U238 for Aerospace Propulsion (Traveling Wave Reactor)

[sanman]

You may have heard a lot of buzz in the news recently about the Traveling Wave Reactor, a concept being developed by TerraPower Inc, which uses depleted uranium (aka U238), instead of the usual rarified U235.

http://en.wikipedia.org/wiki/Traveling_wave_reactor

Apparently, the supercomputer-modeling done by TerraPower has been extensive enough to win over a lot of skeptics, so that they are now garnering the funding they seek to develop the concept to fruition.

I'd like to then ask if the Traveling Wave Reactor could be useful for propulsion purposes?
From a safety standpoint, while U238 is chemically toxic, it is not considered to be a radiation hazard like U235 is. Furthermore, the Traveling Wave reaction is supposed to burn up radioactive decay-chain products, to keep their levels under control.

One might intuitively say that if a Traveling Wave Reactor of reasonable size could be designed for stationary powerplants, then it could probably be adapted for nuclear-powered ocean-going vessels such as aircraft carriers, submarines and ocean-liners. Okay fine, but I wanted to take it further.

Would it be possible in principle to harness the Traveling Wave Reactor for aerospace propulsion? Could the Traveling Wave Reaction be adapted as a particle-bed reactor design, to allow it to serve the higher power demands and variable power demands of aerospace propulsion applications?

For example, could it be used to power a launch vehicle for the ~20minutes it might take to get to orbit?
So far, there have been estimates of Traveling Wave Reactors being built to run for 60 years uninterruptedly.
Could a reactor be constructed with suitable fuel elements to run at a very high power level for ~20 minutes, instead of 60 years at moderate power levels?

So perhaps instead of a moderatable/throttleable fission reactor which is analogous to a throttleable liquid rocket engine, this would instead be the nuclear analog to the Solid Rocket Booster (SRB) which is not throttleable.

 

[Astronuc]

Short answer - no.

One should realize that the TWR uses a fast flux to produce Pu239 from U238. One still has fission products.

Personally, I'm still skeptical about their modeling, and I think TWR is a nutty idea.

 

[sanman]

Hi Astronuc,

So is U238 the only useful starting fuel for a Traveling Wave Reaction? Or are there other possible starting fuels? If so, are there any that might lead to safer reaction products via the Traveling Wave Reaction?

 

[Astronuc]

Hi Astronuc,

So is U238 the only useful starting fuel for a Traveling Wave Reaction? Or are there other possible starting fuels? If so, are there any that might lead to safer reaction products via the Traveling Wave Reaction?

One basically needs a fertile or fissionable material. I'm not sure if thorium (Th-232 => U-233) would be practical as a thermal flux is preferred. In general it appears that one needs isotopes of U or transuranics, but the problem with transuranics is radioactivity, including some spontaneous fission for certain isotopes, plus the fact that they must be synthesized with nuclear reactions. There has been some studies with Cf-252, IIRC.

 

[sanman]

Hmm...

http://tinyurl.com/ycsyvv5

page 20:

http://www.nuc.berkeley.edu/files/TerraPowerGilleland.pdf

 

[Astronuc]

Yes - thorium enriched with U-233, U-235, or Pu-239, is feasible. I was thinking of TWR which uses a 'block' of U-238 (depleted, natural (.7% U-235) or spent (U-238+U-235+tu+fission products).

 

[CrazyEgg]

What about equipping a 747 with a small nuclear reactor such as the hyperion power module (25MW electric), coupled to a supercrtical CO2 generator which in turn would power 'jet engines' without the combustion (so only the pre-combustion compressors)?

The mass of the reactor and the generator equipment could take the place of the fuel mass that you will not need anymore right?

Another reactor option could be a molten salt reactor, similar to the ones built for the aircraft reactor experiment at ORNL in the 1960's.

 

[Astronuc]

Figure out the mass of the Hyperion reactor.

Aircraft nuclear systems were tried, and just didn't work. One difficulty is replacing the advantage of producing the heat of combustion in a small volume, namely the combustor, and passing the hot working fluid through the turbine. The advantage is that the heat is generated directly in the working fluid rather than in a solid/liquid fuel from which is must be transfered.

The reactor is normally located somewhere in the fuselage, and much of the mass is shielding, in addition to the fuel mass. Ideally the center of mass of the aircraft is centered somewhere between the wings for stability purposes. Shielding can add several metric tons to a reactor.

Gas reactors using the Brayton cycle top with a Ranking cycle are attractive from a thermal efficiency standpoint, but in terms of challenges to materials (e.g. erosion/corrosion), they can be difficult. For portable reactors, they are vary challenging.

 

[sanman]

Astronuc,

Are there any potential breakthroughs being researched for radiation shielding?

I've always wondered - neutrons are only neutral with respect to charge, but they do have magnetic spin properties. I've been wondering if somehow a nano-material could be devised which would be composed of tiny atomic/molecular clusters having extremely strong magnetic fields at the nano-scale. Perhaps a bulk material composed of such magnetic nano-domains would be able to act as a "molasses" to stop neutron radiation much more quickly, and maybe even to redirect the flow of neutrons towards or away from certain areas (think of how metamaterials are being used to redirect light for cloaking purposes, etc)

 

[chayced]

Nuclear aircraft engines did work. They successfully produced thrust by direct contact between the cladding and the air. If I remember right the project was canceled after a horrible design flaw caused one of the reactors to melt down.

Shielding is only necessary between the occupants and the reactor. All other exposures can be minimized by time and distance. Not an issue.

The problem with nuclear aircraft engines is that you have the fuel in nearly direct contact with the air. In a meltdown you have a much larger percentage of the core going airborne and fewer safeguards to prevent release. Also conventional reactors don't crash into hillsides.

The plus is that nuclear aircraft engines do not require oxygen to operate. This means that on a planet with a non-corrosive atmosphere you can still have propulsion. If nothing else they are good solid sci-fi.

 

[Hologram0110]

Nuclear reactors seem like a bad choice for aircraft for so many reasons. Shielding is necessarily big and heavy. There is the possibility of accidentally/mechanical failure that would bring a plane down destroying the reactor. They don't have room for a containment system. Although nuclear reactors have a lot of energy / mass, they generally have lower usable power to mass.

Furthermore, the TWR seems like an especially poor choice. TWR designs include much more fuel material than other reactor types. If you want to build a small light reactor, you would likely be better off with some sort of gas cooled, high enriched reactor.

 

[mheslep]

What about equipping a 747 with a small nuclear reactor such as the hyperion power module (25MW electric), ...

What if your 747 crashes?

Figure out the mass of the Hyperion reactor.

10-15 tons

Aircraft nuclear systems were tried, and just didn't work.

I'm not sure that's fair. They were abandoned, doesn't mean they didn't work.

One difficulty is replacing the advantage of producing the heat of combustion in a small volume, namely the combustor, and passing the hot working fluid through the turbine. The advantage is that the heat is generated directly in the working fluid rather than in a solid/liquid fuel from which is must be transfered.

Yes, that's a problem. Doesn't mean the problem is intractable.

The reactor is normally located somewhere in the fuselage, and much of the mass is shielding, in addition to the fuel mass. Ideally the center of mass of the aircraft is centered somewhere between the wings for stability purposes. Shielding can add several metric tons to a reactor.

I doubt this is a show stopping problem any longer. Modern aircraft carry 70ton tanks and the Space Shuttle of all things.

 

[mheslep]

Personally, I'm still skeptical about their modeling, and I think TWR is a nutty idea.

Care to elaborate? There are two parts of the scepticism school. First is the value of the goals they claim if it works, and second is the feasibility of making it work. Do you find the goals, the feasibility, or both nutty? The goals of the TWR include:

• no refuelling after start-up
• an end to the large scale enrichment business
• on site underground storage of waste for the life of the project
• modular off site construction of the reactor
• no water source for cooling
• expands the fuel source time line from a ~hundred years to thousands of years, and the fuel becomes very cheap.

Regards feasibility, the only issue for which I haven't yet seen a very good explanation is what happens to fission products, especially gases like Krypton.

 

[Astronuc]

Regards feasibility, the only issue for which I haven't yet seen a very good explanation is what happens to fission products, especially gases like Krypton.

That's part of my concern - that they don't seem to have addressed the physics of the fission products, and particularly the Xe, Kr, and volatiles such as Cs, I, Cd, and metal compounds. The nuclear properties of the depleted or natural uranium change ahead of the wave, so I would expect the wave to disperse or spread out ahead of the fission zone.

I'd have to see the balance of plant to a make an informed comment on the their heat rejection system.

I'd like to see the details of their core simulation code, and assumptions on conversion/breeding and treatment of fission product. I'm guessing they use a multi-group neutron transport code.

 

[aquitaine]

That's part of my concern - that they don't seem to have addressed the physics of the fission products, and particularly the Xe, Kr, and volatiles such as Cs, I, Cd, and metal compounds. The nuclear properties of the depleted or natural uranium change ahead of the wave, so I would expect the wave to disperse or spread out ahead of the fission zone.

I'd have to see the balance of plant to a make an informed comment on the their heat rejection system.

I'd like to see the details of their core simulation code, and assumptions on conversion/breeding and treatment of fission product. I'm guessing they use a multi-group neutron transport code.

Given that both the Hyperion is both fully automated and factory sealed, I would assume it would have been accounted for in the design.

In anycase I was thinking recently that maybe the original nukes on a plane designers went about it the wrong way. Instead of using the reactor as a heat source, why not use it as a source of electricity and instead use electric jet engines? Weight would not really be a huge issue, the Hyperion system weighs ~18 tons or so, a 737 class or bigger carries about that much fuel or more.

 

[chayced]

A quick google shows that a 747s engines produce about 65MW mechanical just at cruising speed and are capable of about 4 times that. Numbers may not be exact, but that still requires a huge reactor plant just to generate the electricity. That's why they used air cooled reactors for the nuclear jet experiments. No middleman, thermal to mechanical.
http://www.aerospaceweb.org/question/propulsion/q0195.shtml

 

[aquitaine]

A quick google shows that a 747s engines produce about 65MW mechanical just at cruising speed and are capable of about 4 times that

In imperial short tons (2000 pounds per ton), how much does the fuel of a 747 weigh at maximum capacity?

That's why they used air cooled reactors for the nuclear jet experiments. No middleman, thermal to mechanical.

Yes, that's the open cycle approach which is a bit dangerous.

 

[mheslep]

Given that both the Hyperion is both fully automated and factory sealed, I would assume it would have been accounted for in the design.

Hhttp://www.hyperionpowergeneration.com/"; two different things.

 

[mheslep]

A quick google shows that a 747s engines produce about 65MW mechanical just at cruising speed and are capable of about 4 times that. Numbers may not be exact, but that still requires a huge reactor plant just to generate the electricity. ...

What's huge? Hyperion 70MW thermal is 1.5M OD by 2.5M, including shielding.

 

[aquitaine]

Hhttp://www.hyperionpowergeneration.com/"; two different things.

Right, my bad. Still, we're going to have to find a way to fly without using fossil fuels in this century, full stop. Right now I don't see too many other options.

 

[mheslep]

Right, my bad. Still, we're going to have to find a way to fly without using fossil fuels in this century, full stop. Right now I don't see too many other options.

Well there have already been some jumbo jets test flights on biofuel. But https://www.physicsforums.com/showpost.php?p=2292842&postcount=14", I expect, will eventually take over, powered by either batteries or fuel cells. There are just too many advantages (noise, efficiency, flexibility of design) for it not to eventually happen.

 

[aquitaine]

Yeah but what seems to be holding it back the most is our crappy energy storage methods, which is what lead me to think of using nuclear, since it is the only non-combustion source that has a high energy density.

I'm also a bit skeptical of biofuels in general. I just don't think it is a good idea to burn our food, or to use land capable of growing food to burn this. We're also not sure how well oil producing algae will scale up, and what requirments it will have.

Thanks for the link, book marked. :)

EDIT: What we also need is lighterweight radiation shielding. Sure lead and water are effective, but they basically just solve the problem by throwing mass at it. Surely there must be a better way.

 

[mheslep]

EDIT: What we also need is lighterweight radiation shielding. Sure lead and water are effective, but they basically just solve the problem by throwing mass at it. Surely there must be a better way.

As illustrated above weight is not the main problem. The principal problem with nuclear aviation (political or technical or both) is safety, i.e. what are the consequences of a crash?

 

[mheslep]

September Nuclear News interview with the TWR company's CEO and nuclear engineer John Gilleland:
http://www.new.ans.org/pubs/magazines/download/a_647


Per Gilleland:

• Current design 'looks very much like the standard http://en.wikipedia.org/wiki/Sodium-cooled_fast_reactor"
• On fission products: "This is a fast-neutron system, and so it’s the standard reaction, and the plutonium is putting out the spectrum. At the very high end, the neutrons are quite fast, and the reaction is quite insensitive to the presence of the fission products. We cannot do it with a thermal system because that would be much too sensitive to the fission products."
  
• But: "The multiplication that we get in the fast-neutron system is high enough to allow both the breeding and burning of plutonium. We have to be able to sustain this breeding long enough to convert from a breeding phase to a burning phase. That involves a very severe environment for the materials. It’s the reason that this idea hasn’t been done before. "
• Therefore, to maintain the fast spectrum, the trick in the design is to "minimize moderating and loss materials"
• Modeling code was modified from existing Monte Carlo-based neutronics. Led by Charles Whitmer. Checked their codes "against Rebus and other codes. [LBNL] ran tests to compare resutls with us, and [ANL] also provided us with calibrations..."
• Code tracked "about 2500 daughter products"
• Enrichment facilities can disappear, or at least down to 'one per planet'.
• Why the liquid sodium design? Because the wave needs to operate at high-energy density to propagate, and liquid Na has been demonstrated to 200-300 megawatts per cubic meter.
• Timeline: first power producing system 2020.
• Key Personnel: Roger Reynolds (former CTO Areva), Whitmer, Tom Weaver and George Zimmerman (LLNL), Ehud Greenspan (Berkley nuclear prof), Pavel Hejzlar (MIT), Jacopo Buongiorno, John Nuckolls (LLNL director), Charles Ahlfeld, Gevan Weaver (INL), David McAlees (Siemens).
  

So problems I see with achieving a successful design:
-TWR is planned to operate continuously without fuel replacement for decades. They'll have a very high bar to meet in convincing the NRC that the wave containment tube can tolerate fast neutron fluxes for decades.

 

[vanesch]

Personally, I'm still skeptical about their modeling, and I think TWR is a nutty idea.

Indeed. What I never understood in a TWR concept, is how one can have the luxury of wasting half of the neutrons, in a way.
In a "normal" reactor, one needs on average 1 useful neutron per fission, namely for the next fission. It comes down to the Fermi k-value which is theoretically = 1 in a normal reactor.

In a breeder, one needs (slightly more than) 2 useful neutrons per fission: one for the next fission just as in a normal reactor, and (a bit more than) one that needs to be absorbed into a fertile nucleus to make a new 'fuel atom'. If this is less than 1, one produces less fuel than one consumes. If it is equal to 1, one is in principle on the verge of reaching breeding, but a true breeder makes more fuel than it consumes. Now, to get 2 useful neutrons out of each fission doesn't leave much of a margin, as on the average, one produces 2.4 or so neutrons per fission (depending on fast/thermal and on U/Pu). With fast on pure Pu-239, one can reach 2.9 neutrons per fission.

However, in a TWR, of course grossly half of the neutrons goes "the wrong way": will leave the burning slab to the left, into the "ashes", and not to the right, into the fertile fuel. So how can you hope, with 2.9 neutrons to start with, and the need to have more than 2 useful neutrons (one to sustain burning, and somewhat more than 1 to breed), with all "useless" absorptions (as Astro says, in the poisons, in the minor actinides, ...), to have an ongoing breeding if you waste a serious part of your neutrons in the ashes ??

If they do genuine modelling, this should be obvious. Maybe I'm missing something. But it looks, at first sight, totally impossible to me, given the neutron balance one can guess from a start.

 

[mheslep]

.. with all "useless" absorptions (as Astro says, in the poisons, in the minor actinides, ...), to have an ongoing breeding if you waste a serious part of your neutrons in the ashes ??

I think that's where you may be off. With a fast spectrum, absorption by products is much less of a factor. Edit: The absorption by Xenon-135, a high yield product, falls off seven orders of magnitude from thermal to MeV neutrons*, but I see the important issue is the ratio of Xenon capture to Pu-239 fission (or U-239 -> Pu breeding) with fast neutrons and I'm not sure about those ratios.

This is a fast-neutron system, and so it’s the standard reaction, and the plutonium is putting out the spectrum. At the very high end, the neutrons are quite fast, and the reaction is quite insensitive to the presence of the fission products. We cannot do it with a thermal system because that would be much too sensitive to the fission products.

That, and as Gilleland said in the interview above they plan carefully to "minimize [fixed] moderating and loss materials"*For example:

 

[vanesch]

Yes, but my main problem is: what with the neutrons that leave the burning slab "on the left" (in the ashes) ? It is hard to imagine that they are perfectly reflected by ashes (and are not moderated).

 

[mheslep]

Yes, but my main problem is: what with the neutrons that leave the burning slab "on the left" (in the ashes) ? It is hard to imagine that they are perfectly reflected by ashes (and are not moderated).

Hmm. I'm assuming the ashes on the left or anywhere are ~transparent, so then they would want a fixed fast neutron reflector on the walls, if such a thing exists for fast neutrons, I dunno.

 

[Astronuc]

The neutronics is one aspect, and one has to consider the structural materials and coolant, in addition to the fuel.

One may lose as many as half the neutrons from the fission wave volume. I imagine the scattering of fast neutrons from the ash is similar to that of the fuel region. And there is leakage out the sides, and losses into the coolant and structural material.

Besides the fission products are transuranics that themselves are somewhat parasitic. I'd like to see their models.

The bottom line on maintaining criticality is that at least one neutron remains from each fission reaction to cause the next fission - on average. For breeding one needs one neutron to fission, one neutron to be captured by fertile material, and the remainder can be lost to the structure (including coolant) or leak out of the reactor (fuel/fertile materials). If leakeage is already 50%, I'm skeptical about achieving a viable system.


With respect to heat transfer, one factor I see as unattractive is the full length heat transfer system, if I read the design correctly, in which the coolant channels are full length. A lot of heat is transeffered to the fuel where no fission will occur for years. At the end of life, I imagine that unburned fuel will remain.

 

[aquitaine]

As illustrated above weight is not the main problem. The principal problem with nuclear aviation (political or technical or both) is safety, i.e. what are the consequences of a crash?

Part of that depends on how fast the plane hits. When the USS Thresher went down, the impact with the bottom pancaked the hull, but the reactor was still in one piece and still hasn't leaked any radiation. Even if the reactor vessel cracked open on impact, what methods are there to neutralize radioactive isotopes?


Weight still does matter, not just with regards to this issue but also to setup permenant settlements on the moon/mars/whatever we will need it, not to mention if we are going to build "space only" spacecraft .

 

[mheslep]

...
One may lose as many as half the neutrons from the fission wave volume. I imagine the scattering of fast neutrons from the ash is similar to that of the fuel region. And there is leakage out the sides, and losses into the coolant and structural material.

I think originally the idea was to use a fairly large diameter cylindrical core, and start the burn in the center. I recognize heat dissipation limits the core size, but to the degree large size is possible, the losses to the sides of the cylinder shouldn't become significant except where the wave comes close to the containing structure. In those edge regions, perhaps they don't breed or burn up much fuel. In a normal enriched U reactor that would be an expensive inefficiency, but here the fuel is nearly free so perhaps they don't care. In the interview above Gilleland says he never expects to get above 50% U238 burnup. I agree this still doesn't address the cooling or moderating structures and medium.

Besides the fission products are transuranics that themselves are somewhat parasitic. I'd like to see their models.

Isn't the fission yield of transuranics from Pu-239 fission tiny in comparison to Xe, Kr, etc?

If leakeage is already 50%, I'm skeptical about achieving a viable system.

Certainly. The question is whether the TWR does in fact lose 50% of 2.9 neutrons, or not.

At the end of life, I imagine that unburned fuel will remain.

Again, U238 or even natural U. May not matter.

 

[Astronuc]

I think originally the idea was to use a fairly large diameter cylindrical core, and start the burn in the center. I recognize heat dissipation limits the core size, but to the degree large size is possible, the losses to the sides of the cylinder shouldn't become significant except where the wave comes close to the containing structure. In those edge regions, perhaps they don't breed or burn up much fuel. In a normal enriched U reactor that would be an expensive inefficiency, but here the fuel is nearly free so perhaps they don't care. In the interview above Gilleland says he never expects to get above 50% U238 burnup. I agree this still doesn't address the cooling or moderating structures and medium.

Another factor is the increase in volume of the fission products (FP) with two atoms produced per fission. If TWR produces 50% FIMA, then it should expand 50% in volume. It is my understanding that the point is to convert U-238 to Pu-239/240/241 for fissioning.

Isn't the fission yield of transuranics from Pu-239 fission tiny in comparison to Xe, Kr, etc? Certainly. The question is whether the TWR does in fact lose 50% of 2.9 neutrons, or not.

I'm thinking more along the lines of Pu-240, Pu-242, and others that might absorb neutrons but not readily fission. And the key factor is the matter of 50% loss of 2.9 neutrons. Basically one has to have at least one neutron to convert U-238, and one to fission the Pu-239, so the system can only lose 0.9 neutrons on average to other mechanisms, i.e. absorbed by FP, structure, coolant or leak out of the core (absorbed by ex-core structure/shielding).

Again, U238 or even natural U. May not matter.

I'm thinking more about the unburned fissile and fissionable (transuranic) isotopes. Certainly, it's a benefit not to worry about U-237 or Np-237.

 

[sanman]

Belgium has approved work on the construction of an Accelerator-Driven Reactor System (ADS) which uses low-enriched Uranium:

http://www.world-nuclear-news.org/WR_Approval_for_Myhhra_0503101.html

So while it's not Traveling Wave, it's still primarily U238.

Could this idea, if proven to work, then be adapted to a space vehicle?

Alright, I suppose fission products are an unavoidable hazard unacceptable for Earthly use, but for a spacecraft flying far from Earth it seems quite justifiable.

Could something like this be used to power a VASIMR rocket? How about for some kind of lunar shuttle that operated between the lunar surface and orbit, in addition to being a long-range lunar surface transport?

It says low-enriched, but it doesn't say how low. Ideally, you'd like the system to be improved to the point where it could work entirely on depleted uranium - ie. U238 alone.
Could this be conceivable?

 

[sanman]

Furthermore, regarding fission products - given that you're replacing the huge mass/weight of chemical fuel with relatively compact nuclear fuel, you could have spaceships which are much lighter, which again saves you on nuclear fuel mass, which in turn reduces your fission products. An unmanned launch vehicle might not require much reactor shielding, so you'd avoid that mass/weight penalty too.

Instead of direct heat exchange with the flowstream, supposed you used AMTEC (Alkali Metal ThermoElectric Conversion) to convert the nuclear energy into electricity. The nuclear-electric power could be used to power an upper stage, while the lower stage would be chemical.

Furthermore, you could have the chemically-propelled lower stage be a scramjet or something, which would have safer takeoff and landing than a chemical rocket. The scramjet would lift everything to just above the atmosphere, where the nuclear-electric VASIMR propulsion system would take over and propel the upper stage to orbit or beyond (eg. Moon)

With the nuclear-electric VASIMR propulsion, you could send very heavy payloads into orbit around the Moon. The VASIMR module could then ferry this mass in smaller chunks down to the lunar surface.

What's wrong with this idea?

 

[Astronuc]

VASIMR has high specific impulse and low thrust, so it has to be used outside of a gravity field where the local acceleration due to gravity is less than the acceleration provided by VASIMR.

In comparing power conversion concepts, one need only consider the kW/kg (specific power) as a figure of merit. The higher the number the better. Conversly one could look at kg/kW, in which case the smaller the number the better.

AMTEC (Alkali Metal ThermoElectric Conversion) generally has low thermal efficiency, which is tied to the thermoelectric material(s). There are TE materials that have an optimal temperature range. One also most consider how far from the reactor one would have to place the materials so that they are not activated by the neutrons (from a fissile system).

With respect to ADS - one needs a massive accelerator, and they are generally inefficient with respect to energy input vs useful energy out.