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Comment by outworlder | Hacker News Reader

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.

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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.

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.

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/

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.

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.

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