumber9 t1_jc2o0o9 wrote
It can, but by enriching the 235 relative to the 238, you reduce fraction of the fuel that can be jumped from U 238 up to Pu 239 vs the fraction of U 235 that breaks down into barium, krypton and 3 neutrons. There’s still U238 in the fuel rods but with enriched uranium, there’s less of it available to be transmuted
PHATsakk43 t1_jc32sb1 wrote
While you’ve got lots of answers, one that’s been left out is the slower production of “even isotope” plutonium.
Only about 2/3 of U-235 neutron absorption creates fission. The other 1/3 simply does nothing besides creating U-236. Ultimately, U-236 through a series of decay and more adsorption reactions becomes plutonium 238 (or 240) which can’t be separated from the Pu-239 that is used in nuclear weapons (basically, we can’t “enrich” plutonium like we can uranium.)
So, for a given amount of Pu-239 produced, the RBMK with lower initial enrichment has a “cleaner” material.
This is a gross simplification and there are other things that can affect this, but it’s part of the equation.
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