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Old 11-12-2018, 12:47 PM   #31
Ulzgoroth
 
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Default Re: DIY Nuclear Reactors After the End

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Originally Posted by PTTG View Post
I'd say that is a key part of the apocalypse genere. If you can make fuel, bullets, medicine, fuel... your world might suck, but it's hardly ending.

A setting where the dying slaves are trying to husband the last of the energy out of cooling, yet still deadly, fuel rods? Sounds pretty apocalyptic to me.
After The End worlds aren't necessarily ending. A lot of them are at least potentially on an upward trajectory.
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Old 11-12-2018, 01:10 PM   #32
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Default Re: DIY Nuclear Reactors After the End

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Getting a feedstock of enriched uranium does seem to be a problem. Uranium hexafluoride adds corrosion to uranium's delightful properties but it seems to be the only choice for gaseous centrifuging. I'm not sure how large your centrifuging operation needs to be - Oak Ridge's K-25 was around 80K people and used several power plants, but it also produced 125 lbs of 80% enriched uranium in about a year, and that's overkill for a small research reactor's needs.
K-25 was not a centrifuge plant, but a gaseous diffusion plant. This is simpler to build, given a large-scale metals industry, but needs a lot more energy. A centrifuge plant needs less energy, but a lot of high-precision manufacturing.

Neither is really plausible for a post-apocalyptic society. Pakistan has managed to build effective centrifuge plants, but has a GDP of about US$300 billion, and has been able to source some high-tech equipment from other countries. Small compared to the USA, but vast compared to a post-apocalypse society.
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Old 11-12-2018, 01:39 PM   #33
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Default Re: DIY Nuclear Reactors After the End

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K-25 was not a centrifuge plant, but a gaseous diffusion plant. This is simpler to build, given a large-scale metals industry, but needs a lot more energy. A centrifuge plant needs less energy, but a lot of high-precision manufacturing.

Neither is really plausible for a post-apocalyptic society. Pakistan has managed to build effective centrifuge plants, but has a GDP of about US$300 billion, and has been able to source some high-tech equipment from other countries. Small compared to the USA, but vast compared to a post-apocalypse society.
I'm not suggesting that a community build a K-25 equivalent from scratch. I am suggesting it might be feasible to scavenge a few dozen or hundred centrifuges from the surrounding areas and use those in a continuous circuit to produce the comparatively modest amounts of low enriched uranium that a small reactor would need.

Oak Ridge in WWII had a workplace of about 80K people and a power budget of around 250 MW and produced around 140 lbs of 80% enriched uranium in a year (roughly, I may be underestimating the output but that's enough for the Little Boy bomb). Estimating that an AtE society needs around 15 lbs of 15% enriched uranium per year and their scavenged centrifuges are 20x as efficient as K-25, you're looking at a 250 kW power plant (ie, a large diesel engine) and around a hundred workers.

Digging into it more, it seems like Oak Ridge produced that uranium in 3-4 months (production started in March 1945 and was complete by 15 June 1945) which really reduces the required power.

I'm not saying this is highly plausible, just that if you're one of the many US universities that has access to a nuclear reactor and it's AtE, you may make an effort to set up a centrifuge cascade and process some fuel because turning 50-70 kW of burning biodiesel into 1+ MW of nuclear power generation seems like a pretty good deal to me.
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Old 11-12-2018, 10:04 PM   #34
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Default Re: DIY Nuclear Reactors After the End

Do your AtE in the Fallout universe, then you can scavenge as many atomic power plants as you like from cars parked on the freeways.
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Old 11-12-2018, 10:06 PM   #35
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Default Re: DIY Nuclear Reactors After the End

Salvaged centrifuges might work for chemistry, but they would be too imprecise for nuclear science. Anyway, you need tens of thousands of centrifuges operating constantly to provide sufficient sufficient enrichment. I honestly doubt that a reactor could produce as little as 1 MW safely for power, that is the size of experimental reactors that produce short lived isotopes and the like.
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Old 11-13-2018, 01:44 AM   #36
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Default Re: DIY Nuclear Reactors After the End

Also, your centrifuges get power from... what? If you've got a great power source, why not use that? If you need lots and lots more power than you need to power ten thousand centrifuges, is nuclear still going to be a better option than coal (seeing as the environment's probably already ruined...)?
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Old 11-13-2018, 05:27 AM   #37
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Default Re: DIY Nuclear Reactors After the End

You need thousands of centrifuges to produce hundreds of kilograms of highly enriched uranium in a year. But since the goal is much more modest, you can presumably get by with less.

Gas centrifuges are supposed to have a power return of 1500:1. So a 1-2 MW research reactor needs 1-2 kW of ethanol or biodiesel engines to power it's centrifuge array. That's a scavenged lawnmower engine.
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Old 11-13-2018, 05:40 AM   #38
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Default Re: DIY Nuclear Reactors After the End

That level of efficiency assumes industrial process that support a nuclear power infrastructure that generates GWs of energy. When you are talking about smaller batches, you reduce the efficiency of the process. At 1 MW, I would not be surprised if the fuel processing requires more energy than 1 MW due to inefficiencies in fuel processing and the like.
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Old 11-13-2018, 06:52 AM   #39
ericthered
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Default Re: DIY Nuclear Reactors After the End

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You need thousands of centrifuges to produce hundreds of kilograms of highly enriched uranium in a year. But since the goal is much more modest, you can presumably get by with less.

Gas centrifuges are supposed to have a power return of 1500:1. So a 1-2 MW research reactor needs 1-2 kW of ethanol or biodiesel engines to power it's centrifuge array. That's a scavenged lawnmower engine.

centrifuges are tricky and difficult equipment, especially nuclear grade centrifuges. My grandfather tried using them to make oil shale extraction economical at one point and they're very hard to use.



Nuclear enrichment requires higher grade centrifuges than most, and the centrifuges are actually the thing that prevents more nations from getting atomic weapons.
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Old 11-13-2018, 09:07 AM   #40
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Default Re: DIY Nuclear Reactors After the End

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That level of efficiency assumes industrial process that support a nuclear power infrastructure that generates GWs of energy. When you are talking about smaller batches, you reduce the efficiency of the process. At 1 MW, I would not be surprised if the fuel processing requires more energy than 1 MW due to inefficiencies in fuel processing and the like.
Again, Oak Ridge in 1945 produced 60 kg of highly enriched uranium in 3 months with access to a 250 MW reactor and a much less efficient process. GW of power is not required even with the diffusion process, much less the centrifuge process.

Eyeballing it, it looks like you need about 300 stages, each as efficient as K-25's 2892 stages, to refine uranium to 15% enrichment. That's already cutting your power budget by 90%.
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