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LostCosmonaut

Nuclear Reactors in Space: For or Against?

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Personally, I believe that application of nuclear power in space would be very much in our interest. Not only do nuclear thermal rockets offer a major improvement over existing propulsion technologies, but the use of nuclear reactors as power sources for satellites, space probes, and the like could allow for much greater scientific return or utility. However, I realize that nuclear power does have associated risks, and there are others who may feel different. Whether you are for or against the usage of nuclear power in space, I am curious to hear your opinions.

 

For reference, here's an interesting paper discussing the topic.

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Nuclear reactors for space use have some pretty hairy engineering problems.

 

Since you're lobbing this sucker into orbit, it needs to have a good power to mass ratio.  Nuclear reactors are amazing at a lot of things, having high power to mass ratios ain't one of 'em, so you're going to be looking at something at least as complex as a naval nuclear reactor.  Those are usually fast neutron designs, which is a lot harder to design and operate than a thermal type.

 

If it's being used for an NTR the propellant will double as coolant, taking heat away from the core and keeping it at a stable temperature.  If you're using it for electrical power generation... things get exciting.  You need some means of rejecting waste heat, and since convection and conduction don't work in space, that leaves radiation.

 

If you want your radiators to be mass-efficient, the waste heat rejection needs to occur at a high temperature.  If you want to reject waste heat at a high temperature, you end up using really weird, really dangerous working fluids for your turbine... things like mercury.  Also, if you want good carnot efficiency your core temperature has to be super-high since your rejection temperature is so high...

 

Nuclear space power generation is just all kinds of fucked up and weird.  Not saying it's a bad idea overall, but it's considerably more involved than just taking a typical terrestrial nuclear reactor (most of which are absurdly reliable) and flinging it into the sky.

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Parking a satellite on the Earth-Sun L2 with a nuclear reactor onboard might be fun.  You'd always be in shadow, so your radiators would work better.  You could use the reactor to power some sort of electrical rocket (ion, hall thruster, VASMIR, plasma, take your pick) for station-keeping.

 

I can't immediately think of any reason to have a satellite hovering at L2 all the time, but doubtless there is one.

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https://spacenews.com/final-fiscal-year-2019-budget-bill-secures-21-5-billion-for-nasa/

 



Of that total, $180 million will go to Restore-L, a satellite servicing mission also previously threatened with cancellation, and $100 million to nuclear thermal propulsion research, including planning for a flight demonstration mission by 2024.

 

Hype for potential of NERVA returning (plus probably some Timberwind DNA)

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TOPAZ reactor, this international program was a bit too naive.

 

https://www.researchgate.net/publication/266516447_US-Russian_Cooperation_in_Science_and_Technology_A_Case_Study_of_the_TOPAZ_Space-Based_Nuclear_Reactor_International_Program

 

A powerpoint presentation in the post '91 mood, not primarily on space reactors but more on the dangers of the nuclear field.  

https://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-UR-09-00906

The 23 page reminds me of the story of FOGBANK.

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https://breakingdefense.com/2020/02/darpa-doubles-dough-for-nuclear-powered-cislunar-rocket/

Demonstration Rocket for Agile Cislunar Operations (DRACO), formerly known as “Reactor on a Rocket (ROAR)” — $21 million, up from an initial $10 million in 2020. DRACO “will develop and demonstrate a High-Assay LowEnriched Uranium (HALEU) nuclear thermal propulsion (NTP) system.” NASA is working on similar nuclear thermal propulsion rockets, which use low-enriched — between 5 and 20 percent — uranium-235 (U-235). U-235 is the basic nuclear fuel for commercial light-water reactors when enriched to between 3 and 5 percent; the Navy’s nuclear reactors use U-235 fuel enriched to 90 percent. The new rocket would allow the US military to operate spacecraft in cislunar space, which DARPA’s budget documents call the “new high-ground” that is “in danger of being defined by the adversary.” DARPA budget documents say the Air Force is the targeted customer for DRACO. As Breaking D readers know, senior Air Force and DoD officials are increasingly speaking publicly about the need for the United States to expand its military space activities to cislunar space to counter China — which has a robust civil lunar exploration program that many in the US national security community suspect is a cover for military ambitions. Indeed, SDA’s planned space architecture includes sensors in cislunar space. DARPA’s funding boost for the project reflects its intentions to move from feasibility studies this year to an actual demonstration in a testing environment in 2021.

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A consideration with using nuclear generated electricity on a telescope is that the radiative surfaces to cool the scope would have to be large, which would necessitate a wider radius to increase the surface area without overlapping the radiative vectors, which would increase the moment of inertia. This might not be that big of a problem, considering a lot of telescopes just look at one location for long periods of time, but would pose a problem when trying to turn 180 if there is an urgent requirement (asteroids and such). Also, launching such a large  satellite will cause problems, unless we work on our microgravity construction techniques. 
 

Though, on advantage of it being so far out (the Earth-Sun L2), you could indeed use volatile coolants like NaK, not that big of an environmental risk cause space is already inhospitable, though Earth’s gravity might render this point moot if coolant leaves the L2 area at certain vectors. 

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Ok, did some research, and it turns out that the Earth-Sun L2 point is ~1.5 million km from Earth, give or take, and the umbra that Earth casts is only ~1.4 million km in length... which means this point is never in full shadow, and you cannot use the Earth or any vector within the vicinity of the Earth to radiate a spacecraft’s heat.  
 

On a side note, I feel incredibly inconvenienced by these facts. 

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