How can nuclear power produce -electricity- in space when energy must be converted differently?

Question

Hear me out before you downvote; It's not as simple as it seems.

On Earth, nuclear reactions produce heat energy. This is used to heat water, creating steam, powering turbines connected to generators - and voila, electricity.

IN SPACE,
If you attempt to use thermal energy to produce steam, it will not "rise" because buoyancy does not apply; furthermore, liquid water will mix with vapor in messy circumstances that make pressure systems more complex to construct. Therefore, conventional reactors cannot operate to produce electricity the conventional way, in space.

Additionally, as jamesqf mentioned in a comment,
Heat engines (steam turbines) require temperature differences, which, unlike on Earth, cannot easily be catered to in space. Enormous radiators disposing of excess heat are necessary to make turbines run effectively.

So how would a nuclear-powered spaceship, without artificial gravity, generate electricity effectively from raw thermal energy and the radiation associated with nuclear reactions?

Answer 1

Nuclear reactors can work just fine with other coolants, and really high efficiency nuclear reactors here on Earth are designed to use Helium in the primary coolant loop in order to support running the core at a much higher temperature than water cooled reactors generally run at. Other work arounds have included using Sodium (very dangerous because Sodium will ignite in the presence of water, so a leak could become a flaming radioactive nightmare) and metals like lead (the opposite problem has occurred, apparently some Soviet era submarines using high powered, lead cooled reactors have been decommissioned because the operating temperature was allowed to drop too much, resulting in the coolant solidifying in the primary loop).

The biggest reason to avoid "steam" powered generators isn't technical, but rather the laws of physics. Steam generators are considered to be "Rankin" cycle machines, and like most heat engines have a pretty hard upper limit to the amount of energy that can be extracted, known as Carnot's theorem (thermodynamics)

Carnot's theorem states:

All heat engines between two heat reservoirs are less efficient than a Carnot heat engine operating between the same reservoirs.

Every Carnot heat engine between a pair of heat reservoirs is equally efficient, regardless of the working substance employed or the operation details.

The formula for this maximum efficiency is:

efficiency = 1 − TL/TH

where TC is the absolute temperature of the cold reservoir, TH is the absolute temperature of the hot reservoir, and the efficiency is the ratio of the work done by the engine to the heat drawn out of the hot reservoir.

For steam engines (which is what we are talking about), 33% is a high efficiency without reheat or other additional steps (combined cycle systems get much higher efficiency because they use the energy in the fuel several times, i.e. a gas turbine generator which uses the exhaust to heat the steam).

One thing which might work well in space is to use an MHD generator, where the heat of the reactor ionizes a coolant that is them passed through a magnetic field. MHD is not constrained by the Carnot limit. The ultimate expression of that is a "fission fragment" reactor, where the fissile material is introduced into a magnetic chamber in the form of fine dust. The resulting fissioning of the material is captured in the high energy movement of the fission particles (up to .03 c, but in practice usually .01 c due to internal collisions), making a high energy stream of charged particles to be tapped for electrical energy or to be used as a rocket engine. (See this as well)

MHD and fission fragment reactors also require less "plumbing" and usually smaller radiators for the amount of reactor power, which are all advantageous when designing a spaceship or space colony.

Answer 2

You're starting from a false assumption. Steam turbines don't use the buoyancy of steam to generate power, they use its pressure. This works just fine in space -- separating the steam from the water might be a bit tricky, but that's just an engineering detail.

Alternatively, if you don't want to deal with moving parts, you can use a thermoelectric generator to turn heat directly into electricity. This isn't as efficient as a turbine, but it's also less likely to break down.

Answer 3

While nuclear reactors do not require steam to rise, making sure only steam (and no water) gets to the turbine might be a bit difficult in microgravity.

1) Use what we are using now - RTGs. No moving parts = no problem with gravity (or lack of thereof)

2) Generate "artificial gravity" using a centrifuge. (Since centrifuges aren't science fiction, I'm guessing this is not what was meant by "no artificial gravity".) It should be sufficient to place the heat exchanger into the centrifuge - neither the high-pressure primary circuit nor the turbines themselves need gravity, which avoids having to spin the reactor core.

Answer 4

I’ve read that stirling Engine designs are being looked into for this purpose, as being much more efficient and higher power than RTGs. That’s probably what you are looking for. You might find some info in real designs now that you know, but off the of of my head I'm guessing that you can use a design that doesn’t have a phase change like steam (“single-phase working fluid”), and uses active circulation so it doesn’t need convection currents.