The question: Why don’t nuclear reactors use more of the fuel, eg, 50%, 80%, before discarding it? It looks like there is plenty of energy left, and uranium is expensive. Also, there would be an increased concentration of radioisotopes, but there is so much fuel it would last for decades, and by that time they should have decayed away. It seems to me that the current way of using nuclear fuel is less economical than the process described above. Is there any issues with burning all the fuel?
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4$\begingroup$ There are actually two major types of nuclear reactors. One type, as people expect, is to generate power, and the goal is to maximize the fuel usage. The other, although it still generates power, is actually a front and is designed to create weapons grade plutonium as a by-product. Both exist, but the low fuel yield describes the latter type more often than the former. $\endgroup$– NelsonCommented Dec 20, 2023 at 3:43
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3$\begingroup$ Making a reactor run economical is only one of the concerns. Making them run safely is another. The most obvious approach is not always the safest approach. $\endgroup$– MastCommented Dec 20, 2023 at 7:58
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$\begingroup$ Surely you've heard of the problems with radioactive waste from nuclear plants needing storage that will be stable for many thousands of years. "... would last for decades, and by that time they should have decayed away" is wishful thinking. $\endgroup$– BenCommented Dec 21, 2023 at 1:54
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$\begingroup$ Uranium isn't that expensive though. The main reason why reprocessing isn't more common is because just buying more fresh fuel ends up being much cheaper than having to deal with reprocessing. $\endgroup$– ntoskrnlCommented Dec 22, 2023 at 21:23
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4 Answers
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The most important limit is that you can't separate the byproducts from the fuel without reprocessing. Among the byproducts are neutron-absorbers, which poison the desired controllable reaction.
Reprocessing is conceptually simple (just pull out that 95%+ untouched fuel and repackage it). But handling of the radioactive byproducts is not easy or cheap. There are pros and cons doing so.
and by that time they should have decayed away.
As long as you are running the reaction you are creating undesired byproducts. The most problematic ones do not disappear on the timescale of reaction operation.
answered Dec 19, 2023 at 17:00
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The reactor core is designed with a certain amount of reserve reactivity so it can operate at full power for an extended amount of time by adjusting the control rods as the fuel is consumed and the poisons accumulate. However, at some point there's not enough reactivity left to hit full power even with the rods all the way out and from then on the power output of the reactor will fall. That's when it's time to replace the fuel cannisters.
Reprocessing is also the most convenient way to make fissionables for use in atomic weapons, which makes reprocessing a political issue with respect to antiproliferation efforts.
Note that although reprocessing is conceptually simple, the process itself is violently radioactive and has to be performed robotically because the radiation released is lethal to humans.
Finally, note that the transuranic waste products extracted from the spent fuel continue to release lethal radiation for tens of thousands of years, making waste storage a political issue too.
answered Dec 19, 2023 at 17:48
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1$\begingroup$ The conversation about reprocessing is not connected narratively to the first paragraph, and I'm struggling to understand its relevance. Are you saying that "reprocessing" could be used to make new fuel cells appropriate for use in nuclear reactors, and further that it shouldn't be? $\endgroup$– jpaughCommented Dec 20, 2023 at 13:43
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$\begingroup$ It's more that spent fuel has materials that would be useful in a nuclear weapons program. Reprocessing would afford an opportunity to extract those portions of the spent fuel. $\endgroup$ Commented Dec 20, 2023 at 17:03
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$\begingroup$ @jpaugh I think niels mentions reprocessing because that's the only way to use more of the original fuel, which was the OP's question (even though they didn't mention reprocessing at all, probably because they didn't know about poisoning). The connection "the only way to use more of the original fuel would be reprocessing, but ..." is implicit and missing. $\endgroup$ Commented Dec 20, 2023 at 18:10
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1$\begingroup$ @jpaugh Reprocessing is a well known and viable option for making nuclear fuel last much much longer. However, there isn't much difference in reprocessing spent fuel to make it reusable and turning fuel into weapons grade material. So anybody that can reprocess fuel can make weapons. So any efforts for reprocessing essentially face the political argument of making weapons. This has resulted in a political climate that has more or less banned reprocessing for energy. $\endgroup$– David SCommented Dec 20, 2023 at 18:26
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1$\begingroup$ > "which makes reprocessing a political issue with respect to antiproliferation efforts." Which countries are we talking about? Which country has industrial capabilities to do reprocessing but has a political issue because it must not be allowed to gain nuclear weapons? $\endgroup$ Commented Dec 20, 2023 at 22:52
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We must take a numerical example on a specific type of reactor to clearly explain the problem. PWR reactors essentially consume 235U. Natural uranium is enriched in 235U, up to 4 or 5% in uranium 235. Why not enrich it further? Because of a very simple technological problem: the mechanical strength of the zircaloy sheath ( cladding ) in which the fuel pellets are placed. After having consumed the enrichment up to approximately 1%, the reactor core is no longer a critical mass AND the fuel cladding has reached its mechanical limits. So, there would be no point in further enriching the initial fuel: enrichment is expensive... A break in the fuel cladding is a severe accident , fission products escape in the whole primary circuit causing high contamination .
answered Dec 19, 2023 at 17:34
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3$\begingroup$ Reactors running on 40%-50% enriched fuel do exist (e.g. nuclear vessels). They are both expensive and also a regulatory problem (the fuel gets weapon-grade). $\endgroup$– fraxinusCommented Dec 20, 2023 at 9:03
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1$\begingroup$ The more specific reason is that the fuel pellets inside the fuel rods get increasingly weaker and more fractured due to fission product buildup as fuel burnup increases. Thus, the integrity of the cladding cannot be guaranteed anymore in a worst case design basis accident (typically LBLOCA). But in the past 40 years, typical burnup has doubled as we have access to better analyses. Recent developments in accident tolerant fuels (ATF) may allow burnup to be increased further. $\endgroup$– ntoskrnlCommented Dec 22, 2023 at 22:02
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In typical nuclear fuel, about 3.5% of the uranium is fissile (U-235), the rest is not. Only this 3.5% takes part in the energy production. As a side effect, also some of the rest decays, but not much.
Fuel cell production is very little in the price of the creation of the nuclear energy. Thus, it is not a really important question.
The technology is ready to use nearly 100% of the uranium content (google for: breeder reactors) since about the 60ties. Main reason of that it is not widely used, that it is not very needed because (1), but is being labeled "experimental" and causes additional unrest because (2).
