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.

 

[sanman]

Well, let's assume that your chemically-driven lower-stage gets you to escape velocity. Then your nuclear-powered upper stage has all the time it needs to push you higher - even out to the Moon.

Accelerator size (or mass) is an engineering problem - there is no fundamental rule saying that an accelerator has to be of high mass. I would think that the closer the reactor's sub-critical state is to criticality, then the less accelerator input is required to sustain the reaction. So perhaps this could be traded off against accelerator size and power.

As for separation distance between thermoelectric materials and neutron source, well, most rocket designs are pretty long, including even many deep space concept designs, and in space you have a lot of room for separation.

If your vehicle was a lunar shuttle/ferry only operating between the lunar surface and lunar orbit, you'd still essentially be operating in a hard vacuum so that an aerodynamic geometry would not be needed.

While neutrons are electrically neutral, they are still magnetic, and so I've always wondered if it's possible to harvest their kinetic energy via a magnetic field. This would also be beneficial from a radiation standpoint. Maybe a hybrid fission-fusion reactor could do this, exploiting a tokamak's magnetic field.

 

[Astronuc]

Well, let's assume that your chemically-driven lower-stage gets you to escape velocity. Then your nuclear-powered upper stage has all the time it needs to push you higher - even out to the Moon.

A chemical system would get one to orbit (LEO), and the nuclear system would probably be completed on orbit. Then the nuclear system would take one to one's destination.

Accelerator size (or mass) is an engineering problem - there is no fundamental rule saying that an accelerator has to be of high mass. I would think that the closer the reactor's sub-critical state is to criticality, then the less accelerator input is required to sustain the reaction. So perhaps this could be traded off against accelerator size and power.

The accelerator current would drive a subcritical system, i.e. the power produced would be proportional to beam current, and ADS systems are not design with propulsion in mind, but simply to burn transactinides and transmute waste. The system could be designed to go supercritical until it achieves criticality through Doppler effect (resonance absorption), but then that assumes an thermal or epithermal system as opposed to a strictly fast reactor system. Accelerators require bending/deflection magnets for directing and focusing the beam.

The TWR is not designed for propulsion - so it can be rather massive - depending on the energy to be produced, which is given by the integral of power over time.

Generally, to minimize mass, one minimizes the core by increasing the enrichment as close to 100% as desirable. The smaller the core (and with enrichment increasing to > 90%), control becomes a key issue. Dynamic (on-line) fueling of a core would also be problematic.

As for separation distance between thermoelectric materials and neutron source, well, most rocket designs are pretty long, including even many deep space concept designs, and in space you have a lot of room for separation.

One goal is to minimize the heat transport, particularly high temperature heat which requires strong/stable (little or no creep) material at temperature.

If your vehicle was a lunar shuttle/ferry only operating between the lunar surface and lunar orbit, you'd still essentially be operating in a hard vacuum so that an aerodynamic geometry would not be needed.

Aerodynamic profile is only important in minimizing energy loss in an atmosphere. The discussion is strictly about propulsion.

While neutrons are electrically neutral, they are still magnetic, and so I've always wondered if it's possible to harvest their kinetic energy via a magnetic field. This would also be beneficial from a radiation standpoint. Maybe a hybrid fission-fusion reactor could do this, exploiting a tokamak's magnetic field.

Neutrons have a magnetic moment, which is not the same as being magnetic (they have a very weak field), and one is unlikely to collect a large mass of neutrons, and they are neutral, so they essentially pass through a magnetic field without much loss of energy.

 

[sanman]

Astronuc, thanks for your great replies as usual, which always give me pause for thought.

Well, since the neutron's cross-sectional area is small, it's too bad we don't have some material with extra-large cross-section to intercept it. I've wondered if nuclear isomers might be capable of cross-sectional geometries that might allow greater neutron-interception capabilities relative to their nuclear mass. Is it possible that a suitable nuclear isomer exists which has higher neutron-capture capability, just as some ions have higher electron affinity?

While I understand that isomers are expensive to manufacture, just think of the problems that could be solved if such a desirable material were made.

 

[Astronuc]

Astronuc, thanks for your great replies as usual, which always give me pause for thought.

Well, since the neutron's cross-sectional area is small, it's too bad we don't have some material with extra-large cross-section to intercept it. I've wondered if nuclear isomers might be capable of cross-sectional geometries that might allow greater neutron-interception capabilities relative to their nuclear mass. Is it possible that a suitable nuclear isomer exists which has higher neutron-capture capability, just as some ions have higher electron affinity?

While I understand that isomers are expensive to manufacture, just think of the problems that could be solved if such a desirable material were made.

Ah - now one appreciates the challenges of designing a nuclear reactor, particularly a small one with high power density.

Reactor design involves a tradeoff of nuclear properties and the other physical properties. There are a limited number of fissile fuels (isotopes) from which to choose. There core must maintain a stable geometry, which becomes more challenging as the temperature is increased. Then the core must contain all the fuel to be consumed, which would limit the lifetime to seconds to years depending on size and utilitization, unless the core can be refueled. Refueling usually means shutting down a reactor and replacing some fuel elements. CANDUs are designed to be refueled on-line whereby the refueling machinces operated within the pressure boundary. One machine inserts new fuel while the other extracts/receives an equivalent number of spent fuel. But CANDU are large machine not designed for propulsion.

With respect to nuclear properties, neutrons originating from fission have energies on the order of several MeV, ~ 1-10 MeV, with an average of about 4 MeV. They are considered fast, and they can travel a distance on the order of meters before interacting with most materials. However, the lighter elements can readily interact with neutrons, with hydrogen being the best at slowing down neutrons, because the proton is about the same mass and conceiveably, if a neutron hits a proton 'smack-on-center', then the neutron essentially loses all it's energy, nearly coming to a stop. It would then likely drift into a nearby nucleus and be absorbed. Deuterium is the next best stopping or slowing down medium, then Li, Be, B, C . . . . according to mass. Another property with which one must be concerned is the absorption cross-section, since the absorption by structural materials competes for neutrons with the fission process. In other words, the desirable absorption leads to fission, while the undesirable absorption leads to parasitic losses. Of course, to control the reactor, it is desirable to have a neutron aborber that can be inserted in the reactor to shut it down (make it subcritical) quickly. A physical property that goes along with neutron cross-section is the atomic density - which for most solids is on the order of 10²² atoms/cm³. Density decreases with temperature, which is an important factor when designing the fuel.

In addition to structure, a reactor systems requires a means of heat removal, otherwise the core will eventually melt or vaporize if the temperature exceeds melting or boiling points. So a core design needs a means of passing a 'working fluid' though the core and transporting the heat to a thermal conversion system - and doing so in a controlled (stable and reliable) manner. The fluid maybe single phase (gaseous or liquid), or two phase (vapor + liquid).

For turbomachinery, vapor is the preferred phase. Liquid is preferred to minimize the work used to return the coolant to the core. If the working fluid is gaseous, a compressor would be required to force the gas back to the core.

With respect to reactor cores, one can design a fast reactor core with a reflector using something like a beryllium-based reflector. Basically compact cores (particularly those using a fast neutron spectrum) need to be reflected in order to maintain a margin of controllablility.


In terms of reactor design, the following indicates the issues that must be addressed:
http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-appa.html
Part 50 Domestic licensing of production and utilization facilities
http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/

10CFR - http://www.nrc.gov/reading-rm/doc-collections/cfr/ - addresses a whole host of issues and requirements for nuclear technology.

 

[Frame Dragger]

To respond to people thinking of nuclear aircraft... I don't see how that is worse than a nuclear reactor at sea; in fact I'd much rather see a meltdown in air than water. Politics is one thing, but if the plane crashed into a body of water, THAT is the issue...
...
...
...

And yet we have many nuclear powered submarines and other naval vesels. We ALREADY live with this risk, it just feels more remote.

Add this http://www.nuclear.com/n-plants/index-Floating_N-plants.html and I think we have something to be genuinely terrified of. I can imagine cleaning a LARGE region of contaminated land, but I can't imagine the full consequences of a meltdown at sea, of that magnitude.

Aviation is frightning, because people DO think of something falling from the sky... we tend not to instinctively fear what we do to our oceans, and what the effect of the steam would be. Oh, it would be very very bad.

So, it's all political, because we're happy to live with nucelar weapons aimed down our throats at all times, because of our confidence they will never be deployed. We seem not to care if we stick nuclear weapons and reactors under or on the water... of course, planes crash in countries OTHER than ones they originated from. The politics of that *shudder*.

 

[Astronuc]

To respond to people thinking of nuclear aircraft... I don't see how that is worse than a nuclear reactor at sea; in fact I'd much rather see a meltdown in air than water. Politics is one thing, but if the plane crashed into a body of water, THAT is the issue...
...
...
...

And yet we have many nuclear powered submarines and other naval vesels. We ALREADY live with this risk, it just feels more remote.

Add this http://www.nuclear.com/n-plants/index-Floating_N-plants.html and I think we have something to be genuinely terrified of. I can imagine cleaning a LARGE region of contaminated land, but I can't imagine the full consequences of a meltdown at sea, of that magnitude.

Aviation is frightning, because people DO think of something falling from the sky... we tend not to instinctively fear what we do to our oceans, and what the effect of the steam would be. Oh, it would be very very bad.

So, it's all political, because we're happy to live with nucelar weapons aimed down our throats at all times, because of our confidence they will never be deployed. We seem not to care if we stick nuclear weapons and reactors under or on the water... of course, planes crash in countries OTHER than ones they originated from. The politics of that *shudder*.

Aircraft, ships/submarines/barges, and terrestrial power plants are all completely different systems with different engineering/technical requirements. The engineering/technology are completely different.

 

[Frame Dragger]

Aircraft, ships/submarines/barges, and terrestrial power plants are all completely different systems with different engineering/technical requirements. The engineering/technology are completely different.

Yes, and that's why the Russian approach I cited is so troubling, because they are ignoring that. Nothing that has come out of Sevmash should inspire confidence, especially the lack of care to shielding the hull against a 'burning' nuclear core, despite the consequences of a possible failure. They're just repurposing some KTL-40 (naval), and ABV-6M reactors! They're even considering a VBER-300 for later models.

That's not just warming the oven for propulsion, and the "switchover" to providing heat, power, and desalination is also a bit unclear. I'd offer more, but there is no more that has been made public, that I'm aware of.

 

[Astronuc]

Yes, and that's why the Russian approach I cited is so troubling, because they are ignoring that. Nothing that has come out of Sevmash should inspire confidence, especially the lack of care to shielding the hull against a 'burning' nuclear core, despite the consequences of a possible failure. They're just repurposing some KTL-40 (naval), and ABV-6M reactors! They're even considering a VBER-300 for later models.

That's not just warming the oven for propulsion, and the "switchover" to providing heat, power, and desalination is also a bit unclear. I'd offer more, but there is no more that has been made public, that I'm aware of.

The Russians are not ignoring that fact. The KTL-40 is a marine reactor, not an aircraft reactor. If they were to deploy it, they would have to meet the regulatory, safety and security requirements for the nation in which it was deployed, and they'd likely have restrictions due to proliferation due to the 90% enrichment.

This technology would not work in an aircraft as it is too massive. An icebreaker weighs a heck of a lot more than a 747.

As for information - please refer to a report from Norwegian authorities - http://www.nks.org/download/pdf/NKS-Pub/NKS-138.pdf

3.3 THE KLT-40 PLANT
The latest version of Russian maritime reactor plants is the KLT-40. It has been installed in the
icebreaking freighter Sevmorput and in two icebreakers, Taimyr and Vaigatch, all with one
reactor only. Much is known about this plant, because the Russian government submitted the
safety report for NS Sevmorput, [Information], to the Norwegian safety authorities in 1991 before
a visit of Sevmorput to Tromsø in 1991. This report has been the basis for many studies of
Russian marine reactors. The KLT-40 plant contains a pressurized water reactor with power
levels of 135 MWt (Sevmorput) and 171 MWt (Taimyr and Vaigatch). The information given
below has been obtained from [Information] and, strictly speaking, applies only to the Sevmorput
plant. However, data for Taimyr and Vaigatch are presumably not very different even though the
power level of their reactors is somewhat higher. The KLT-40 is in many ways similar to the OK-
900.

3.3.1 Reactor
Figure 3.7 gives a vertical cross-section of the reactor. The coolant enters the reactor tank at the
top, flows downwards through the reflector/thermal shield, up through the reactor core and from
the top of the reactor tank to the steam generator. From here, the coolant flows through the
canned circulation pump back to the reactor. The design is very compact, completely welded with
a tube-inside-tube arrangement whereby the length of the piping and number of flanges etc. of the
primary circuit is kept to a minimum, reducing the risk of leakage. The reactor tank is on the
inside provided with a stainless steel layer. The thermal shield consists, in the radial direction, of
steel-water layers and, at the top above the tank lid, of a concrete shield.
The core height is 1 m and the diameter 1.21 m. The 241 fuel elements are arranged in a
triangular lattice with a spacing of 72 mm. The fuel elements are placed in a removable insert or
basket inside the reactor tank, and movement is prevented by fixing them both at the bottom and
at the top.

Core height 1 m
Core diameter 1.21 m
Mass of U-235 in core 150.7 kg
U-enrichment 90% (weapons grade)

3.3.5 Primary Cooling Circuit
Figure 3.8 shows the primary system. The reactor is provided with four cooling loops, each of
which contains one steam generator and one circulation pump. Pressure in the primary system is
controlled by a gas pressurizing system connected to the four pressurizers. This system is based
on injection/discharge of gas. According to [Kuznesov1], coolant inlet temperature is 278° C and
outlet temperature is 318°C. According to [OKBM] and [Information], inlet temperature is 78°C,
outlet temperature 312°C and the pressure of the primary system is 130 bar. The temperature and
pressure of the steam leaving the steam generator is 290°C and 40 bar. There is an emergency
cooling system, but in addition, the reactor can run by natural circulation at 25–30% full power.

Certainly one would have to be careful with the design.

The 747-8 Freighter will be longer than the 747-400F by 5.6 m (18.3 ft) and have a maximum structural payload capability of 140 metric tonnes (154 tons) with a range of 8,130 km (4,390 nmi). Also powered by 787-technology engines, it will achieve the same environmental benefits as the 747-8 Intercontinental. The 747-8 Freighter will have nearly equivalent trip costs and 16 percent lower ton-mile costs than the 747-400, plus 16 percent more revenue cargo volume than its predecessor. The additional 120 cu m (4,245 cu ft) of volume means the airplane can accommodate four additional main-deck pallets and three additional lower-hold pallets. Operating economics of the 747-8 Freighter will be significantly superior to the A380F. The 747-8F's empty weight is 80 tonnes (88 tons) lighter than the A380F, resulting in a 24 percent lower fuel burn per ton, 21 percent lower trip costs and 23 percent lower ton-mile costs than the A380F.

. . . .

http://www.boeing.com/commercial/747family/747-8_facts.html

And compare to the An-225
Empty weight: 285,000 kg (628,315 lb)
Max takeoff weight: 640,000 kg (1,323,000 lb)
http://en.wikipedia.org/wiki/Antonov_An-225#Specifications_.28An-225.29

Of course, with the KTA-40, one would need shielding, a turbine-generator set, and high speed motors. The shielding, T-G, and motor(s) add substantial mass, such that it would be impractical for an aircraft.

The other major factor would be designing an impact resistant containment systems. Since ships massive and don't travel at high speed or high altitude, there not much of an impact to design for as there is for an aircraft. One can build a substantial containment on a ship or barge, but not so for an aircraft. Certainly one would also have to consider designing containment for missles, both natural and man-made - and this would be a simpler proposition for a ship/barge than for an aircraft.

Various governmental research organziations, e.g., US DOE, are aware of the KTA-40 and other systems, and their potential application.

 

[Hologram0110]

Do people really think that a nuclear power plane crashing on land would be better than sea?

While I really don't like the thought of any reactors in situations where their containment could conceivably fail especially in uncontrolled areas. If a reactor were to fail outside of containment, out at sea seems like one of the better places to me. Please correct me if I'm wrong, but because the oceans are so large, if an accident occurs far from land, the contamination would have been significantly diluted by the time it reaches populated areas.

Also, speeds at sea are lower than air speeds. A sinking ship will hit the sea floor much more softly than an air craft crashing on land or sea. Conceivably, we could then work to recover, or contain the reactor lost at sea before it leaks.

Also, the risks of using nuclear reactors on submarines is somewhat more justifiable because they need high energy density to stay underwater. Nuclear reactors in space also seem justifiable to me for similar reasons. There are no real reasons planes in the atmosphere would need anything other than chemical and/or electrical energy sources. They simply are not in the air long enough to justify the need. You could just use fossil fuels, biofuels, hydrogen, batteries, super/ultra capacitors ect.

Currently, solar and radioisotope generators are able to provide enough power for propulsion. I wonder if you could create a hybrid radioisotope + fission source similar to accelerator driven systems.

 

[Frame Dragger]

The Russians are not ignoring that fact. The KTL-40 is a marine reactor, not an aircraft reactor. If they were to deploy it, they would have to meet the regulatory, safety and security requirements for the nation in which it was deployed, and they'd likely have restrictions due to proliferation due to the 90% enrichment.

This technology would not work in an aircraft as it is too massive. An icebreaker weighs a heck of a lot more than a 747.

As for information - please refer to a report from Norwegian authorities - http://www.nks.org/download/pdf/NKS-Pub/NKS-138.pdf
Certainly one would have to be careful with the design.

http://www.boeing.com/commercial/747family/747-8_facts.html

And compare to the An-225
Empty weight: 285,000 kg (628,315 lb)
Max takeoff weight: 640,000 kg (1,323,000 lb)
http://en.wikipedia.org/wiki/Antonov_An-225#Specifications_.28An-225.29

Of course, with the KTA-40, one would need shielding, a turbine-generator set, and high speed motors. The shielding, T-G, and motor(s) add substantial mass, such that it would be impractical for an aircraft.

The other major factor would be designing an impact resistant containment systems. Since ships massive and don't travel at high speed or high altitude, there not much of an impact to design for as there is for an aircraft. One can build a substantial containment on a ship or barge, but not so for an aircraft. Certainly one would also have to consider designing containment for missles, both natural and man-made - and this would be a simpler proposition for a ship/barge than for an aircraft.

Various governmental research organziations, e.g., US DOE, are aware of the KTA-40 and other systems, and their potential application.

Ah, I think you're misunderstanding me... I wasn't talking about the possiblity of this reactor being used for aircraft, or any reactor. I for one, believe batteries and capacitors will lead the way before we start lofting reactors to anything less than orbit.

As for the Russians respecting treaties and the countries they'll be using this in... my sum-total response is... I don't find that comforting. In fact, I find that about as comforting as a whale might a treaty while being hunted by the Japanese. We've developed "nuclear bunker busters" in theory at least, which is fancy for "GIANT ******* Groundburst from hell". I don't see personal or national responsiblity being a factor in this, especially over a long and uncertain future.

As I said Astronuc, I'm more concerned with how they're repurposing these reactors, the AVB-6M, but the 300MW reactor is the one that really makes me want to cry.

"Useful for oil exploration"... sounds like Siberia and the arctic to me, and the last time I checked no one saw eye to eye on who follows what rules where regarding he latter. The former, is of course, still Russian, as would be many places they send this to. Meanwhile the ocean and atmosphere seem less respectful of those lines on maps as well.

Worst case scenario: A 300MW reactor experiences catastrophic failure, and an exposed core ends in the damned ocean. It doesn't take nuclear engineer to figure out the result of that... just look at our naval testing of nuclear weapons, and why we stopped blowing them underwater. Airbursts are bad, Groundbursts are terrible, an burning core in the ocean would be beyond catastrophe.

 

[Frame Dragger]

Do people really think that a nuclear power plane crashing on land would be better than sea?

While I really don't like the thought of any reactors in situations where their containment could conceivably fail especially in uncontrolled areas. If a reactor were to fail outside of containment, out at sea seems like one of the better places to me. Please correct me if I'm wrong, but because the oceans are so large, if an accident occurs far from land, the contamination would have been significantly diluted by the time it reaches populated areas.

Also, speeds at sea are lower than air speeds. A sinking ship will hit the sea floor much more softly than an air craft crashing on land or sea. Conceivably, we could then work to recover, or contain the reactor lost at sea before it leaks.

Also, the risks of using nuclear reactors on submarines is somewhat more justifiable because they need high energy density to stay underwater. Nuclear reactors in space also seem justifiable to me for similar reasons. There are no real reasons planes in the atmosphere would need anything other than chemical and/or electrical energy sources. They simply are not in the air long enough to justify the need. You could just use fossil fuels, biofuels, hydrogen, batteries, super/ultra capacitors ect.

Currently, solar and radioisotope generators are able to provide enough power for propulsion. I wonder if you could create a hybrid radioisotope + fission source similar to accelerator driven systems.

Exposed reactor + Water = Radioactive Water + LOTS of thermal energy = Radioactive Steam boiling out of the ocean. Forget what leaking does to the immediate environment... I worry about a continuous plume of what would essentially be fallout.

 

[Astronuc]

Exposed reactor + Water = Radioactive Water + LOTS of thermal energy = Radioactive Steam boiling out of the ocean. Forget what leaking does to the immediate environment... I worry about a continuous plume of what would essentially be fallout.

Reactors, including Naval Propulsion Reactors, are designed to SCRAM, i.e., rapid shutdown (subcritical). If the reactor scrams, there is no power generation, except for decay heat. In the ocean, if the ocean water gets to the core (i.e., containment and primary cooling circuit are breached), that means a lot of cooling.

Conventional LWR reactors scram in about 3 seconds.

FD, I was responding to your comment "To respond to people thinking of nuclear aircraft... I don't see how that is worse than a nuclear reactor at sea; in fact I'd much rather see a meltdown in air than water."

A meltdown in the air would be certainly problematic to anyone close to the aircraft. A crash of an aircraft would have severe radiological consequences in the vicinity of the crash, much more so than a ship sinking at sea, ostensibly with an a scrammed/shutdown reactor.

 

[Frame Dragger]

Reactors, including Naval Propulsion Reactors, are designed to SCRAM, i.e., rapid shutdown (subcritical). If the reactor scrams, there is no power generation, except for decay heat. In the ocean, if the ocean water gets to the core (i.e., containment and primary cooling circuit are breached), that means a lot of cooling.

Conventional LWR reactors scram in about 3 seconds.

FD, I was responding to your comment "To respond to people thinking of nuclear aircraft... I don't see how that is worse than a nuclear reactor at sea; in fact I'd much rather see a meltdown in air than water."

A meltdown in the air would be certainly problematic to anyone close to the aircraft. A crash of an aircraft would have severe radiological consequences in the vicinity of the crash, much more so than a ship sinking at sea, ostensibly with an a scrammed/shutdown reactor.

If the reactor SCRAMs then yes, I have minimal concern. That said, as with an aircraft or any other situation, when I say "catastrophic failure" I'm thinking of a total failure of all possible safety measures. I realize how vanishingly unlikely that is, when you can just pump neutron poisons into the chamber, but then, presumably any airborne reactor would be shielded to withstand a crash.

My point is simply that the worst-cases don't even come close. I'm not against nuclear power, but this just seems needlessly riskly, and I don't know how quickly VBER-300 could SCRAM.

Let me ask you flat out: You have a VBER-300 in the air, or in the ocean, exposed, and critical. Which do you prefer in 'that worst of all possible worlds', that I concede is incredibly unliklely barring an added human element?

 

[Hologram0110]

Someone correct me if I'm wrong, but I would suspect that naval reactors would default to an off position in the event of any sort of failure. I know that many land reactor designs require power to keep the control rods out of the core by water pressure against a spring. If water pressure is lost because of loss of power for example, the rod returns to the off position without any intervention. I would assume that similar safety measures would be included for a naval reactor.

My concern for radioactive release would be due to physical damage to the core from outside source. IE Sub hit with torpedo, plane hit with missile, that destroys the reactor core spreading the fission products that are already in the core. In that case, I'd still rather it be in the ocean then air. The ocean can better dilute things and gives more time for radioactive decay before the release could get to populated areas.

Even if somehow the reactor were rigged to stay on, I think a reactor in water would better than air.

 

[mheslep]

If the reactor SCRAMs then yes, I have minimal concern. That said, as with an aircraft or any other situation, when I say "catastrophic failure" I'm thinking of a total failure of all possible safety measures. I realize how vanishingly unlikely that is, when you can just pump neutron poisons into the chamber, but then, presumably any airborne reactor would be shielded to withstand a crash.

My point is simply that the worst-cases don't even come close. I'm not against nuclear power, but this just seems needlessly riskly, and I don't know how quickly VBER-300 could SCRAM.

Let me ask you flat out: You have a VBER-300 in the air, or in the ocean, exposed, and critical. Which do you prefer in 'that worst of all possible worlds', that I concede is incredibly unliklely barring an added human element?

I'll comment on one aspect: the reactor actually in the ocean, submerged, is not going to stay critical very long, nor are the most radioactive fission products going to become airborne and travel hundred of miles over human populations as in the possible case of a aircraft crash on land.