An idea for a Pu-238 replacement for NASA space missions

[Evanish]

So I was reading this article about http://www.wired.com/2013/09/plutonium-238-problem/all/ and it got me thinking about possible substitutes. This is what I came up with.

You make thin sheets of beryllium mixed with some kind of alpha particle emitter. You also make thin sheets of some substance that is good for capillary action which doesn’t absorb a lot of neutrons. I think carbon nanotubes might work. You then layer these sheets together in even proportions, and maybe roll the result up into a cylinder. When this gets to space you can expose it to a solution containing uranium enriched to whatever degree makes sense. It will get sucked up into the nanotube sheets through capillary action.

This way you would have three sources of thermal energy. The alpha emitter (maybe Americium-241 would work) will release energy, When some of those alpha particles fuse with beryllium it will release more energy plus neutrons and some of those neutrons will encounter uranium-245 causing it to fission releasing even more energy. This way you can get fission energy without needing a chain reaction.

 

[Vanadium 50]

Did you work out the numbers for this idea?

 

[Evanish]

Did you work out the numbers for this idea?

I'm afraid that is beyond my ability (I have a B.S. in accounting, I wouldn't even know where to start) so instead I stuck it here hoping someone else might work out if it's doable or not.

 

[Drakkith]

I'm afraid that is beyond my ability (I have a B.S. in accounting, I wouldn't even know where to start) so instead I stuck it here hoping someone else might work out if it's doable or not.

I hope this doesn't sound rude, but most ideas people put forth about a subject they have little knowledge about turns out to be a bust. We see it here all the time.

 

[Evanish]

I hope this doesn't sound rude, but most ideas people put forth about a subject they have little knowledge about turns out to be a bust. We see it here all the time.

I understand that, but it's so boring to keep my thoughts to myself all the time. At least sharing it here I might learn a thing or two.

I was thinking maybe it would be simpler to use solid fuel. You would have to have some mechanic way of inserting the two together because you wouldn't want fission to start happening while it's still on earth. Maybe you could include some burnable neutron poison to keep the power relatively constant like they do on ships.

I was wondering how would I find out what I needed to know in order to evaluate this idea myself?

I figure I would need to know the cross sections for alpha particle beryllium for alpha particles from Americium-241. I also think I would need need to know the cross section for the neutron produced and all the various thing they may encounter. How do people figure stuff like this out?

 

[Drakkith]

Usually they spend about 6-8 years going to school to learn about this stuff, followed by a career in the nuclear industry.

 

[mfb]

The idea is certainly not new - small neutron sources are used to "ignite" fission weapons. If they would be scalable and cheap enough, we would run nuclear reactors with them as you could easily avoid criticality issues. For sure someone (probably multiple independent groups) checked if they could be used for space probes. They are not used in space, so something has to be too problematic. I don't know what the main point is, but some issues I can think of:

- realistic fission rates could be too low to make it practical
- the system could be too expensive
- intense neutron and gamma radiation is bad for electronics, you would probably need some shielding which adds to the total mass

Pu-238 is almost a pure alpha emitter, so shielding is trivial.

 

[Evanish]

The idea is certainly not new - small neutron sources are used to "ignite" fission weapons. If they would be scalable and cheap enough, we would run nuclear reactors with them as you could easily avoid criticality issues. For sure someone (probably multiple independent groups) checked if they could be used for space probes. They are not used in space, so something has to be too problematic. I don't know what the main point is, but some issues I can think of:

- realistic fission rates could be too low to make it practical
- the system could be too expensive
- intense neutron and gamma radiation is bad for electronics, you would probably need some shielding which adds to the total mass

Pu-238 is almost a pure alpha emitter, so shielding is trivial.

Thanks for the info!

 

[Astronuc]

This way you would have three sources of thermal energy. The alpha emitter (maybe Americium-241 would work) will release energy, When some of those alpha particles fuse with beryllium it will release more energy plus neutrons and some of those neutrons will encounter uranium-245 causing it to fission releasing even more energy. This way you can get fission energy without needing a chain reaction.

The problem is that the Am and Be would be mixed and one would have a fissioning system working during launch. Ideally, fission systems are shutdown.

Primary neutron sources have use Pu-Be in the past for startup of nuclear reactors. Now Cf-252 sources are used. Secondary sources use Sb-Be in which the Sb is activated and a decay product is a high energy gamma ray which dissociates the Be and produces a neutron.

Pu-238 has the right half-life and power density, and is currently preferred of small thermal generators in space.

 

[QuantumPion]

It is possible to design a small fission reactor that works with no moving parts (e.g. controlled by material properties i.e. thermal expansion) and produces a constant power output once activated. This would be a lot more practical.

 

[Hologram0110]

I've thought up something like this before too. It is basically a sub critical reactor driven by an internal neutron source.

There are some positives and negatives to such a system. A big disadvantage is the radiation from neutrons and fission products which can damage to some equipment. This can be problematic for the thermal generator materials (basically thermal couples). Neutron cross-sections are also annoyingly small meaning unless you have a lot of fissile material or a moderator a lot of your neutrons will leak without providing any power at all. As your effective multiplication factor approaches 1 you get significant increase in power for the same source neutrons. However, to start getting close to 1 you end up with a small nuclear reactor. For a sufficiently large multiplication factor it becomes questionable why include the external source for anything other than start up. Basically this design ends up being optimized towards a small nuclear reactor.

Now if you could find an isotope that functioned both as the alpha source and fissile target this might make it worth it. In this case the only increase in weight is from including the Be. This might make it worth it provided your electronics on board are sufficiently radiation hardened.

 

[Evanish]

The problem is that the Am and Be would be mixed and one would have a fissioning system working during launch. Ideally, fission systems are shutdown.

Primary neutron sources have use Pu-Be in the past for startup of nuclear reactors. Now Cf-252 sources are used. Secondary sources use Sb-Be in which the Sb is activated and a decay product is a high energy gamma ray which dissociates the Be and produces a neutron.

Pu-238 has the right half-life and power density, and is currently preferred of small thermal generators in space.

You wouldn't want to have fission until it was in space, could you prevent it by only introducing the uranium after it gets into space?

It is possible to design a small fission reactor that works with no moving parts (e.g. controlled by material properties i.e. thermal expansion) and produces a constant power output once activated. This would be a lot more practical.

That's really interesting. If I'm understanding it right you're saying it becomes critical expands looses it's criticality cools off shrinks then becomes critical again.

I've thought up something like this before too. It is basically a sub critical reactor driven by an internal neutron source.

There are some positives and negatives to such a system. A big disadvantage is the radiation from neutrons and fission products which can damage to some equipment. This can be problematic for the thermal generator materials (basically thermal couples). Neutron cross-sections are also annoyingly small meaning unless you have a lot of fissile material or a moderator a lot of your neutrons will leak without providing any power at all. As your effective multiplication factor approaches 1 you get significant increase in power for the same source neutrons. However, to start getting close to 1 you end up with a small nuclear reactor. For a sufficiently large multiplication factor it becomes questionable why include the external source for anything other than start up. Basically this design ends up being optimized towards a small nuclear reactor.

Now if you could find an isotope that functioned both as the alpha source and fissile target this might make it worth it. In this case the only increase in weight is from including the Be. This might make it worth it provided your electronics on board are sufficiently radiation hardened.

Your post is really interesting. Thanks.

I’ve been thinking about it some more, and I thought maybe it would be a good idea to add a moderator to the fuel. I did some searching and I came up with Uranium hydride.

What I’m picturing now is two different parts.

One part is a cylinder with a whole in the middle. The outer part of the cylinder is graphite to reflect neutrons inward and moderate them. Next is a mixture of an alpha producer and beryllium. In the middle there's a whole.

The other part is also a cylinder but longer and thinner. This cylinder would be composed of two parts. The output part would be structural maybe composed of beryllium. The next part would be the fuel, Uranium hydride, and some kind of burnable neutron poison to keep the balance as long as possible.

You would then need some kind of mechanical device to slowly move the skinny cylinder through the whole in the thicker one.

Even if this idea turns out to workable the radiation would still be a problem. I guess it makes more sense to just manufacture more plutonium 238.

 

[Hologram0110]

I’ve been thinking about it some more, and I thought maybe it would be a good idea to add a moderator to the fuel. I did some searching and I came up with Uranium hydride.

What I’m picturing now is two different parts.

One part is a cylinder with a whole in the middle. The outer part of the cylinder is graphite to reflect neutrons inward and moderate them. Next is a mixture of an alpha producer and beryllium. In the middle there's a whole.

The other part is also a cylinder but longer and thinner. This cylinder would be composed of two parts. The output part would be structural maybe composed of beryllium. The next part would be the fuel, Uranium hydride, and some kind of burnable neutron poison to keep the balance as long as possible.

You would then need some kind of mechanical device to slowly move the skinny cylinder through the whole in the thicker one.

Even if this idea turns out to workable the radiation would still be a problem. I guess it makes more sense to just manufacture more plutonium 238.

This is just a reactor where instead of a control rod for reactivity control it is a booster rod (positive reactivity instead of negative). Such a system has all the complexity and weight of a reactor (there have been reactors in space reactors).

RTGs fulfill a niche role of low/moderate power requirements for a long time mostly where solar isn't a viable option. They are excellent for the energy to weight density. Adding fuel, moderator and shielding destroys the weight benefit.

If you wanted to build a small lightweight reactor for space you would probably end up with a lightly reflected bare sphere of U235 (like the bit of a nuclear weapon). You could then design in some feedback mechanism that keeps the temperature under control automatically. This could be something like a material between the quarters of the pit which thermally expands (decreasing reactivity) when it is too hot. Such a system would probably produce far more power than an RTG and more than is required for most missions.

Pu-238 provides a power of 568 Wth/kg. The minimum critical mass is around 10 kg of Pu-239 (smaller for some exotic isotopes, maybe a reflector would reduce this but probably wouldn't help with the total weight). So the smallest critical reactor would have to produce around 56.8 kWth to be competitive with RTGs on a by mass basis. However this comes with the extra radiation, constrains on geometry (which makes cooling difficult) and also provides a single point of failure.

 

[QuantumPion]

Pu-238 provides a power of 568 Wth/kg. The minimum critical mass is around 10 kg of Pu-239 (smaller for some exotic isotopes, maybe a reflector would reduce this but probably wouldn't help with the total weight). So the smallest critical reactor would have to produce around 56.8 kWth to be competitive with RTGs on a by mass basis. However this comes with the extra radiation, constrains on geometry (which makes cooling difficult) and also provides a single point of failure.

The SAFE-400 is designed to produce 400 kWt at 512 kg (including gas turbine) to produce 100 kWe. The GPHS-RTG weighs 57kg and produces 300 W. So 10 times the weight for 300 times the power.

 

[Hologram0110]

@QuantumPion Does that include shielding?Even if it doesn't there is an obvious niche. If your probe only needs 300 or 400 W of power a full blown reactor is overkill (and likely adds more weight and safety issues).

 

[QuantumPion]

@QuantumPion Does that include shielding?Even if it doesn't there is an obvious niche. If your probe only needs 300 or 400 W of power a full blown reactor is overkill (and likely adds more weight and safety issues).

Probes only need 300 W because they are designed around the constraint of having that much available from an RTG. With kilowatts of power you can use more efficient and powerful electric propulsion drives, higher bandwidth transmitters, more power hungry experiments, etc.

 

[Hologram0110]

Probes only need 300 W because they are designed around the constraint of having that much available from an RTG. With kilowatts of power you can use more efficient and powerful electric propulsion drives, higher bandwidth transmitters, more power hungry experiments, etc.

Absolutely. I'm not denying that space based reactors have huge potential. They certainly scale to high power far better than RTGs. I was just making the point that if you only want 300 W than an RTG is far lighter, simpler and more reliable than a 300 W reactor (for such a limited power). Right now probes mostly use RTGs to charge batteries which allows for greater peak power.

In my opinion any sort of deep space manned mission will likely require a nuclear reactor. That much power is simply too useful for propulsion and life support systems.

 

[Petermaster]

The idea of a sub-critical-RTG was actually proposed in 2011 by researchers at MIT, and published at the British Interplaneary Society, as "Advanced Subcritical Assistance Radioisotope Thermoelectric Generator: An Imperative Solution for the Future of NASA" ... F. J. Arias (2011), JBIS, 64, 314-318. An proposed at NASA and then developed in Idaho nuclear space center 2013.
Basically, the Pu-238, being an alpha-emitter is use to assist the production of energy from the direct radiative decay by subcritical multiplication.
Initially you could use any other alpha-source, as Americium or Curium, or reducing the amount needed of Pu-238, However, Americium or Curium, have the problem of Gamma radiation and then heavy shielding needed. So, reduction of Pu-2238 is preferable.
Because the energy from fission is on 200 MeV and the energy of alpha decay is on 5 MeV, then, even considering the very low neutronic production from the subcriticality a 5-10% additional energy is obtainable.