Mars rover mission will use pioneering nuclear power source | Hacker News Reader

Mars rover mission will use pioneering nuclear power source

nature.com

82 pointsgmays58 comments

That seems like a big deal.

RTGs have been constrained by short supply, the department of energy is the only producer in the US.

This is 'just' a heating device for now, but if they start delivering RTGs, as alluded to in the article, that would be fantastic.

Americium is the same stuff that's in smoke detectors, albeit in tiny amounts.

Short supply, specifically, because the stuff has to be more meticulously tracked than elephant ivory. If the wrong people get their hands on enough of it, it is, at best, a dirty bomb.

The short supply is because plutonium-238 is difficult to make in relatively pure form. It's found in spent power reactor fuel, but only in tiny concentrations and mixed with much larger quantities of other plutonium isotopes.

The United States started making Pu-238 in useful quantities again last year:

https://www.ornl.gov/news/pu-238-shipment-quantity-opens-tap...

https://www.energy.gov/ne/articles/us-department-energy-comp...

The process requires irradiating neptunium-237 (itself a small portion of spent nuclear fuel, requiring special chemical separation) in a government reactor.

It's also true that Pu-238 is dangerously radiotoxic if inhaled or ingested, but that's true of every material that provides a useful quantity of decay heat, including the americium-241 alternative used in this new mission.

The primary advantage of using Am-241 over Pu-238 is that it's easier to recover from spent power reactor fuel; there's more of it in spent fuel than Pu-238, and it's the most abundant isotope:

https://en.wikipedia.org/wiki/Isotopes_of_americium

Which means that Am-241 can be separated chemically from old fuel and does not require any additional nuclear treatment before use as a radioactive heat source.

That's not why it is in short supply. Plutonium based RTGs use Plutonium 238, a non-fissile isotope. Being an alpha emitter, it is a terrible choice for a dirty bomb - certainly no better than any other potential RTG fuel candidate. Obviously it is a radioactive isotope, and a hazardous one at that, but it is not meticulously tracked for security purposes. Indeed up until lithium batteries rendered them obsolete, Plutonium-238 was used in nuclear powered pacemakers.

Plutonium-328 is a trace impurity in general plutonium production, but production of RTG-grade Plutonium-328 needs to be done in special reactors, where Neptunium-237 is bombarded with neutrons. The US shut down its only Pu-238 producing reactor in 1988, along with the rest of the Savannah River Site. Since then, Russia has been the sole source of Pu-238. Since 2013 the US has been trying to set up a new Pu-238 production plant at Oak Ridge National Labs, but even at full scale production it can still only produce 1.5 kg per year.

Uh huh, yeah, uh-huh,

> nuclear powered pacemakers

?!?!

Thank you for a bit of shocking weirdness I had no idea about. Good article: http://large.stanford.edu/courses/2015/ph241/degraw2/

Suppose we could try grabbing one of the hundreds of abandoned generators floating around Northern Russia.

https://bellona.org/news/nuclear-issues/radioactive-waste-an...

Those use strontium-90, which decays much faster than americium-241. That makes it hotter, at first, but its useful life is less. Being more than 4x less dense, you might carry enough more to make up the difference (and discard the extra heat, at first) if you have the room.

Strontium-90 decays by beta emission, ending up as stable zirconium, meaning you don't have to vent helium.

For more efficiency than an RTG, slap a Stirling engine on the heat source (https://en.wikipedia.org/wiki/Stirling_engine). Probably not practical, since the necessary radiators would be huge and it introduces mechanical complexity.

The radiators don’t have to increase in size really. If it’s more efficient you don’t have to reject as much heat.

But definitely a large increase in complexity as Stirling engines are mechanical devices and thermoelectrics are solid-state. The Stirling engines can be over 25% efficient rather than like 6% for thermoelectrics.

NASA has operated a Stirling engine in a lab for 17 years so it’s feasible, at least.

Only one of 16 has lasted this long. Another lasted 12 years. I can’t find numbers for all of them.

Got any links? I wonder what fails.

ESA finally using nuclear power? Hallelujah. They actually learned from the mistake of Rosetta's Philae landing in the shade on Comet 67P, where its solar panels couldn't produce any power and it died humiliatingly.

Anyone know what temperature this runs at internally?

If the black body radiation is high enough frequency maybe we could tune a special solar panel to pick it up and be dramatically more efficient than thermal electric.

They've built those in the lab decades ago [0]. I don't know what the large drawback is that prevents them from being more widely used.

In the thermophotovoltaic case they were looking at emitter temperatures between 1,200 and 1,350 K.

[0] https://sci-hub.se/https://doi.org/10.1063/1.1867178 ("Thermophotovoltaic Converter Performance for Radioisotope Power Systems", 2005)

Blackbody radiation is the wrong approach. If you are going to slap a PV panel onto a radioactive source then tune it for the emitted radiation rather than the blackbody heat. Radiation from nuclear decay should be very narrowband, allowing for a more efficient PV solution.

There is no emitted radiation: the heat source self-absorbs its own alpha radiation and converts it into heat.

Arrayed as a thin film on a substrate, you can use the kinetic energy of the alpha particles directly for thrust. The substrate will need to dissipate some heat from nuclei emitted in the wrong direction.

What is the status of making power directly from e.g. alpha radiation?

You could also do https://en.wikipedia.org/wiki/Betavoltaic_device

A Wikipedia page[1] claims that Chandrayaan-3 used Am241 but the referred article does not mention anything.

[1] https://en.wikipedia.org/wiki/List_of_nuclear_power_systems_...

Reading the linked source the tells you that the RHUs were not installed on the rover due to mass constraints and might instead be used in future missions.

The propulsion module carried an experiment, though, which was wasn't used for providing actual heating and a test only.

- "It is thought[by whom?] that national security reasons delayed the disclosure to the media."

Ludicrous. More realistic is they were afraid of negative media coverage and protests about the potential public health hazard to their people. In the US, NASA goes to extreme lengths to qualify their space radioisotope devices, their survivability and how they fail in a failed rocket launch or unplanned atmospheric reentry. All of which is transparent to the public, and to independent review. There's AFAICT the opposite of such transparency and accountability here. It's not clear at all what degree of risk this (at-the-time secret!) radioisotope launch had, how the containment device was engineered and what potential (secret) flaws it could have.

So the new part is they didn't bother generating power in the RTG because they only need the heat? That doesn't seem very pioneering to me, it's just a pile of plutonium.

it's not plutonium. it's americium

The new part is that it's using Americium. That's it. It's a challenge to use the material for various reasons but the wonderful thing about it is that it's easier to get hold of apparently.

The UK, in fact, has a problem with its plutonium stockpiles being polluted increasingly by Americium and now there's something it can be used for.

There are significant disadvantages (lower power per weight, high gamma ray activity) and some upsides (longer half-life by a big factor).

Piles of plutonium are pretty difficult to come by.

I figured by 1985 it’d be available in every corner drugstore.

If you have good contacts to Gadaffi and the Libyans, you might get it from them.

Just have some pinball machine parts ready to trade.

While this wasn't available in 1985, if it's a pile of parts that add up to, say, Addams Family Pinball, then that's the going rate for at least 1.5 grams of weapons-grade plutonium from the DoE last I checked (NRC and possible state fees not included).

With that much and some careful engineering, I can probably generate 1.21 jigawatts of power.

Nawh, it's already obsolete with Mr. Fusion. Just add banana peels.

I was hoping there would be something here about using Stirling engines that use decay heat, but looks like they are just using the decay heat itself and not generating electricity. The space stirling engines that NASA has worked on in a few iterations made a lot more electrical power than the older RTGs, with the tradeoff of moving components and associated issues.

It's striking that this is only used on the lander to keep it warm until the rover deploys. Seems overkill if the lander is only used briefly.

If they only need heat, could they use nuclear waste?

That’s exactly what they’re doing. FTA:

americium-241, a by-product of plutonium decay

Doesn't talk about the downsides, which are mainly that Am-241 has about five times lower power density than Pu-238, and also needs additional radiation shielding.

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

There's no spacecraft-related reason to select this—it's strictly worse—it's just the US/EU are in a decades-long ongoing shortage of Pu-238, and this is the only available alternative. I don't think this is "pioneering" in any positive sense of the word. IMHO, it's embarrassing.

The half-life of americium-241 is about 432 years, that’s a huge advantage. The Voyager probes at half power and because of exponential decay it’s not easy to simply start with more Pu-238 to compensate. You need to deal with dumping exponentially more waste heat at the beginning when you want to extend their lifespan. Not everyone needs to last that long, but Opportunity died 15 years after launch plenty of time for a significant reduction in power output.

When the cost per pound to orbit was higher it was an obvious choice, but these days it’s more questionable.

Plutonium has a half life of almost 90 years which isn't too shabby. If you want ten more years at full power then you only need to add about 10 percent more. The exponent is not very big.

An americium system that starts off bigger is going to be worse for most kinds of mission. Especially Mars landers that are going to wear out one way or another.

It's still boosting the science profile of the mission to use it as test bed for Americium. There's a lot of science to be done in the Kuiper belt and further out where longevity may matter eventually simply because we wouldn't be sending repeat missions to each dwarf planet out there anytime soon.

So instead of 10% more of the high power density material, you want to use 500% more of the low power density material?

Weight of a single component isn’t the only consideration. Having less heat output on the launch pad is a real benefit. It may take a 200 year mission for the weight to balance out, but having less variation in output is a benefit on day 0.

Further there’s many reasons why someone managing a project may say no to 10% extra plutonium even if the project could benefit from a longer lasting power supply.

You can’t just slap 10% more without consequences. You now need to design a spacecraft to dissipate 10% more heat on all phases of the mission including when it’s well insulated sitting on a rocket ready for launch, for a mission that you want to last 20 years the initial heat load can be quite problematic.

And picking a specific end date means giving up on the long tail of overly successful missions. Spirt was sending good data from May 1, 2009 to March 22, 2010 while stuck and NASA kept trying to contact it until May 24, 2011. https://en.wikipedia.org/wiki/Spirit_(rover)

And you can't just slap in americium without consequences either.

Any energy source needs consideration initially. However it is a real advantage be able to modify one parameter without needing to adjust several others.

When it comes to an RTG lower power density is the same as more stable output and longer life since that's the other side of the half-life coin. So it's not "strictly worse", it actually has attractive features.

Yes, but not really, because the crossover point is at around 247 years.

The crossover point depends on how the weight is distributed between the radioactive material, the shielding, and the radiators.

The first two favor plutonium, the last one favors americium.

That's an very good point about the radiators—you need some overcapacity with a fast-decaying radioisotope, so you will need more heat dissipation.

It's still not a close call, as far as I can tell (I'm not a domain expert). If you plan for even 20 years operating life, then your initial heat dissipation (for Pu-238) would be 1.17 of the nominal power at t+20 years, due it its radioactive decay. The largest RTG, GPHS-RTG, has 13.0 kg of "housing and fins" [0]; if you scale that linearly by 1.17 that would add +2.2 kg.

On the flip side, the isotope mass that emits 4,500 Watts of heat goes from 8.3 kg of Pu-238, to 39.5 kg of Am-241—that's +31.2 kg.

This is relative to a total mass (GPHS-RTG) of 56.0 kg.

(To preempt anyone complaining that this are small quibbles: the total dry mass of New Horzions [1], which contained one of these RTG's, is 401 kg).

[0, pdf] https://ntrs.nasa.gov/api/citations/20080003866/downloads/20... (table 1 on page 3, "State-of-the-art RPS. Comparing existing GPHS-RTG with near-term MMRTG and SRG110")

[1] https://en.wikipedia.org/wiki/New_Horizons

Strontium-90 has advantages. At density 2.6 vs 12, you can afford to loft more of it to compensate for its 28y half-life, and still come out ahead provided you can dispose of the extra heat at first. Few missions need to run decades.

Any details on this number? I wonder if it accounts for the contributions from the decay products of Pu-238. Would the decay products even matter much?

Plutonium-238 decays to uranium-234: https://www.planetary.org/space-images/decay-of-pu-238-to-u-...

U-234 has a 246,000 year half life and can be safely ignored when calculating the remaining heat output.

Heh. I got confused by the article:

> ESA’s heater units will not only be a first for Europe, but the first anywhere to use americium-241, a by-product of plutonium decay that packs less power per gram than its predecessor

... except that decay occurs from Pu-241, a different isotope which is not what seems to be commonly used for RTGs, and has a half-life of only about 14 years.

Wow, such negativity.

> There's no spacecraft-related reason to select this—it's strictly worse—it's just the US/EU are in a decades-long ongoing shortage of Pu-238, and this is the only available alternative. I don't think this is "pioneering" in any positive sense of the word. IMHO, it's embarrassing.

No, but there are practical reasons to select this. It doesn't matter _why_ Pu-238 is unavailable, it matters that it is unavailable and this has been a heavy constraint on space missions. New Horizons almost got postponed (with a danger of cancellation) due to this. And even then it launched with 80% fuel.

The question is, can they deliver americium RTGs ? That would be great.

>It doesn't matter _why_ Pu-238 is unavailable

Why not exactly? Because we used to buy it from Russia because we stopped making it ourselves?

Anyways they reference here that it's expected to finally increase production to a reasonable amount soon.

[1]https://en.wikipedia.org/wiki/Plutonium-238

There is no spacecraft related reason to use this unless what you want is for your spacecraft to actually fly on schedule. Risk assessment from supply chain issues is part of engineering.

> the US/EU are in a decades-long ongoing shortage of Pu-238

Europe is and will continue to be. America is ramping up production [1].

[1] https://www.scientificamerican.com/article/behind-the-scenes...

The isotope used in space missions is plutonium-238. That Scientific American article is about weapons pits made of plutonium-239.

The US has a large surplus of stockpiled Pu-239 for weapons:

https://www.energy.gov/nnsa/articles/nnsa-issues-final-surpl...

Sadly, it is of no use for space missions.

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