Is there limit on how small fusion/fission reactor can be??

[raxo2222]

I was playing Interstellar mod for KSP where you can have 0.625m fission/fusion/exotic reactor, not all kinds of them though.

[Tex_NL]

Fusion: Humanity has not been able to control sustained fusion in ANY reactor yet. Once a big one is build they might be able to shrink it but until then nobody knows.
Fission: You will need a certain amount of nuclear fuel. The so called critical mass. Anything less than that will not sustain a reaction. And to prevent it becoming a bomb you will need control rods, cooling shielding. Don't count on a nuclear fission powered can any time soon. They will remain building size for quite some time.

Edited November 27, 2016 by Tex_NL

[raxo2222]

25 minutes ago, Tex_NL said:

Fusion: Humanity has not been able to control sustained fusion in ANY reactor yet. Once a big one is build they might be able to shrink it but until then nobody knows.
Fission: You will need a certain amount of nuclear fuel. The so called critical mass. Anything less than that will not sustain a reaction. And to prevent it becoming a bomb you will need control rods, cooling shielding. Don't count on a nuclear fission powered can any time soon. They will remain building size for quite some time.

Well (space-worthy) molten salt reactor from Interstellar has minimum size of 0.625m - this means diameter of 1.25m and similar height.

Thermal generators/engines adds to height too.

[magnemoe]

Just now, raxo2222 said:

Well (space-worthy) molten salt reactor from Interstellar has minimum size of 0.625m - this means diameter of 1.25m and similar height.

Thermal generators/engines adds to height too.

This sounds pretty right for an tiny reactor.
An fusion reactor will be larger but this depend on who designs who works out. 

[Tex_NL]

13 minutes ago, raxo2222 said:

Well (space-worthy) molten salt reactor from Interstellar has minimum size of 0.625m - this means diameter of 1.25m and similar height.

Thermal generators/engines adds to height too.

And the Death star can blow up a planet. That's just how the story goes.
In the game EVERYTHING is possible because it is just 1's and 0's. A mod may LOOK realistic, that does not mean it IS realistic!

Edited November 27, 2016 by Tex_NL

[raxo2222]

9 minutes ago, Tex_NL said:

And the Death star can blow up a planet. That's just how the story goes.
In the game EVERYTHING is possible because it is just 1's and 0's. A mod may LOOK realistic, that does not mean it IS realistic!

lol

 

but this Interstellar mod for KSPI wants to be as realistic as possible

[Tex_NL]

Just now, raxo2222 said:

lol

 

but this Interstellar mod for KSPI wants to be as realistic as possible

All very true. It can be as realistic (or unrealistic) as it wants. It's just coding.
Even if you want your Kerbals to ride pink elephants that shoot chocolate rainbows when they fart. It's just coding.

[Steel]

24 minutes ago, raxo2222 said:

lol

 

but this Interstellar mod for KSPI wants to be as realistic as possible

It may want to be realistic, but at the end of the day, space-going small form-factor fusion reactors are pretty much in the realm of science fiction right now, and 0.625 m fission reactors are non much better. Thus, "realism" is as much a guess at some vaguely OK looking numbers as actually based on any solid science or engineering.

Edited November 27, 2016 by Steel

[ChainiaC]

Soviet Topaz II reactor is about four meters in length and weighs just over one metric ton. Thats a lot smaller then a building. 

https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor

[kerbiloid]

IRL a fission reactor itself (its active zone) sometimes is less than a meter in size (still producing tens of megawatt).

But its anti-radiation protection is at least 5-6 meters thick in every direction. Does not depend much on its power. And this is unavoidable due to the nature of U fission.

Fusion reactor would have the same problem, unless you use aneutronic one (in KSPI-E it's boron-fueled). But the aneutronic one needs much greater temperature (billions K), so you should use a multiple times larger cooling system. Probably radiator-based, because you can't use convection - it's too slow. So, again at least meters.

[cantab]

Realistically, fission reactors can be made pretty small. The minimum requirement is that you need a critical mass, but that can be as low as about 10 or 20 kilos. There are certainly reactors you could fit in your fridge, such as the https://en.wikipedia.org/wiki/SNAP-10A that actually flew in space.

[Stargate525]

https://en.wikipedia.org/wiki/SNAP-10A

 

Given that this was built in the 60s, I'm pretty certain we could make it smaller. Granted, this is just the reactor. Including all the cooling and shielding is a different question; it's like asking what the smallest internal combustion engine you can build is, but then also including the transmission, exhaust, and car electrics as well.

[Tex_NL]

2 minutes ago, Stargate525 said:

it's like asking what the smallest internal combustion engine you can build is, but then also including the transmission, exhaust, and car electrics as well.

Indeed it is. An engine without the equipment to make it actually run (fuel and ignition systems etc.) is only fit the serve as a boat anchor. Same goes for a reactor without the equipment to run it.

[raxo2222]

6 minutes ago, Stargate525 said:

https://en.wikipedia.org/wiki/SNAP-10A

 

Given that this was built in the 60s, I'm pretty certain we could make it smaller. Granted, this is just the reactor. Including all the cooling and shielding is a different question; it's like asking what the smallest internal combustion engine you can build is, but then also including the transmission, exhaust, and car electrics as well.

I was talking about smallest possible rocket with engine powered by nuclear reactor - either by thermal energy or using electricity like VASMIR or something on steroids.

 

 

[todofwar]

So is fission/fusion just a way to get heat energy? No more efficient ways to harness it? 

[wumpus]

2 hours ago, kerbiloid said:

IRL a fission reactor itself (its active zone) sometimes is less than a meter in size (still producing tens of megawatt).

But its anti-radiation protection is at least 5-6 meters thick in every direction. Does not depend much on its power. And this is unavoidable due to the nature of U fission.

Fusion reactor would have the same problem, unless you use aneutronic one (in KSPI-E it's boron-fueled). But the aneutronic one needs much greater temperature (billions K), so you should use a multiple times larger cooling system. Probably radiator-based, because you can't use convection - it's too slow. So, again at least meters.

A lot depends on what you want, and how you can build it.  Shielding in space can easily be limited to a single direction (mostly shielding crew, and possibly using fuel as shielding if you are willing to eject your reactor before burning the fuel).  Cooling is probably the biggest issue, and is proportional to power.  Don't think of things like nuclear subs flowing through arbitrary amounts of cooling water, think more like Chicago Pile-1 (step 0 of the Manhattan Project: create *any* nuclear reaction) which didn't appear to have any cooling whatsoever (nor shielding).  What you have is more like an RTG with a bit more control of when the radioactive fuel is emitted ("real" RTG's simply take the power as plutonium decays at its fixed rate).

Building a reactor on the Moon or on Mars is still wildly different from in space.  You get at least *something* of a heatsink there, in vacuum you are limited to radiating heat into space.  Don't be too surprise if your Heat->electric power conversion is so inefficient that you wind up carrying more radiators (that are heavier) than solar panels (assuming you are within Mars or so).

[raxo2222]

7 minutes ago, todofwar said:

So is fission/fusion just a way to get heat energy? No more efficient ways to harness it? 

Well if @FreeThinker is right, then we can use charged particles to generate electricity from fusion reactors and some very advanced fission reactors.

Charged particle generator (producing electricity from charged particles) efficiency is between 50% - 86.5% efficient.

This question is more of "what if" nature - inspired by KSP Interstellar Extended mod.

 

4 minutes ago, wumpus said:

A lot depends on what you want, and how you can build it.  Shielding in space can easily be limited to a single direction (mostly shielding crew, and possibly using fuel as shielding if you are willing to eject your reactor before burning the fuel).  Cooling is probably the biggest issue, and is proportional to power.  Don't think of things like nuclear subs flowing through arbitrary amounts of cooling water, think more like Chicago Pile-1 (step 0 of the Manhattan Project: create *any* nuclear reaction) which didn't appear to have any cooling whatsoever (nor shielding).  What you have is more like an RTG with a bit more control of when the radioactive fuel is emitted ("real" RTG's simply take the power as plutonium decays at its fixed rate).

Building a reactor on the Moon or on Mars is still wildly different from in space.  You get at least *something* of a heatsink there, in vacuum you are limited to radiating heat into space.  Don't be too surprise if your Heat->electric power conversion is so inefficient that you wind up carrying more radiators (that are heavier) than solar panels (assuming you are within Mars or so).

This is actual problem in early KSPI science mode, where reactors don't produce much energy, radiators have low maximum temperature and there are thermal generators being at 36% or so efficiency.

Edited November 27, 2016 by raxo2222

[kerbiloid]

8 minutes ago, wumpus said:

think more like Chicago Pile-1 (step 0 of the Manhattan Project

Which was 6 meters in size with total output 200 W.

10 minutes ago, wumpus said:

nuclear subs flowing through arbitrary amounts of cooling water,

https://www.researchgate.net/file.PostFileLoader.html?id=54d0ed06cf57d786418b4631&assetKey=AS%3A273690091032577%401442264211285

Still several meters of reactor shielding. Just not a concrete.

[pincushionman]

The PUR-1 reactor at Purdue is only 1 ft. x 1 ft. x 2 ft, but it's only the reactor core itself. It's the 8 ft. x 17 ft. pool it's in that makes it big. It also only produces 1 kW of heat, none of which is useful for power generation.

You also need to keep in mind how you're generating power with it. You can't just slap a reactor core on a satellite and say "Hey, I have power!" - you need the heat to do work somehow. Terrestrial power reactors usually use steam turbines, which are pretty large and complex themselves, but they do the job very well. They're also worthless in space.

[cantab]

1 hour ago, todofwar said:

So is fission/fusion just a way to get heat energy? No more efficient ways to harness it? 

There are but they're all paper concepts. Using the reaction to create heat and converting that heat into electricity is the only proven way.

[Tex_NL]

22 minutes ago, pincushionman said:

The PUR-1 reactor at Purdue is only 1 ft. x 1 ft. x 2 ft, but it's only the reactor core itself. It's the 8 ft. x 17 ft. pool it's in that makes it big. It also only produces 1 kW of heat, none of which is useful for power generation.

You also need to keep in mind how you're generating power with it. You can't just slap a reactor core on a satellite and say "Hey, I have power!" - you need the heat to do work somehow. Terrestrial power reactors usually use steam turbines, which are pretty large and complex themselves, but they do the job very well. They're also worthless in space.

Exactly. That PUR-1 reactor you're talking about may only be 1x1x2 ft but without all the peripherals it's just a 1x1x2 ft nuke. It may not actually explode but all you get from it once it is activated is a meltdown and a big ball of radioactive death.

[wumpus]

1 hour ago, pincushionman said:

You also need to keep in mind how you're generating power with it. You can't just slap a reactor core on a satellite and say "Hey, I have power!" - you need the heat to do work somehow. Terrestrial power reactors usually use steam turbines, which are pretty large and complex themselves, but they do the job very well. They're also worthless in space.

I'd be shocked if vaporized material turbines don't convert heat into electricity wildly more efficiently than pretty much any other proposed conversion (although I wouldn't expect water steam to be the final vaporized material, and I suspect at least a cascade between two materials (water as the first, followed by freon)?  There's also sterling engines, but I suspect that would be for the "smallest reactor we could build" type solution.

RTGs might use peltier-type junctions or thermocouples, but I suspect that they want ultra-simple operation for years without maintenance, and are willing to sacrifice power for it.  Once you start bothering with full blown reactors, I'd suspect that standard heat engines will be used.

[Streetwind]

The smallest "complete" reactor we've ever built might be from the "Safe Affordable Fission Engine" program by NASA in the early 2000's. The core of the later, larger SAFE-400 model was a cylinder 50cm tall and 30cm across, which makes it small enough to easily hold in your arms... except, it weighed over half a metric ton. Just try to wrap your head around that for a moment. As its name suggests, it produced 400 kW heat load, and had all the necessary equipment to feed that heat into a brayton cycle turbine that generated 100 kW electrical power.

However, before they went to that one, they built an even smaller one, called SAFE-30. Unfortunately I can't find any reliable numbers that aren't behind a paywall, but given the much smaller output (only 17 kW) and the already tiny size of the 400 kW system, I'd reckon this one was a fair bit smaller.

Edited November 27, 2016 by Streetwind

[magnemoe]

40 minutes ago, wumpus said:

I'd be shocked if vaporized material turbines don't convert heat into electricity wildly more efficiently than pretty much any other proposed conversion (although I wouldn't expect water steam to be the final vaporized material, and I suspect at least a cascade between two materials (water as the first, followed by freon)?  There's also sterling engines, but I suspect that would be for the "smallest reactor we could build" type solution.

RTGs might use peltier-type junctions or thermocouples, but I suspect that they want ultra-simple operation for years without maintenance, and are willing to sacrifice power for it.  Once you start bothering with full blown reactors, I'd suspect that standard heat engines will be used.

You could use thermocouples on larger reactors too, however they are much less efficient than an steam turbine, benefit is no moving parts. 
NASA had an project with an higher effect RTG who used an stirling steam engine rather than thermocouples, main benefit would be more power and you could use other radioactive materials. 
 

[KSK]

Bimodal nuclear thermal engines (can be run as engines or generators) can be quite small. According to the introduction to this paper, the enhanced SNRE design (which I believe stands for Small Nuclear Rocket Engine) is  59cm in diameter by 132cm in length and can generate 25kWe through a Brayton cycle turbine.

So a 0.625m fission reactor is entirely realistic. Any size of rocket sized fusion  reactor is currently entirely fictional.