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NASA to test prototype Kilopower nuclear reactor
- nickhalfasleep 9y agoThermo-acoustic stirling engines have long been a niche product. Seems like a great environment for them to shine, and far better efficiency than RTG's.
- jessriedel 9y agoWhat's the advantage over direct thermoelectric conversion?
- nickhalfasleep 9y agoAn RTG uses the Seebeck effect, which gets you microvolts per degree kelvin. A Stirling engine can theoretically extract many times more energy from the same temperature differential, but with a larger complexity.
- jessriedel 9y agoActually, when I mentioned thermoelectric conversion I wasn't referring to an RTG (which is just nuclear decay) but rather to the true nuclear reactors that the Soviets put in space: https://en.wikipedia.org/wiki/Romashka_reactor https://en.wikipedia.org/wiki/Romashka_reactor https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor They used thermoelectric conversion rather than a Sterling engine with moving parts (although I guess TOPAZ is technically "thermionc coversion").
- curtis 9y ago> What's the advantage over direct thermoelectric conversion? Stirling engines can approach 50% efficiency [1], whereas thermocouples are usually less than 10% [2]. [1] https://en.wikipedia.org/wiki/Stirling_engine https://en.wikipedia.org/wiki/Stirling_engine [2] https://en.wikipedia.org/wiki/Thermoelectric_generator#Efficiency https://en.wikipedia.org/wiki/Thermoelectric_generator#Effic...
- jessriedel 9y agoCool, thanks! It's really surprising to me that inserting a mechanical intermediary has such a large efficiency advantage.
- brians 9y agoWhy does mechanical stuff suck? Because of friction of moving parts. Too much energy is lost to heat. For a thermal engine, if you can arrange to put the moving parts mostly on the hot side, that’s no longer loss.
- deleted 9y ago[deleted]
- Groxx 9y agoHuh, interesting device: https://en.wikipedia.org/wiki/Thermoacoustic_heat_engine https://en.wikipedia.org/wiki/Thermoacoustic_heat_engine Is this actually a thermo-acoustic engine, or some other kind of sterling engine? Or are they equivalent? The article doesn't seem to mention "acoustic" (or an equivalent).
- baybal2 9y agoAnother thing to mention is sodium/lithium salt thermochemical generators that run at 15 percent efficiency with liquid salt being the only moving part
- curtis 9y agoThis is similar to the "Stirling radioisotope generator" [1] but it is an actual nuclear reactor (albeit a small one). Both systems are a response to the same fundamental problem, the dwindling supply of Plutonium-238 [2]. The Stirling radioisotope generator uses Pu-238 more efficiently than thermoelectric RTGs, and the nuclear reactor from the article dispenses with Pu-238 altogether. [1] https://en.wikipedia.org/wiki/Stirling_radioisotope_generator https://en.wikipedia.org/wiki/Stirling_radioisotope_generato... [2] https://en.wikipedia.org/wiki/Plutonium-238 https://en.wikipedia.org/wiki/Plutonium-238
- gozur88 9y agoThe other problem with RTGs is they scale very badly.
- Faaak 9y agohow ?
- andygates 9y agoThey're very low power, make a lot of waste low-grade heat, and use an inefficient electricity generation approach.
- masklinn 9y ago> make a lot of waste low-grade heat That's probably the biggest issue for deep space it's extremely difficult to shed heat in hard vacuum (without ejecting mass and its heat with it) as you can only radiate it away.
- acidburnNSA 9y agoAnd radiative heat transfer scaled with temperature to the fourth power so it would be easy to do if you had materials that could handle really high temperatures for long times.
- GlenTheMachine 9y agoThis is a very cool technology, but frankly I have a hard time imagining that it will see much use. NASA sees public protests and scare mongering whenever it tries to fly an RTG, which is based in the radioactive decay of (usually) plutonium, and does not use a nuclear chain reaction. They did fly one on Mars Science Lab, because it was the only way to get enough power for the rover. But it takes a PR hit every time it does so. RTGs are therefore only used when no other technology will work. It think it's likely that the public relations nightmare NASA would have to go through to fly an actual reactor will be through the roof.
- JumpCrisscross 9y agoHas anyone studied these protesters? My off-the-cuff sense is they’re mostly older environmentalists.
- sp332 9y agoIt's because sometimes a rocket explodes on the launch pad or a few seconds later, and it could spread radiactive material over a very wide area.
- ethbro 9y agoThe alternatives still seem worse.
- rwcarlsen 9y agoThe half-life of fuel for these reactors is very long - over 100,000 years. So for all practical biological effects they are basically stable. If it exploded before it was activated (far into space), it would be about the same danger to people and the environment as the rest of the exploding metal the rocket was made of.
- bildung 9y agoThe half-life of the currently used fuel (Plutonium 238) is 87 years.
- giarc 9y agoAnyone know the approximate size of that thing? Hard to tell from the picture.
- GlenTheMachine 9y agoHard to tell what the weight is, of course. Judging from the size of other objects in the picture (the lift point and the connectors) it's very roughly around four cubic feet. Which is actually pretty awesome; the equivalent solar panels would be significantly larger.
- red75prime 9y agoHeat sink for space operation will be quite large though.
- jacquesm 9y agoAccording to a NASA pdf the thing weighs 1150 Kg or thereabouts. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170002010.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201700...
- dgoodell 9y agoI saw a full scale prototype/model in the Stirling lab. I think a strong person could have lifted it up. It’s just several lbs of uranium with several heat pipes coming out of it connected to Stirling engines. I don’t think they have enough actual Stirling engines because the ASRG project to make them was cancelled so I think they use simulators for the heat load.
- jimmcslim 9y agoStrap it to an EmDrive and head for the stars!
- yummybear 9y agoI thought one of the advantages of the normal RTG is that it is non-mechanical. I would think a mechanical engine has a lot higher fail rates?
- wefarrell 9y agoThe problem with normal RTGs is they're dependent on Pu 238 which is in short supply.
- ChuckMcM 9y agoOkay, this is awesome. 10kW is a serious enough amount of energy to do some fun stuff. Things like rovers that move at km/h rather than m/h and all day operation. It also solves the 'base load' problem on deep space missions which would like to use more power for radio communications.
- hutzlibu 9y ago"It also solves the 'base load' problem on deep space missions which would like to use more power for radio communications." Not really, since "moving parts" ...
- dgoodell 9y agoOne of the primary uses are deep space probes. The stirling engine has exactly two moving parts and uses non contact seals. In theory the lifetime should be limited by the deformation of the piston at elevated temperatures. Still, you’re right. They certainly have to do a lot of reliability testing to be able to show that these stirling engines will last 10+ years before they use them in a mission. They have several stirling engines that have been running for multiple years in simulated conditions for just this purpose at NASA GRC.
- hutzlibu 9y agoBut on deep space missions, aren't you aiming for 50+ years, or ideally potentially unlimited if you want to hit alpha centauri? Voyager 1 is going for 50 years ...
- ChuckMcM 9y agoYour point about moving parts is good, it is absolutely the place to look for possible failure. That said, it is also possible to build such things in a durable way. For example, at the science museum in London there was a display with a Stirling engine that over 75 years old and still works fine. I expect the challenge will be to keep the materials within the range of temperatures and pressures that do not cause structural change in the material.
- tobyhinloopen 9y agoput it in a car
- gambiting 9y ago1kW isn't driving you anywhere though(even if we ignore all the dangers of having a nuclear reactor in a car). A typical automotive engine produces 100-300kW.
- rijoja 9y agoBut it would be perfect for a well isolated home wouldn't it?
- gambiting 9y agoHmmmm a normal boiler for a 4-person family home is about 8-10kW so again, probably not. At least not for heating. But for normal power consumption should be ok, especially if paired with some storage device to account of periods of higher consumption(running a 3kW kettle for couple minutes for example).
- rijoja 9y agoWell yeah not the average household obviously but if you really wanted to get rid of your dependency on the grid it would be somewhat possible. As opposed to say drive a at least 70 HP ~ 50 KW car. Not a proposal for everybody of course but for lets say political idealists. As far as the 3kW kettle is concerned having a battery to provide for these surges is entirely viable.
- maxerickson 9y agoHighway speed power draw is closer to 25 kW, so a small scale up (they also discuss 10 kW) and you have a self charging battery. But apparently the 10 kW is modeled to weigh about 1800 kg, so no need to worry about one in a car any time soon.
- GuB-42 9y ago
- jlebrech 9y agothe cold of space and the heat of a nuclear reactor would be the perfect combination for a peltier reaction. not very good for a rover though.
- wiredfool 9y agoSpace is cold, but there's not much there, so your flux will be very small. Of the three ways to move heat (radiation, convection, and conduction) you're basically reduced to radiation, which is the least efficient.
- gruturo 9y agocold of space? Unfortunately, no - space is an extremely effective insulator, so it's extremely difficult to get rid of heat. You need large heat sinks (which adds weight) and they have piss-poor efficiency in space. So your cold side would get hot very quickly
- jlebrech 9y agoare you saying it would be better on a planet with an atmosphere to use as heat conduction medium?
- hutzlibu 9y agoMy first thought was "deep space mission". But because of moving parts involved, this is probably not the use-case. So a bit OT, I was wondering if anyone has yet thought about the solution of still using solar panels for them, but to also use a mirror to focus more light in the direction of the probe or a laserbeam? I mean, theoretical I don't see why not, appart from being more expensive? Andd you could also offset the laser/mirror cost, because you only need them later on .... (but to be on the safer side, I still would add a RTG)
- monob 9y agoInverse square law. Every n AU you go out from the earth you will need n2 are of mirror to focus on a solar panel to get the same power. That assumes you can even build a structure that can keep that shape. Say you have a mission to Uranus, you will need a mirror with 400 times the area of the solar cells to get as much power as they would in earth orbit.
- hutzlibu 9y agoHm, ok the mirror is probably out of the question then. But if you use a laser? I imagine it will be easier to focus it more precisely?
- philjohn 9y agoSaw a documentary recently and they're doing just this.
- contact_fusion 9y agoLasers are still subject to the diffraction limit, regardless of the quality of focus. We can examine this limit in the context of delivering energy to a remote object. This analysis will be simple - we assume that the optics are perfectly aligned (dubious - pointing is difficult); we assume nothing about the absorption properties of the object, which will necessarily need to be very high efficiency. The angular size T of a laser beam of wavelength L emitted by an aperture of diameter D_a is roughly L/D_a. Similarly, using the small angle approximation, this angular size T at the object itself will be the width of the beam, W, divided by the distance between the object and the aperture, D_o: T = W/D_o. Ultimately, this gives us the width: W = L*(D_o/D_a). What does this tell us about the practicality of such a system? Visible light, wavelength roughly 500 nm, is perhaps a solid guess for a real system. Realistically the aperture size is probably limited to about 10 meters, but we can go even further and assume a synthesized aperture of a realistic system being 100 meters. You would want to get all of your beam for power transmission, so lets assume an upper limit for the beam width at the object to be 100 meters as well - probably unrealistic, but maybe solar sail/ultralight absorbers could get there. Throwing these numbers in gives a maximum range of... 2x10^10 meters. This is roughly a tenth of an AU. In comparison, this is about 50 Earth-Moon distances... and only a quarter of the distance between Earth and Mars at their closest approach. Coincidentally, this is also about one light-minute. Any real power delivery system, using current tech and without assuming convenient fictions, will have a much more limited range. In short, lasers are not really perfect rays, even though they are approximately so over scales we typically encounter; at astronomical scale, diffraction always wins. This is why it is usually way better to bring the power with you - especially as you lose solar irradiance as you get further from the Sun. And for bringing power with you, nothing beats nuclear for energy density.
- iliis 9y agoMore details in this german article: https://www.golem.de/news/kilopower-ein-kernreaktor-fuer-raumsonden-1712-131418.html https://www.golem.de/news/kilopower-ein-kernreaktor-fuer-rau... I'll summarize some of the interesting points: - The nuclear core (75kg of enriched uranium/molybdenum [1]) is designed to not go critical, even if it accidentally falls into the sea and is surrounded by water (which is a good neutron reflector). It only starts when you surround it with a neutron reflector made of beryllium (an even better neutron reflector, mainly due to less absorbtion). Combined with the fact that the reactor only gets nasty when it's been running for a while (and thus is already far away from earth) it is a lot safer than plutonium fueled RTGs. - It would be very useful to reach far away destinations (like the orbit of Uranus, Neptun or Pluto) using ion drives, as they need to run for years and solar panels aren't effective far away from the sun. - While there have been other attempts at developing nuclear reactors for space, most of them didn't go far. They could use an existing research reactor (Flattop [2]) for this project which already has all the required permissions to run, so a lot of paperwork could be saved for the Kilopower experiments. - The Kilopower reactor is the first to use heatpipes instead of pumps for the heat transport and stirling engines for the energy generation. The first experiment was thus to show that the cyclic heat draw of the stirling engine would be safe, because usually nuclear reactors reach an equilibrium between heating up (and thus expanding slighty which slows down the reaction) and cooling down (which accelerates the reaction). - Instead of the planned eight 125W Stirling engines, they're currently using two 70W ones from the Advanced Stirling Converter Project [3]. The other ones will be simulated using simple heatsinks. - Theoretically it could run for hundreds of years (after 500 years less than 1% of the uranium will be used), but the Stirling engines will break much sooner than that. [1] http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/33/034/33034319.pdf http://www.iaea.org/inis/collection/NCLCollectionStore/_Publ... [2] https://en.wikipedia.org/wiki/Flattop_(critical_assembly) https://en.wikipedia.org/wiki/Flattop_(critical_assembly) [3] https://tec.grc.nasa.gov/rps/stirling-research-lab/advanced-stirling-convertor/ https://tec.grc.nasa.gov/rps/stirling-research-lab/advanced-...
- fsloth 9y agoI first read 'NASA to test KILLpower nuclear reactor' and thought are they starting to use Space X naming conventions...
- Gravityloss 9y agoFrom https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160012354.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201600... 2-5 W / kg. Solar cells seem to be about 150 W/kg. This is relevant for outer system exploration (beyond Jupiter) or maybe for night power on planetary / moon surfaces.