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

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/Te...rGilleland.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 if your 747 crashes?

10-15 tons
I'm not sure that's fair. They were abandoned, doesn't mean they didn't work.
Yes, that's a problem. Doesn't mean the problem is intractable.

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

mheslep

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

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

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...on/q0195.shtml

aquitaine

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

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