How to design a zero-g nuclear reactor?

Question

My worldbuilding involves a reasonably near-future, high-realism space station research outpost, and I'd like it to be powered by a nuclear reactor. However, I need as much detail on the actual reactor design as possible. (Also, if its totally impractical, should I move to fuel cells?)

Are any of today's reactor designs generally suitable for operation in zero-g? Or are there experimental/theoretical reactors out there which may be suitable? Where do current designs assume the presence of gravity, and what other factors complicate the use of existing designs when deployed in space? What aspects of the designs can be totally discarded in space (containment, biological shields, etc)?

So far, I've only found detailed info on commercial, land-based power reactors but those are probably overkill for a space station. I only need a few MWe. Can I simply scale down these reactors (I am using these as a starting point because the info on the commercial power plant systems is plentiful, compared to ship- and submarine-based reactors).

How can I provide adequate radiation shielding for my crew, that doesn't involve huge amounts of dead weight? Can the crews water or LOX tanks be arranged as shielding, and the turbines and other heavy reactor components? Do tokamaks and stellarators require elaborate shielding? Do they create torques on the supporting structures?

Let's assume that a resupply mission can provide fuels to the reactor if necessary, and if in-flight refueling is possible; that limited mining of resources is available on-site, and that we're not particularly concerned about dumping waste products overboard.

A good answer will provide deep reference material on a candidate reactor design, with discussion on the modifications necessary to adapt it to zero-g usage. Or, if nuclear power isn't going to work, provide similar resources for fuel-cell or other technology.

Answer 1

There have indeed been fission reactors used in orbit: both the Soviet Union and NASA launched experimental fission reactors on satellites - a Soviet one even malfunctioned and ended up deorbiting above Canada, contaminating a zone with its fission fuel, which ended up with the Soviet Union paying compensation to Canada.

For example, the TOPAZ II massed 1061 kg for a power output of 300 kW. It wasn't flown, but it was extensively tested by both Russian and Western engineers, so it should be a pretty solid baseline. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980234591.pdf

You will also find lots of information on this website, which is a collection of resources for hard-SF worldbuilding: http://www.projectrho.com/public_html/rocket/basicdesign.php#id--Power_Generation--Nuclear_Fission_Reactors

Note that the TOPAZ II generates 6.8 MW of thermal power for those 300 kW of electricity power, so you will indeed have to get rid of lots of heat. This will be done by heat radiators. The hotter the radiators, the smaller they can be to radiate a given power. The size reduction is massive, as it scales with the fourth power of the temperature (in K). On the other hand, you need a temperature differential between the reactor and the radiator to actually generate power, so you can't have it running too hot either. Radiators can take different shapes, but the most common will be large plates sticking out of the reactor module. You will probably want to have those in pairs, each on a side of the module: if you put more of them around, they will start radiating into each-other, which makes you loose efficiency very fast. Another solution is radiator spikes if you don't need too much surface. Being long and thin, those have less inter-reflection problems, but those would be rather uncommon anyway.

As for radiation shielding, the most probable configuration is having the reactor module at the far end of a long broom, to keep it as faraway from the station proper as possible: the further it is, the less radiation is reaching the station. On the station-facing part of the reactor, there will be a shadow shield: that is, a partial radiation shield put in a way that the station is in its shadow. This avoids having to put a shield all around the reactor, with the enormous mass penalty it represents for anything space-based.

On the other hand, it means that your spacecrafts will have to be careful approaching the station from the other side, so they are inside the shadow once they are close enough for radiation to be dangerous. And you don't want anything to stick out of the shadow: even if an inert object isn't by itself damaged by radiation, it can reflect radiation back to the station. On the other hand, the Powers that Be may decide to grace the space program with enormous budgets (this is science-fiction after all) and decide they can afford completely shielding the reactor and avoid those problems.

Answer 2

There is a lot to learn from the aircraft reactor experiment.

In a modern treatment, this would likely be expressed as a LFTR design variant of some form.

This reactor was designed to work right-side up or upside down since planes do that sometimes. As such, it was not dependent upon gravity. This reference paper describes a 2.5 MW liquid fuel reactor. As liquid fuel, near room pressure reactor, there was no need for a containment pressure vessel that adds considerable weight and size to traditional nuclear reactors. Shielding is still needed of course.

Although designed for a plane, this also meets most of the design features you would desire for a space-based reactor. Not dependent upon gravity, comparatively lightweight, high temperature operation (therefore efficient and less heat rejection require for the same load), no need for shutdown every 6 months or so to re-arrange fuel rods, no need for large amounts of water.

The experiment only lasted a few hundred hours, but the viability of the reactor was well-established by the experiment. A nuclear plane is pretty insane, but if you really need a high-power space mission, you could justify the risk.

The launch risk of a nuclear reactor falling on your head should be considered in all such designs, but as the primary fuel in a LFTR design is Th-232, there is no need for tons of enriched fuel. You do need some of the "nasty stuff" (Pu-239, U-235, U-233) to bootstrap a LFTR reactor, but much less that a traditional nuclear design. This could be kept in a hardened vessel during launch.

Answer 3

Polywell fusion

You state (emphasis added)...

My worldbuilding involves a reasonably near-future, high-realism space station research outpost, and I'd like it to be powered by a nuclear reactor.

Well then, you have a great option there in the Polywell fusion reactor. Not only is it symmetrical in all three dimensions in a manner that seems very fit for a micro gravity environment, it also looks the part. In my opinion Polywell is æsthetically gorgeous. It is also very elegant from an engineering perspective with pretty much no moving parts and — as opposed to crude old fission reactors — not dependent on gravity for anything. As such Polywell certainly fits as representation of a science fiction space reactor. The only suspension of disbelief you need to achieve here is to say "Robert Bussard was right, Polywell works", which in real life could happen as soon as the year 2020.

Wiffleball 8 in operation. (Source)

Answer 4

"Go solid" on purpose

If you read the Three Mile Island analysis (or The China Syndrome movie, very similar incident)... what spooked them was overfilling the reactor and causing the top part of the reactor to be water instead of steam. Why does that matter since a PWR type doesn't use steam in its primary loop? That steam bubble has the same purpose as the blue pressurizer tank in a house with a well - a volume of compressible gas to buffer changes in pressure, and avoid damage to piping and the reactor vessel.

There's no "top" in zero-gee, so you can't rely on a steam bubble. Instead, you use that same style of pressure accumulator, and make your PWR "go solid". All the space in the reactor is occupied by liquid water.

Why not make steam in the reactor vessel? BWRs do that, and they are a fine design. (China Syndrome was a bit confused about whether it was a BWR or PWR). Steam and water must be separated. The only way to do that in zero-gee is spin the reactor, which pushes the water out to the edge... The fuel rods need to be in water, making your core donut-shaped and a lot harder to achieve critical mass. So a BWR type is right out.

Energy from hot water (or other coolant)

Using steam power in a secondary loop (as in the PWR) is probably right out, same reasons as the BWR: you'd have to spin the steam generators and condensers. Also, the condensers would need to have an ultimate heat sink cooler than 100C, which may not be practical in space. Better to take the water to Peltier devices at the ultimate heatsink, and return the water to the reactor still quite hot. This is normal in a PWR primary loop. It is also cheap to pump, since you are only recirculating coolant at same pressure, not injecting 1-psi feedwater into a 1000 PSI boiler.

Of course, you also have the problem that unlike terrestrial reactors, the environment is likely to freeze water which is not actively heated, ergo, no shutting down for maintenance. (Sort of like a Soviet Alfa sub whose sodium primary-loop coolant will freeze in the pipes). You may want a different coolant, that either shrinks when frozen and/or has a suitable freezing point. Most materials shrink when frozen, and the Alfa can be restarted in a shipyard facility.

But water is really good

Be careful in your choice not to lose water's best property - not its absolutely superb thermal density, but rather it being a good moderator in normal state, and a really terrible moderator in abnormal state. When a reactor makes excess heat, it makes steam bubbles which slow the reactor down - giving a useful "cruise control", and also passively shuts down fission if coolant is lost. This is such a desirable trait that I wanted to mention it. Chornobyl did not have this.

Another desirable feature of a coolant is that it turns solid and congeals when it escapes into space, that way the ship is not lost and can recollect its coolant.)

If you wonder why I'm keeping reactor design so "close to home", it's because experience matters. The Boeing Dreamliner was proposed as a flying wing. But they went with the conventional shape of the ME262 and 737 because it is so well understood.