Our current fission reactors on land and at sea use nuclear heat to turn water into steam to turn turbines. Nuclear fusion plants – if they ever come online – will turn water into steam. Is there any kind of nuclear reactor concept that could use energy directly from the reactor to power a ship, power grid or rocket? I’ve heard of something called fission fragment, but my knowledge of it is limited.
Turning water into steam to drive turbines is also how things like coal and gas fired power plants work, not just nuclear power plants.
I dunno - maybe something like solar power? Solar power involves a type of electromagnetic radiation (i.e. "light’) hitting a voltaic cell to generate electricity. I have no idea if that’s even possible to do with ionizing radiation, but it’s the only thing that occurs to me.
Radioisotope thermoelectric generators would do the trick but they are not very efficient, about 10% vs. a turbosteam plant’s 35%. What do you have against steam?
I don’t believe anyone has anything against steam, but it is a legitimate question whether it is possible to practically extract energy more efficiently than a simple heat engine. I do not have data on the thermal efficiency of specific nuclear reactors currently operating, but let’s say they are about 35-45%. Therefore at least half the energy is wasted! 
Take a beta source. Put it in the center of a spherical capacitor. Charge the capacitor up to a voltage just shy of the energy of the beta particles produced (positive terminal inside, negative outside). Bleed off charge at the same rate that it’s replenished by the beta particles. That’s about as direct as you can get, and it’s highly efficient. It’s even been used in some applications, like pacemakers that don’t need to be recharged for decades. The problem is that it’s tough to get a significant power out of it. Oh, and the best isotope for it is tritium, which doesn’t occur naturally and has to be synthesized, so it’s more like an energy storage system than an energy source, and once you consider the cost of making the tritium, it’s a very inefficient energy storage.
Solar Power is of course power from a fusion reaction.
What comes from a reactor is energy, in the form of heat and radiation.
The trick is to turn this energy into something convenient for us to use, usually electricity, though using the energy to power rockets has been tested, but never flown.
The traditional method is a steam cycle, and it is about as efficient as it is going to get. We’ve been using steam turbines for quite a while, and they’re pretty well engineered.
Downside is is that you are limited to the thermal properties of water, which means if you want a high temperature differential, you have to deal with a really high pressure.
And alternative is the Brayton cycle, using CO[sub]2[/sub] as the working fluid in its supercritical state.
This is more efficient, and runs at a much higher temperature, but, you obviously have to have higher temperature reactor to take advantage of that, and that comes with its own engineering challenges.
You also have fun concepts like the Orion drive, where you are detonating nuclear bombs being your spaceship, and using their shock waves as propulsion. That would be as close to utilizing nuclear energy directly for a task as we could really get.
It’s not that anyone has anything against steam. More like the radioactive decay > steam > turbine > generator process currently in use has a Rube Goldberg vibe to it. There should be a more efficient way to capture that energy, convert it into electrons, and shove them in the end of a wire.
Let’s turn the question the other way around. The maximum thermodynamic efficiency for a source (reactor core) at say 1000C (1273K) and a sink ( water) at say 27 C (300k) is about 76 %. So say you make a device that works at this maximum efficiency for a 1 GW plant. That means around 0.2 GW has to dissipated as waste heat. Where would you put this massive amount of heat ? In the atmosphere ? Or water ?. As you probably realize, putting it in the atmosphere will need large surface area and will not be economical.
A typical US Nuclear Plant producing 1.2 Gigawatts at 35% efficiency is putting about 2.2 Gigawatts in the river around it. Often the ecology of water bodies around a power plant is permanently altered.
Radioactive decay processes usually do not use steam as a medium, but use either a thermocouple or a sterling engine to produce power from the heat generated. They are usually low power (hundreds or maximum of thousands of watts), and the trade off of small compact size makes up for their inefficiency.
Nuclear reactors do not rely on passive nuclear decay (well, not for 93% of their power), but rather active nuclear fusion. This produces many orders of magnitude of power more, usually measured in the hundreds of megawatt to gigawatt range.
There is not a more efficient way of capturing that power, if there were, then we would be using it.
Just as an interesting note, take a look at this diagram. That first turbine, the HP (high pressure) side, produces 2/3rds of the power delivered to the shaft, with those three giant low pressure turbines picking up the rest. We’ve optimized turning heat into electricity at those sorts of temperatures as much as possible, they are pretty close to maximum theoretical efficiency there.
As I said earlier, with different reactors running a higher temperature, then you can possibly take advantage of the brayton cycle. This is more efficient, and produces more power with a smaller turbine.
As a follow up to my comment above, one of the highest thermodynamic efficiency achievable (about 100%) is from sunlight since the source is at approximately infinite temperature. Even then the average solar panel operates at about 44% efficiency.
Like a wise man said, you can’t win, you can’t even break even and you can’t quit the game.
Nice thread/idea, and even better new fact I (and others, probably) never knew.
You say “has been used in some applications”: can you flesh that out (heh) regarding the actual use and technology of pacemakers nowadays?
It’s no longer used in pacemakers due to concerns about the radioactive stuff getting out of containment during cremation.
What did the Russians use in their nuclear-powered satellites?
There is a fundamental misapprehension here that “energy”—specifically the randomized kinetic energy or “heat”—that is the result of radioactive decay in a nuclear fission reactor is a physical thing that can be exchanged like apples or pork bellies. In reality, what is really being exchanged are states; that is, from a more orderly and lower temperature state to a higher and less ordered state from which a heat engine like a turbine can be used to convert the difference between states into useful mechanical work, such as driving a rotating shaft or providing propulsive thrust.
Nuclear fission of nuclear fuels (primarily uranium and plutonium, although other unstable fissile and fissionable isotopes such as thorium are possible) produce gamma radiation, high energy neutrons, and some daughter products which themselves will decay releasing neutron, neutrino/antineutrino, and ionizing (alpha, beta, gamma) radiation. In theory, the charged alpha (helium nucleus) and beta (electron or positron) radiation could be captured by an electrostatic grid of some kind and converted directly to electric charge or moving current, but the reality is that most of the energy released is in the form of high energy neutrons which have no charge, and the only way to efficiently convert their energy into useful work is by colliding them with some working fluid such as water or helium which then operates on a heat engine of some kind, generally through a heat exchanger into a steam turbine, but other applications, such as a nuclear thermal rocket or direct high temperature gas turbine using helium or carbon dioxide are possible. The other possible use is by using the neutrons to irradiate a blankets of fissionable isotopes which can release alphas and betas, but generally the yield is tiny compared to resulting neutrons.
Although radioisotope thermoelectric generators (RTG) are solid state devices that do not work on a working fluid heat engine principle, they are still heat producers and their conversion efficiency is pretty terrible compared to a theoretical Carnot thermal cycle, and they are used almost exclusively where simplicity and reliability over a long mission duration with no maintenance is critical beyond all other considerations. Most RTGs, such as the NASA Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) use [SUP]238[/SUP]Pu which is expensive and difficult to produce (and in fact availability and cost have been tentpole issues in NASA planetary exploration mission planning), although other isotopes are possible if mission durations are shorter. For comparisons, RTGs have thermal efficiencies of about 3 to 7 percent initially and decline with age, while nuclear fission power plants using a steam cycle have around 30 to 35 percent thermal efficiencies.
There are aneutronic nuclear fusion cycles such as the deuterium-[SUP]3[/SUP]helium and the proton-[SUP]11[/SUP]boron reactions which produce mostly charged particles (there is still a small fraction of neutron yield with p-[SUP]11[/SUP]B) but they require much higher triple product conditions and are of lower overall energetic yield. Given that we have yet to even be able of sustaining a normal D-D and D-T fusion reaction for long enough to get breakeven yield, getting a workable aneutronic fusion process without some kind of fundamental breakthroughs in fusion physics seems unlikely at this point. Even if it were, the efficiencies of yield through a series of electrostatic grids might still be less than a high efficiency heat engine process like a Brayton cycle and a high temperature fluid like helium, or some hypothetical solid state thermal conversion process.
A fission fragment rocket engine is still a thermal heat cycle, albeit one that is open loop; in this case, one that uses the working fluid as reaction mass. Although it is certainly more efficient than chemical rockets or nuclear pulse propulsion a la Project ORION, the cited propulsive efficiencies often stated in popular science articles are based upon extremely high temperature reactions requiring materials that are at or beyond the limits of material science and fail to account for dealing with residual waste heat (or otherwise would require gigantic heat exchangers and radiators). In any case, this is no more “get[ting] energy directly from nuclear reactors” than a closed steam cycle or other conventional heat engine.
Stranger
A few have alluded to the limit of efficiency for heat engines. Here it is: If your engine takes heat from a hot source and gets to release it to a cold sink whose absolute temperature is x% of the hot source temperature, then at least x% of your heat energy will remain heat energy and at most (100-x)% will have been converted into some other kind of energy. That’s what entropy is about. One nice thing about radioactive heat sources is that you can run them as hot as you like, as hot as you can hold onto whatever they’re made of, so you can get a high efficiency limit.
Various heat engines running under good conditions, like you’d design a power plant for, get pretty close to this theoretical limit.
I think you meant to say, “but rather critical fission” or “self-sustaining chain reaction”
Yeah, the biggest virtue to an RTG is that it has no moving parts (well, at least, no macroscopic ones), and so there’s very little that can go wrong with it. There aren’t very many man-made devices which can reliably run for fifty years with no maintenance whatsoever.