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Old 01-07-2017, 07:36 AM   #1
Crystalline_Entity
 
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Default Smallest possible fission generator?

I've been looking through Ultra-Tech and noticed that at TL9 there's a semi-portable fission generator which weighs 1,000 lbs and "fits in a trunk bed" (p.20) which can generate external power. Compared to the small modular reactors mentioned in Disasters: Meltdown and Fallout (p.6) this seems awfully small (in both volume and mass) even if they're TL8 and the fission generator in Ultra-Tech is TL9.

I can imagine that Ultra-Tech assumes "external power" is less than the 10-100 MWe that Disasters: Meltdown and Fallout says so the reactor in Ultra-Tech may be somewhat smaller, but is the TL9 fission generator in Ultra-Tech reasonable, or it is borderline superscience? How small can you plausibly make a fission reactor?
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Old 01-07-2017, 08:10 AM   #2
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Default Re: Smallest possible fission generator?

I know that powering automobiles with fission reactors was seriously modeled. So, off the top of my head a half-ton TL9 reactor doesn't sound too out of line, especially for single-digit megawatts given some of the cutting-edge ideas floating around for them, but I'm absolutely not an expert.

I mean, just one megawatt is a lot of power. That's 14000 lightbulbs, or 1000 microwave ovens, or 700 hairdryers on high heat. In fact, if my maths are correct it's enough to continuously power 66 average US households (10812kWh/month) which sounds like "external power" to me.

Last edited by acrosome; 01-07-2017 at 08:23 AM.
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Old 01-07-2017, 08:20 AM   #3
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Default Re: Smallest possible fission generator?

The Soviet BES-5 weighed 53kg and produced 2kW circa 1967, the TOPAZ weighed 320kg and produced 5kW circa 1987. Both were used in satellites.

There are a bunch of US one too, of course.

10kW under 500kg seems like something you could purchase today, if it was possible to make it commercially viable.
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Old 01-07-2017, 08:24 AM   #4
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Default Re: Smallest possible fission generator?

Quote:
Originally Posted by Purple Haze View Post
The Soviet BES-5 weighed 53kg and produced 2kW circa 1967, the TOPAZ weighed 320kg and produced 5kW circa 1987. Both were used in satellites.

There are a bunch of US one too, of course.

10kW under 500kg seems like something you could purchase today, if it was possible to make it commercially viable.
Aren't those technically RTGs? (Technically. It's never critical.)

But the US equivalent is the SNAP family, which definitely are RTGs. Did we ever make such small reactors?

Granted, I guess they are still fission "generators." The OP didn't say "reactors" except in reference to the SMRs.

And what the hell does "reactor" mean anyway? I assume it means criticality, which an RTG does not reach. I assume that a criticality reactor will produce significantly more power that a similarly-massed RTG.

Last edited by acrosome; 01-07-2017 at 08:30 AM.
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Old 01-07-2017, 08:28 AM   #5
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Default Re: Smallest possible fission generator?

The Soviets built some pretty small reactors -- really reactors, not RTGs -- for use on satellites. The TOPAZ reactor was 6KW in 1987, but that line of development ended will the fall of the USSR.
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Old 01-07-2017, 08:31 AM   #6
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Default Re: Smallest possible fission generator?

It's based more on 3e numbers than current understanding, but for my Vehicles conversion project, I have the smallest efficient fission generator at TL 9* as weighing around 15 tons, while you can get a smaller one that is less efficient that weighs around 5 tons. That large one produces 9 MW, while the smaller one produces 1.5 MW.

*Minimum size is based on TL. At TL 8, it's 50 tons, then 15, then 5, then 1.5, and finally 0.5. That is, my system has that 1,000 lb fission reactor as being either TL 11 (for low efficiency - 1 MW) or TL 12 (for high efficiency - 2 MW). If those numbers look like they don't quite add up, that's because TL 10+ reactors get better output than TL 9 ones.

EDIT: That's for a fission reactor - TL 9 RTG's have a minimum efficient size of 1.5 tons, which is tantalizingly close. That would produce 150 kW. My system would have a TL 9 half-ton version producing only 1 kW. At TL 10, that half ton version (thanks to both being of efficient size and the boost for TL 10 output) would produce 100 kW, which may be enough to justify external power. Of course, those last for 15 years - TL 9 also has Nuclear Power Units, with double output and only a 7 year endurance (close to the 5 for the UT one). Of course, my system also has a 0.5 ton RTG only costing $150 (the NPU would cost $300), rather than the $100,000 of the UT version...

Last edited by Varyon; 01-07-2017 at 08:41 AM.
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Old 01-07-2017, 09:20 AM   #7
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Default Re: Smallest possible fission generator?

Quote:
Originally Posted by Purple Haze View Post
10kW under 500kg seems like something you could purchase today, if it was possible to make it commercially viable.
Well maybe not purchase, all light reactors use high enrichment fuels that nonproliferation agreements really restrict the sale of, and really light spacecraft designs get a major cheat compared to virtually any other application in that you only need radiation shielding between the reactor and the sensitive instruments - it's perfectly OK to irradiate empty space.

Sure, hundreds of kilowatts-years in a hundred kilograms is physically possible if you have an unlimited budget and no concerns about using nuclear weapons grade fuels or killing anyone foolish enough to get close to your reactor to read the warning label for a century after the first time you turn it on, but that's not really a commercial design.

The shield is the big weight issue. You can get gamma rays from an experimental reactor down to survivable for hours levels with less than a meter of concrete (say 20 cm of lead or similarly dense metals), down to something OSHA might let people work in the same building with half again that thickness. For 20 or 30 cm of lead, well, that would be about 400 kg or 1300 kg for omnidirectional shielding even if your reactor had zero volume.
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Old 01-07-2017, 11:39 AM   #8
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Default Re: Smallest possible fission generator?

Quote:
Originally Posted by johndallman View Post
The Soviets built some pretty small reactors -- really reactors, not RTGs -- for use on satellites. The TOPAZ reactor was 6KW in 1987, but that line of development ended will the fall of the USSR.
Hmm. Interesting, so that combined with things like this do imply that a 2000 lb fission reactor isn't outside the realm of possibility, though at <1 MWe (given that an Ultra-Tech 400-sf solar array generates "external power", "external power" for Ultra-Tech is presumably also <1 MWe).

Quote:
Originally Posted by malloyd View Post
Well maybe not purchase, all light reactors use high enrichment fuels that nonproliferation agreements really restrict the sale of
That hadn't occurred to me, but if these small reactors use highly enriched fuel that would explain the reason that they're Legality Class 2 in Ultra-Tech whereas a semi-portable fusion reactor (TL10) is only LC3.
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Old 01-07-2017, 02:37 PM   #9
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Default Re: Smallest possible fission generator?

I remember asking the very same question months ago. I was seriously surprised at just how tiny they could be with modern or even outdated tech... assuming Fallout level disregard for weapons proliferation and irradiation safety.
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Old 01-07-2017, 02:41 PM   #10
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Default Re: Smallest possible fission generator?

To create a fission reactor all you really need are a couple pounds of processed fissionables. Everything else a reactor is made out of is to regulate the reaction, cool the reactor, get useful energy out of the reactor, and shield everyone from the radiation. Depending on how willing you are to let safety slide there's a lot of weight in a fission reactor that can be removed. At higher TLs the entire regulation process and energy extraction may be able to able to be miniaturized considerably. Shielding, unfortunately, can't really be improved without TL^ superscience - but it can be ignored.
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