Ultimate power source? - Pebble-bed reactors

Rxke · 2004-09-08 22:55:04

http://www.wired.com/wired/archive/12.0 … ad_7]Wired

Being discussed in the 'China...' topic, but I want it seeing discussed in relation with manned missions.

The pebble bed reactors have a lot of going or them:

-cheap, reliable, lightweight, very hassle-free, waste is only mildly radioactive etc..

IMO, it should be looked into for manned missions, the power/weight ratio is really good, if you run out of fuel, just send up another sack of pebbles... You won't have sleepless nights mulling the possibility of a meltdown... And if you're able to make Portland cement, you can even make the package to launch lighther than it already is...

Oh, and it's an excellent power-source to generate hydrogen, to boot...

Austin Stanley · 2004-09-09 04:25:58

Good article, I normaly don't like wired's reporting but this one was pretty decent.

However, I am uncertian as to if a Pebble bed reactor is optimal for an early mars mission. As this reactor has to be sent up from earth, it needs to be pretty weight efficent in terms of kW/kg. I have a feeling that a conventional water cooled reactor is going to beat it here.

In terms of safety I don't see a pebble bed or other gas-cooled reactors are going to have much of an edge either. Since the martian air is unbreathable and the planet is already bathed in higher than normal radiation a meltdown is of little concurn. The biggest worry with any reactor malfunction is going to be loss of necessary electrical power. I don't see how a pebble bed reactor is going to be any more reliable than a conventional one.

OTOH, the thing will probably be easier to fix if it doesn't blow-up in the first place, but modern well designed PWR (pressurised water reactors) don't do that in the first place. Worst case scenario, the vent radioactive steam, which is not a scary issue in the martin enviroment. As a plus, it would probably be easier to replace the vented water then it would be vented helium.

Just some thoughts.

GCNRevenger · 2004-09-09 17:54:55

There won't be any water-cooled reactors sent to space any which way... the name of the game now is fast neutron reactors cooled by liquid metals, usually Sodium or Potassium, sometimes Cesium. The reason being, you can make a very very compact reactor this way. The liquid metal then passes its heat on to a less efficenct but light & reliable thermoionic converter or a more efficenct but complex gas/turbine loop.

Austin Stanley · 2004-09-10 11:38:35

Ahh well whatever is most efficent in terms of kW/Kg, be it PBR, PWR, BWR, whatever. In terms of safety and most other considerations they will all probably come out about the same for any non-terrestial enviroment.

A more intresting question to me is what sort of reactor would be simplest to build on mars. I have a feeling that it will be some sort of water cooled operation if only because the cooling medium will be easy to come buy and replace. I also tend to think that convential fuel rods will be easier to construct on mars rather than fuel pellets a PBR uses.

Any ideas where I can find some more good info on other liqued metal cooled reactors? I know the russians used them some (I think they even put some in there submarines), but thats about it.

RobS · 2004-09-10 14:13:30

At the Mars Society annual meeting an MIT team gave a presentation. They were proposing a pebblebed reactor for the Martian surface that used carbon dioxide air as coolant, because it was free and could be exhausted straight back into the environment. If radioactive containment were an issue, the reactor core could be cooled by a closed CO2 loop and that could be cooled by an open CO2 loop. That was their advice. I wouldn't dismiss the danger of radioactive contamination at a Mars or moon base. Radioactive dust could still find its way inside on spacesuits.

-- RobS

Austin Stanley · 2004-09-10 23:03:17

Using CO2 as a coolent would fix the problem of aquiring coolent, but as it is vuneriable to changing into various radioactive isotopes, a secoundary cooling loop would probably be required. Or perhaps the CO2 could be vented frequently and replaced, although I am unsure as to how this contanimation might effect the mission.

I had also not considered radioactive dust inflitrating it's way back into the closed enviroment. I suppose this would be possible. Of course dust control is going to be a major issue in any event. some method of controling it will have to be found.

This would only be an issue if the reactor had a major meltdown. Which is possible, but unlikely for PWR and other types (they would need to both completely lose control of the reaction and lose coolent. More likely is some sort of coolent leak, in which case water is superior as it would quickly freeze and would be unlikely to find its way into the closed life-support system. Radioactive CO2 might pose a small risk of inflitration, but probably not a major one. It might complicate some of the plans for using the nearby martian air for fuel and such applications though.

However, even with the threat of nuclear contamination, I still think the biggest concurn would be loss of power. An early mission would be hard pressed to find juice for life-support without their nuclear reactors. And lack of air and heat would kill long before minor radiation contamination would. I'm not trying to downplay the threats posed by a possible nuclear accident, just saying that when added to the loss of power and life-support, radiation is a secoundary concurn on the martian enviroment. Any reactor will have to be reliable first and lightweight secound. The decision should not be made on the basis of which reactor has the potential to meltdown, but on which one is overall the most reliable and lightest.

Rxke · 2004-09-11 05:36:36

Wouldn't the use of CO2 reduce the efficiency?

Of course, using CO2 has the potential to make the design simpler, so probably a good choice after all?

Austin, good point, it will take a while to get good, objective numbers about the reliability of the PBR, i guess..

John Creighton · 2004-09-11 10:34:56

Wouldn't the use of CO2 reduce the efficiency?

Over what coolant and why? I assume at pressures at or bellow a one bar I assume CO2 would behave as an ideal gas and the main differences between the gasses would be their heat capacity. The heat capacity would be related to the gas constant. The main efficiency loose I see is the need for a compressor. Or would it be possible to build a condenser for CO2 on mars given its low temperature. I assume the pressure would have to be fairly high for the CO2 to liquefy. And if you had a condenser can you build a pump to pump liquid CO2. If you could build such a pump how reliable would it be.

GCNRevenger · 2004-09-11 13:07:41

At the Mars Society annual meeting an MIT team gave a presentation. They were proposing a pebblebed reactor for the Martian surface that used carbon dioxide air as coolant, because it was free and could be exhausted straight back into the environment.

This, I think, is a case where MarSoc has picked a technology we need on Mars not because it is the best technology, but rather because it uses some Martian reasource just for the sake of using a Martian reasource.

A nuclear reactor is not somthing you want to be doing anything with on Mars, it ought to be a ready-made complete power system where you just plug in your wires & hoses and push "activate" on the control computer. Adding the coolant, which is a smaller portion of the devices' mass, is not a simple task and isn't worth the weight savings compared to bringing the ready-to-use reactor from Earth.

A pebble-bed reactor is awfully bulky and heavy for an initial Mars power plant, a pressurized water system isn't all that efficent per-watt and has a fairly heavy cooling system, but a liquid metal reactor cooled by Helium/Argon can be built very compact and pretty light while retaining excelent thermal efficency.

Carbon dioxide used as a coolant has a number of issues, such as it will liquify under pressure if it gets too cold and solidify if you lose pressure and temperture, which might make startup difficult. It will also limit the operating temperatures, as the gas may simply decompose if you get too hot, which would ruin the moving parts. Helium or Argon have very high specific heats too, which makes them more efficent, and remain a gas over any concieveable operating range. Helium is, as you know, pretty light weight too nor can it become radioactive.

John Creighton · 2004-09-11 13:12:01

At the Mars Society annual meeting an MIT team gave a presentation. They were proposing a pebblebed reactor for the Martian surface that used carbon dioxide air as coolant, because it was free and could be exhausted straight back into the environment.
This, I think, is a case where MarSoc has picked a technology we need on Mars not because it is the best technology, but rather because it uses some Martian reasource just for the sake of using a Martian resource.

I think the idea is to more the just use Martian resources for the coolant. Anyway, if you are only using mars resources for the coolant then sure, bring the whole system from earth.

Grypd · 2004-09-11 17:15:12

We wont have that many people to keep an eye on any energy production gear so if we want a good safety measure we ought to ensure that the system we do use is a bit idiot proof. And it does seem that a pebble bed reactor has that function over a sophisticated liquid metal cooled system.

GCNRevenger · 2004-09-11 21:10:42

On the contrary, a liquid metal cooled system could be even more trouble-free than a pebble bed. The only big advantage to the pebble bed system is its safety, that even with a catastrophic coolant loss, reactor vessel damage, or even fuel element damage that the radiation release would be minimal. Thats really about it, as if the thing leaks, it will stop producing power just as readily as other reactors... so, the Mars crew freezes to death. The radiation release risk is simply not that big a factor. The reactor is pretty efficenct, but it is also bulky and heavy per-watt because of its safety-minded fuel elements and all-gas cooling.

But a liquid metal cooled fast neutron reactor, you can do a neat trick: A hollow control rod fixed in place is filled with Lithium-6 and inert gas in a configuration much like a glass thermometer. When the reactor heats up, the Lithium expands and moves up the tube, blocking more neutrons and slowing the reaction. Thusly, you have a reactor which is self-regulating without control, power, or solid moving parts that is also more compact then other configurations.

Austin Stanley · 2004-09-12 04:54:28

Wouldn't the use of CO2 reduce the efficiency?
Over what coolant and why? I assume at pressures at or bellow a one bar I assume CO2 would behave as an ideal gas and the main differences between the gasses would be their heat capacity. The heat capacity would be related to the gas constant. The main efficiency loose I see is the need for a compressor. Or would it be possible to build a condenser for CO2 on mars given its low temperature. I assume the pressure would have to be fairly high for the CO2 to liquefy. And if you had a condenser can you build a pump to pump liquid CO2. If you could build such a pump how reliable would it be.

The only real problem I see with using CO2 as a coolent is it's potential to transfmutate into radioactive isotopes. If it were to leak back into the atmosphere, or even worse into the life-support systems, that might be an issue, but on the whole, a rather low-priority one. Of course, the same thing is true for praticly any other coolent you might care to use (water, argon, whatever), except for helium which has an extreamly low neutron cross-section and does not transmutate very easily. But as nuclear reactors are designed NOT to leak there coolents, and it doesn't seem likely that any coolents leaked in such a manner could find there way into the life-support systems, this is all pretty much a non-issue.

It would be pretty much impossible for a reactor to get CO2 up to the temperatures at which it would decompose. If you DID get it hot enough, you probably be more worried about what was happening to your fuel and there containment (are they melting?) then the CO2 decomposing. It just doesn't decompose very easily. For use inside a reactor it should be just fine.

Now CO2 going into a liqued state is more of a concurn. Most mission are planned for the "tropical" regions of mars, where the temperature should stay above CO2 freezing point, but there could always be a cold-snap. It could also potential turn into a liquid if it is under pressure, but not heat, this might happen if you had to kill the reactor, for example. But since (as far as I know) CO2 is only being planned to be used as a coolent, and not a moderator, neither of these issues is truly critical. When the reactor resume opperation, it would naturaly heat-up and return to the gasous phase.

In addition, the Specific Heats of gasous CO2, Helium, Argon, and Nitrogen are all fairly close to one another. You might get a little more efficency out of Helium or Argon, but the diffrence isn't all THAT great.

All that said, I agree with Mr. GCN, you don't want to be adding coolent to you reactor during a mission. It a realitivly small weight saving (especialy if you are using a gas), and it introduces a whole other dimension of things that could go wrong.

Edit: posts by RobertDyck
http://newmars.com/forums/viewtopic.php?id=6101]Nuclear Rocket

SpaceNut · 2004-09-15 10:23:16

Resource of NASA’s Nuclear Systems Initiative

http://www-rsicc.ornl.gov/ANST_site/sco … 2NERAC.pdf

Antius wrote:

RobertDyck wrote:
I have argued for the "Direct Iron" method of smelting. It uses less energy, works at lower temperature (but still +900°C or slightly higher), when used on Earth it consumes less coal, produces fewer carbon emissions. Business uses it because it's lower cost, not for environmental reasons. Environmentalists should like it because it produces fewer emissions, but they never seem to like anything. On Mars it's useful because temperature is below that to melt stainless steel, so you can use the heat of a nuclear reactor directly. If you have to convert heat of a reactor to electricity, then use electricity in the smelter, it's very inefficient. It's far more efficient to use the heat directly. The catch is this method only works with very high grade ore. The good news is "hematite concretions" are very high grade ore.
But you will still need a lot of electricity. Carbon content in steel can be so excessive that it makes the steel brittle. The "Direct Iron" method uses a combination of CO and H2 to smelt. More H2 and less CO means less carbon in finished steel. H2 combines with oxygen from ore to form water, and CO becomes CO2. Use RWCS to recycle: CO2 + H2 → CO + H2O. Water is recycled via electrolysis to become H2. That means a lot of electrolysis, so a lot of electricity.
So you'll need a nuclear reactor for heat as well as electric production. Again, I suggest a thorium reactor because MGS identified thorium deposits on Mars. Produce nuclear fuel locally.
I would agree that steel production on Mars would benefit from a nuclear heat source. But the thorium reactor is unlikely, as there are too many uncertainties in long-term materials behaviour. Stainless steels and nickel alloys are both susceptible to stress corrosion cracking and in a molten salt reactor, you have a high temperature mix of literally hundreds of fission product compounds, many of which will migrate along the grain boundaries. On the plus side, it isn't pressurised. But engineering a container that will survive for decades will be a very difficult challenge.
To produce the sort of temperatures you are talking about is very difficult, as the outlet coolant temperature is always lower than cladding temperature, which again, is always lower than peak fuel temperature. Liquid metal cooled reactors have a relatively low film temperature drop, due to excellent thermal conductivity. But they lose that advantage in their heat exchangers. Gas reactors using SS cladding only work for coolant temperatures up to about 700C, after which mechanical strength declines to the point where clad thickness and neutron losses become excessive. Pebble-bed designs are reported to be capable of 900C outlet temperatures, but they have low power density largely resulting from the poor thermal conductivity of the pebble. And fuel burn-up tends to be quite low. Maybe not so bad for something you can build on Mars and never really have to decommission, but not good if you need to ship it from Earth. On the plus side, Mars has lots of CO2 that can be used as coolant.
A nuclear assisted process is achievable, with nuclear heat providing the first 700C of temperature rise and electric heating the remaining 250C.

The heat source could be pebble reactor, rtg or even a kilowatt reactor as we are not just looking for the heat source but also some electrical to make things all work together. Josh believes that this can be a low level heat source to make the dry ice go through phase change to gaseous and be pressured enough for use. I just do not know....

I do not know how well the water would work to capture co2 ..

We will be most likely going with a kilopower reactor which is a beefed up RTG design or more like a pebble reactor....as it must bring its working fluid with it as well as the radiators to make it work not just the generator system...

Antius wrote:

This is quite an old article, but it makes interesting reading.
http://www.osti.gov/scitech/biblio/5262838
I am not sure how well this would work. Basalt has a liquidus temperature of ~1200C, which is a bit hot for a nuclear reactor. A liquid metal cooled reactor using some sort of PRISM or pebble fuel might be capable of doing this. The magma would both corrosive and abrasive and would somehow need to be removed from the tunnels, maybe by casting into bricks in situ?
Another option is to leave the reactor on the surface and use an electrically powered melt device. The problem then is the need to trail a power cable behind the device.
All the same, this provides a good solution for lunar base construction, as it practically eliminates the need to spend large amounts of time carrying out EVA on the surface. Habitation can be subsurface from the start, shielded from hard radiation, temperatures extremes and micro-meteors.

J.J. Moesker · 2005-01-27 13:34:33

You might want to check the appendix of the following ESA report. It adresses some of the issues concerning the selection of space rated nucluar fission reactors.

ftp://ftp.estec.esa.nl/pub/aurora/Human … on.pdf]Esa report concerning human mission to mars

JoshNH4H wrote:

Oh, seeing the long durations thread, now I understand the purpose. Actually, I would suggest a two stage, ammonia decomposition/ LOX first stage, and a LANTR (Lox Augmented NTR) using U-233 pebble bed reactor to orbit, then refuel with NH3 in orbit, then go to the moon in Nerva mode. On the moon, you can bake O2 out of the large majority of SiO2 and other oxides in the soil, and use that in your NTR to go home.
How long do you think the stay should be?
How many people ?
ETC.
The Isp of an Ammonia fueled NTR is appx. 5/8 of Hydrogen.

Antius wrote:

Kbd512, Zubrin's nuclear salt water rocket would seem to represent the best of all worlds. No one in official NASA circles appears to take it seriously. To my knowledge, it has never been subjected to serious modelling. Yet it would appear to allow unlimited access to the solar system within timescales of weeks or months. What is your opinion on its plausibility?
I have been working on a lower performance concept for a nuclear thermal engine that uses natural uranium as fuel. Whilst this is much more bulky than NERVA, it could be constructed on Mars using local materials. The engine is only really workable in space, as it's T/W ratio is too low to achieve take-off from a planetary body. However, T/W would be substantially greater than any electric propulsion system and ISP would be 800-1000. It should therefore be possible for a freight carrying vessel to traverse from low Mars orbit to high Earth Orbit and back again with a single tank of hydrogen propellant.
There are two ways that the engine could be built. The most technically easy option for a mars colony would be a pebble bed reactor with natural uranium carbide slugs embedded within graphite spheres. However, the low moderating power of carbon would result in an excessively large core. The second option would be a hybrid, hydrogen cooled, heavy water moderated core. This would have higher power density, but requires a more complex design. Uranium carbide fuel must be housed in graphite sleeves within magnesium alloy tubes running through a tank of D2O. The graphite is in place to insulate the D2O from the hot hydrogen gas used to cool the fuel. Because the moderator remains cool, the neutrons remain fully thermalised even as the uranium reaches temperatures of >2000C.
An obvious problem with a hydrogen cooled, deuterium moderated rocket is that the propellant has a 641x higher neutron absorption cross-section than the moderator. This could both dampen the reaction and could make core physics unstable. However, the propellant atom density at 10bar and 2500K is still 1000 times lower than that of the moderator. Also, because phase-change does not occur in the propellant, any power transients will be relatively slow and should be dampened by the Doppler effect and active control systems.
Because the burn-up of natural uranium is limited to ~500GJ/kg, the fuel must be replaced after about 2 round trips. This would presumably be carried out in Mars orbit at the same time as hydrogen propellant refilling.
The purpose of the core would be to power large volume freight transport (1000s of tonnes) between Earth and Mars orbits. Freight must travel in both directions cheaply, because beyond a certain point, a Mars colony must be capable of paying for its imports using exports.

SpaceNut · 2020-06-28 17:11:51

Something has been mentioned by

Robertdyck wrote:

The particle bed was not fluidized. Liquid hydrogen flowed through pebbles of uranium, boiling to hydrogen gas. The problem was some spots between particles produced a gas bubble that stayed in place, preventing liquid hydrogen from cooling the pebble. This caused the face of both pebbles to overheat, causing that face to melt. This caused the pebbles to weld together, agglomerating the pebbles into a solid mass. That made the engine not restartable.

Calliban wrote:

kbd512 wrote:
Even if the rockets end up being heavier as a result, from an energy input perspective LOX/LCO still looks like a much better deal to me if energy and resource input per unit of output thrust produced is taken into consideration. We already know that we can obtain as much CO2 as we need and this propellant combination is very simple to make, requiring no new technology development. Unless humans will be breathing the atmosphere outside the rocket without pressure suits, which will never happen, or the propellant lines intrude into the pressurized compartment of the vehicle, which shouldn't happen given proper design, then this is also one of the most benign realistic propellant combinations available.
Agreed. If the production can be carried out thermochemically, using nuclear or solar heat, then LOX/LCO becomes even more attractive as a propellant.
https://www.frontiersin.org/articles/10 … 00601/full
It would be difficult to develop nuclear fuel with an operating temperature of 1500C. A pebble bed could probably reach those temperatures and could run on natural uranium. Maybe a pressurised sodium reactor using unclad UO2 fuel. Now there's a scary thought :-)

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#1 2004-09-08 22:55:04

Re: Ultimate power source? - Pebble-bed reactors

#2 2004-09-09 04:25:58

Re: Ultimate power source? - Pebble-bed reactors

#3 2004-09-09 17:54:55

Re: Ultimate power source? - Pebble-bed reactors

#4 2004-09-10 11:38:35

Re: Ultimate power source? - Pebble-bed reactors

#5 2004-09-10 14:13:30

Re: Ultimate power source? - Pebble-bed reactors

#6 2004-09-10 23:03:17

Re: Ultimate power source? - Pebble-bed reactors

#7 2004-09-11 05:36:36

Re: Ultimate power source? - Pebble-bed reactors

#8 2004-09-11 10:34:56

Re: Ultimate power source? - Pebble-bed reactors

#9 2004-09-11 13:07:41

Re: Ultimate power source? - Pebble-bed reactors

#10 2004-09-11 13:12:01

Re: Ultimate power source? - Pebble-bed reactors

#11 2004-09-11 17:15:12

Re: Ultimate power source? - Pebble-bed reactors

#12 2004-09-11 21:10:42

Re: Ultimate power source? - Pebble-bed reactors

#13 2004-09-12 04:54:28

Re: Ultimate power source? - Pebble-bed reactors

#14 2004-09-15 10:23:16

Re: Ultimate power source? - Pebble-bed reactors

#15 2005-01-27 13:34:33

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