For those curious, this uses a nuclear reactor to power a Stirling engine.Stirling engines are a good choice because there is no mass transfer to create the work (i.e. no fuel needed, just heat). It only needs a temperature differential and sealed gas/liquid to work. I highly recommend to read more about Stirling engines since they are of one the most efficient form of work generation with just a temperature differential.
Stirling engines aren't that efficient but they are very reliable and capable of running for many years with zero maintenance. They compete with bimetallic solid state thermoelectric generators which also require no maintenance but are less efficient than Stirling engines and also degrade slowly over time.
If our civilization figures out how to turn heat into electricity with a high efficiency and a small solid state device, it changes the economies of so many things.
But it’s a hard problem, mostly because of “entropy”.
Seems like this would be limited in space applications by how much you could radiate away. Wouldn't you need to get rid of ~2x the desired output power? So to get 1kW you'd need to dissipate ~2kW?
The upper limit of any heat engine (turning heat into useful work) is called the Carnot efficiency and is limited by the temperature differential. It's usually not more than 70%, though for a Stirling engine, a few percent would be considered excellent efficiency.
A Stirling engine easily needs to radiate 20 times as much heat as it produces as power.
> for a Stirling engine, a few percent would be considered excellent efficiency.
Wikipedia says "Stirling engines have a high efficiency compared to internal combustion engines, being able to reach 50% efficiency," which seems to contradict that. What's the discrepancy?
Yeah, even thermoelectric piles (Peltier effect devices) can easily get ~~double digit efficiencies. Not sure what that guy is talking about~~.
Thermoelectric piles are basically the worst form of generating electric power with heat (it has 0 moving parts, so it's used in various places), and it's still not that bad.
edit: Never mind, apparently they are worse than what I thought they were, at 5~8%. But they are still the least efficient way to turn heat into electricity, and yet they can achieve what that guy said.
Yes. Though cooling sails are relatively feasible. Assume you can cool to ~250K: the back of my envelope and a Stefan-Boltzmann constant cribbed blindly from wikipedia tells me that such a sail would radiate ~200W/m^2, which is very doable for a spacecraft with a 1kW power budget.
Ormat does large Stirling engines for geothermal power. The binary units mean you don't run the risk of taking too much of the reservoir fluid. I wouldn't be surprised if tech like this ramps up for the same reason battery tech went nuts when people wanted longer life electronics - except the driver is climate change and carbon pricing.
Stirling engines are not efficient. Using that heat to make steam to turn a turbine is much more efficient. But it would be harder to make that portable.
You may be interested in reading Allan Organ’s books. He claims that aspirations to Carnot efficiency are a red herring and claims (with Much Math) that the peak efficiency is closer to 50% of The Carnot cycle.
They invented and are very close to commercializing a direct heat-to-electricity converter that uses acoustic waves. No moving parts. Close to 30% efficiency.
Turbines are only efficient at large size (see failed gas turbine cars), while Stirling engines are very dependent on construction details for efficiency.
Nope. It is easy to make an efficient heat engine if temperature differential is so huge that it is sufficient for a steam turbine. And efficiency of a turbine will be nowhere like Carnot efficiency. Stirling engines work on much lower temperature difference, providing efficiencies dead close to Carnot levels (which is still, on that temperature difference, quite low).
For those interested in learning more, I've found the Beyond NERVA blog to have really good articles! Their most recent post (and the 3rd in a series on Kilopower) [0] is on this set of tests, but there's also posts on more modern Nuclear Thermal Rocket designs [1] as you may have guessed from the blog's name.
I think someone should develop a 2 module design for this, where the Stirling engine is separated from the fission reactor with insulated pipes. Why? Weight savings. Insulated pipes can be made very efficient and relatively light. Radiation shielding can't easily be made light. By separating the modules into two sections, martian soil can be more easily used to provide the radiation shielding. It could even be built out of bricks, which we already know can be made out of martian soil.
That might be a good second phase setup for a larger base, but this current one would be good for initial setup since it could be ready without having to produce anything on planet. That makes it good for rovers or other unmanned setups.
They nailed all their milestones. This is tremendously important work with regards to a future outpost anywhere and possible outposts on Earth. (Imagine how much fuel you wouldn't have to ferry down to Antartica each year to prepare for winter operations, for example).
Now to ruggedize the heck out of it and make it easily transportable.
Conspicuously not mentioned: what kind of design the reactor uses. Thanks engadget, 10/10 reporting.
Based on the wikipedia article, it looks like a passive decay heater attached to a bunch of sterling engines. Pretty much a souped up RTG that trades more energy output for having moving parts. Neat.
It is not based on radioactive decay but on fission, the usage of Sterling engines is correct. [1]
[...] the core of the reactor will be of solid, cast uranium-235 surrounded by a beryllium oxide reflector. This reflector focuses neutron emissions and returns their energy back into the core to minimize nuclear gamma radiation [...].
Nuclear reaction control is provided by a single rod of boron carbide which is a neutron moderator that is initially fully inserted, [...]. Once the moderator is extracted the nuclear chain reaction will start but can not be stopped completely, although the depth of insertion provides a mechanism to adjust the heat output [...].
Passive heat pipes filled with liquid sodium then transfer the reactor core heat to one or more Stirling engines, which converts heat into rotary motion that drives a conventional electric generator.
It is self-regulating (load following) to some extent so it doesn't have many moving parts. It has two main control mechanisms: a neutron absorbing control rod and a Beryllium neutron reflector.
Passive decay is still fission, it's just not artificially sped up fission :)
it's probably more complex than that. Is high speed natural decay of an artificially created or enriched heavy element natural or artificial? I'm not sure how that gets classified.
Decay is not fission. Some time fraction of decays may be spontaneous fissions but as a rule generally nobody calls decays fission reactions unless they are specifically mentioning spontaneous fission reactions.
Artificial and natural is probably not the best terminology to use [0]. What is true is that this will almost certainly contain enrichment of one or more isotopes (NASA uses Pu-238 for some of their missions).
My bad, I had assumed fission was splitting atoms by adding neutrons, but actually fission is just the "splitting" part, so both cases are indeed fission....
The fission process releases neutrons, so you effectively "add neutrons" by concentrating self-fissioning material in a small space.
To put another way:
If you increase the size of a sphere of fissionable material, its rate of "natural" decay stays the same, but more of these decays lead to neutrons triggering additional fission reactions. So doubling the size of the sphere leads to >>2x fission events due to neutrons having more fissile material to hit on the way out.
A large fission reactor is just a scaled up version of this with control rods to adsorb extra neutrons to control secondary "induced" fission.
Some background on this: NASA deep space missions have historically used radioisotope thermal generators powered by the decay of the exotic plutonium isotope Pu-238. This isotope has a good balance of lifetime (87.7 year half life) and specific energy (0.5 watts/gram). It is non-fissile -- no risk of criticality. It also decays solely by alpha emission, so there are no problems with shielding the rest of the systems from e.g. gamma or neutron radiation.
American plutonium 238 was formerly produced using Savannah River Site reactors that primarily produced materials for nuclear weapons. With the retirement of those reactors in 1988, and the American nuclear weapons program going to maintenance mode, NASA lost the side-benefit of Pu-238 production using the weapons infrastructure. Deep space missions requiring RTGs had to subsist off of historical Pu-238 stockpiles and additional material purchased from Russia. But the Russian supply has run out too now -- apparently they aren't producing more of it either.
American plutonium 238 production efforts resumed in 2013:
With NASA now paying the full cost, including fixed startup costs, Pu-238 is extraordinarily expensive:
NASA and DOE have estimated the rebooting will cost between $75 million and $90 million over five years. According to NASA officials, the agency expects to have 1.5 to 2 kilograms produced per year, starting 2018.
The high cost and limited supplies of Pu-238 have spurred the search for alternatives to Pu-238 RTGs. A few years ago I remember reading that the European Space Agency was going to try to design its own RTGs using the less powerful but more abundant americium 241 instead of plutonium 238. But a quick search just now doesn't show any concrete development effort.
This Kilopower reactor is an alternative to RTGs for some mission profiles -- in fact offers more power than RTGs and uses cheaper nuclear materials. (Highly enriched U-235 isn't cheap in an absolute sense, but it's far less expensive than Pu-238. And the security of supply is effectively backstopped by its use in reactors for the US Navy.) It can offer ample power for very deep space missions at a cost significantly less than using years' worth of Pu-238 production.
Improved photovoltaic cells have also enabled missions further from the Sun than they could have supported in 1988. The Juno mission, which entered Jupiter orbit in 2016, relies on PV instead of RTGs. It seems plausible that further evolution will eventually push PV's reach out to Saturn missions. But PV is not currently plausible for missions beyond Jupiter, and it is reliant on slowly-evolving battery technology for surface missions on Mars. Martian missions using PV also face significant problems from cell-obscuring dust. This reactor seems too large to be conveniently integrated in a Martian rover, but enabling non-surface missions to avoid Pu-238 use may mean more can be reserved for future rovers akin to the RTG-powered Curiosity.
The Kilopower reactor presents far lower radiological risks than RTGs in the event of complete disassembly within Earth's environment.
Pu-238 has a specific activity of 634 billion Bq/g, nearly 8 million times that of the Kilopower reactor's U-235 fuel (80 thousand Bq/g). Once the reactor attains criticality, it produces fission products that have even higher specific activity than Pu-238. But the reactor can remain safely inactive until the risky launch phase is over. There is no way to inactivate the decay of Pu-238 for the launch phase.
The risks of RTG launch are handled by using designs with high mechanical/thermal robustness to encapsulate the plutonium ceramic. I think that they were already safe enough. But the Kilopower reactor is inherently low-radiotoxicity before criticality, which makes it safer yet during the launch phase.
To simplify the other more detailed response, It's just Uranium at that point. Even highly enriched Uranium isn't that radioactive by itself, it's once you start operating the reactor that it starts generating highly radioactive waste.
The new fuel rods going into it are totally safe from a radioactivity perspective. Here's a picture of a man holding a fuel rod bundle with nothing more than gloves on. http://nuclearstreet.com/images/img/dw037.jpg
One can't help but notice from his shirt and his hair that the picture was taken in the 1970s. Lots of awful practices still existed then; who's to say this isn't a picture of that?
At all stages the material can be handled with no more protection than gloves. It is roughly as dangerous as handling lead fishing weights until the fuel actually attains criticality.
I don't know why people downvoted this, it's entirely correct. Although I guess the larger concern than the radiation would be the heavy metal poisoning but in any case it's still fine, just treat it like you would any other heavy metal.
The Soviets launched over 30 satellites with nuclear reactors in the past. The US also launched one. Most of them are still in orbit and I only know of one that scattered nuclear waste all over Canada.
Edit: it looks like there was one launch failure that ended in the reactor dropping into the ocean, and one end-of-life failure that also resulted in the reactor dropping into the ocean. However, normal procedure was to decommission them by boosting into a higher orbit, which means debris and radiation from them has been an ongoing problem for other satellites - i.e. radioactive droplets of sodium coolant.
Funny you should use the phrase "black helicopter." During the Cold War, the military used tiny nuclear reactors to power automated remote sensing stations in the Arctic. We called them "Power Pigs" and they were delivered by helicopter to very remote places.
They were fascinating little devices, and because of where they were placed, used the outside air/ice/snow/rocks for cooling.
Don't know if they are still being used, or what happened to the old ones, but they were very clever technology.
I guess these are the Strontium-90 RTGs made by the Soviets. There's about 1000 out there still - some have been vandalised, some washed out to sea, some sitting rusting in a lighthouse outhouse etc.
http://bellona.org/news/nuclear-issues/radioactive-waste-and...
I am guessing that you are talking about betavoltaics [1]. They were used to power pacemakers and are very useful for remote devices. They can't power much, but they are smaller than PV and don't have to be exposed to the sun (reflective).
Though there are larger devices that were used too.
They are not safe. They lack their own biological shield, and thus require the core section to be buried underground, as can be seen in various demo images. Even then, I suspect you wouldn't want to be too close while operating. They also lack other safety systems a commercial nuclear power station reactor has, having only a single control rod. This makes sense for an extraterrestrial reactor, as transporting mass to Mars is very expensive.
>They also lack other safety systems a commercial nuclear power station reactor has, having only a single control rod.
They also have a very strong negative thermal reactivity coefficient, meaning that as the reactor gets hotter it produces much less power. [0] From what I understand, with any reasonable amount of cooling this reactor won't melt down even with the control rod entirely removed, where "reasonable amount of cooling" means "not left to float in a vacuum with no radiators". (Also, if you remove the neutron reflector it'll stop working)
Granted, you'd probably want a bit more shielding before putting one in your basement. But it's not going to melt down.
I wonder, can you make a stirling with plutonium pentafluoride working fluid? Eg, cold chamber with neutron poison, hot chamber out of a neutron moderator material?
The free piston stirling engines required also operate via magic and few people understand them well enough to make one suitable for this purpose. The engines required for a reactor like this haven’t been designed yet and it would take years of effort before They would be designed and approved for flight use.
For this test they used a couple of undersized (<100W) stirling engines and the rest were just thermal simulators.
Stirling Engines exceed their competition by percentage points. Nuclear energy exceeds its competition by orders of magnitude. I stand by my statement that the magic comes from the nuclear half of this relationship -- so long as we accept a definition of magic related to, you know, energy production, rather than effort or hipster points.
How do they manage to not have the reciprocating mass (the piston) throw off a space vehicle's trajectory? IIRC the Space Shuttle used magnetic tape because a hard drive would have acted as a gyroscope.
Keep in mind that in the space shuttle era hard drives were a good deal more massive than today. Presumably a modern hard drive wouldn't present much of a problem.
In any case a rotating mass isn't too big of a problem, it's changes in rotational speed that'll mess with your heading -- not your trajectory though. The piston itself shouldn't be too much of a problem either because any motion it makes one way it shortly after makes the other way balancing out the forces over time.
can this be used for car/boat/etc-fuel somehow one day, then all gas-station will be run out of business and middle-east-plus-texas-oil-field all file for chapter-11?
There's a few problems with making Highly-Enriched Uranium commercially available, even before we get to radiation.
Thorium reactors [1], on the other hand, might be more feasible due to their lack of weaponization potential, cheaper fuel, and reduced waste. However, Thorium reactors are much more complex, and will likely never approach the small scale of KiloPower; they also have significant startup costs, remain commercially unproven, and have a more dangerous fuel cycle (in terms of radioactivity).
Something that's often left out in the discussion of nuclear because it's not an issue yet, is that nuclear fissile materials are also rare. If nuclear became the standard for energy production, the rarity of these materials would, in the longrun, end up creating more stupid games similar to what we play with fossil fuels today.
'Time do bring democracy to Australia!'
'Wait guys... what?? We are a democracy!!'
'Sorry, can't hear you over the sounds of FREEDOM coming your way!'
It's usually left out of the discussion because it really is a non-issue long term as well. With breeder reactors (technology which already exists), Th in addition to U, U/Th extraction from seawater, we have enough fission fuel to power the entire planet at current levels for tens of thousands of years, if not indefinitely.
That's vastly understating the amount of Uranium and Thorium we can access. Without a breeder reactor it might only be tens of thousands but with a breeder reactor the amount of material we could use goes up by several orders of magnitude.
You can't just handwave away the economics or practical issues here. Seawater extraction is incredibly expensive. Breeder reactors are also expensive, ignoring the countless other technical and practical issues that they come with. For instance the output level for optimal efficiency with breeder reactors is quite low. You'd have literally thousands of breeder reactors spread around (with a proportional number of fuel reprocessing plants) when you run into the practical problem that, while failure rates are low, with the number of facilities scaled up by orders of magnitude they are currently intolerable due to the consequence of failure.
But back to cost your argument is like saying that "Bah, there's no concern about a lack of freshwater. We can just desalinate the sea!" You sure can, but that's completely irrelevant as the costs make it an unreasonable solution. Using conventionally obtained and utilized material, we only have enough nuclear material for about 200 years of consumption. [1] And in looking at the longrun it's obviously quite myopic to take as an assumption that humanity has reached the highest energy usage it will ever have.
> For example, cutting off the system's cooling or ramping it up to its maximum level both resulted in a core temperature change of just 15 degrees or less.
You can't extract energy from a heat engine without cooling. Useful work is done in the process of moving thermal energy from a high temperature location to a low temperature location.
Stirling engines drive because of a temperature differential. If you don't cool then those two temperatures will tend to equalize, becoming less efficient as the entire device warms up.
What would the effects be of releasing nuclear waste in space? Could we sent up all of our nuclear waste, put a rocket on it and just let it disappear into the distance?
I think the drawback for space disposal is the risk of rocket failure. If a rocket explodes with nuclear waste on board, the waste would spread over a large area. It's safer at the moment to consolidate nuclear waste in small areas away from civilization.
Think what the argument against it is that the rocket explodes during launch, before actually leaving earth behind. Then it would most probably be noticeable.
In the 1950s they thought that Nevada was very big and that the waste wouldn't even be noticeable.
They also thought the Pacific Ocean was very big and that the waste wouldn't even be noticeable.
Car drivers still think this way with their toxic emissions. It is very human to think 'out of sight out of mind' rather than 'take only photos, leave only footprints', so maybe it is actually fine to trash space with waste.
This... isn't really relevant for space. The amount of emptiness so massively outweighs the quantity of matter and energy it would be literally impossible to utilize a nontrivial portion of the volume available.
The only case in which releasing waste into space is problematic is when it's within a highly-utilized area, for example LEO. In the future, we may potentially use various transfer orbits and/or the Interplanetary Transport Network [1], so we should presumably avoid dumping there, but even within the solar system, the sheer volume of vacuum dwarfs any possible use.
You would need a huge fleet of huge rockets to do that. Of the order of 50 Saturn V - sized rockets per year just to keep up with the current production of nuclear waste. And about 1000 of them to get rid of what we accumulated so far. Only 13 Saturn V’s have ever been launched.
NASA and the DOE did a study on this and they determined that it was too expensive to do it, though safe enough but that we might want the waste later. Vitrification and storage was deemed cheaper, safer and more 'reversible'.
I think that's just short term thinking: When the sun expands into a red giant, the gas drag will slow down the moon. I think that means it will crash into the earth if it's not removed before.
Regardless of whether or not what you say is likely to happen (I think not, but anyways), if humans can even last long enough to see the Sun as a red giant, we'll have larger problems on our hands. In any case, we'll likely have destroyed ourselves or evolved into something else by that time.
The Moon orbits the Sun, and is only mildly perturbed by the Earth. Gas drag would decay the Moon's orbit, but it's not at all clear to me it would crash into the Earth. It seems far more likely it would simply fall into the Sun.
The Earth and Moon system orbit the Sun, but the Moon is very much in orbit around the Earth.
A more likely scenario is that the Earth's orbit will move outwards as the Sun expands, since stars in the red giant phase(s) lose a lot of mass. It's plausible that the Earth/Moon will not be destroyed, physically. But in that case all life would have been obliterated due to the immense temperatures associated with having the surface of the Sun within a few million miles (or less) of Earth.
On resiliency “cutting off the system's cooling or ramping it up to its maximum level both resulted in a core temperature change of just 15 degrees or less”
Umm. . . it probably wouldn't be cost effective for you: the NRC is pretty brutal in it's regulations. I'm very gun-ho on nuclear power, but that is one bureaucracy I don't mind charging ~$4-500/hr for people to review submitted designs for approval and then painfully inspecting operating plants.
Would you mind backing up the assertion about convection being the mechanism of sodium transport? e.g. on earth are you saying the sterling part wouldn't work if the hot fluid had to traverse say down (a gravity gradient) instead of up?
The wiki page you linked specifically mentions deep space applications as a design goal.
I'm not sure about the sodium heat pipes but the alcohol vapor heat pipes you see in electronics typically use an internal wick to return the working fluid.
They should team up with Tesla to develop storage batteries for the reactors in case a reactor fails you would still have some temporary emergency power.
Tesla doesn't make the batteries, Panasonic, Samsung and Hitachi does. IIRC, Panasonic is the firm building batteries at Tesla's new gigafactory facility. Tesla simply slaps their name on the battery packs.
plenty of companies already make batteries for space applications. and the environment is demanding enough, the budget large enough, and the production volume low enough that more exotic battery chemistries are used. https://en.wikipedia.org/wiki/Batteries_in_space
