Half a kilo of cosmic nuclear fuel reignites NASA's deep space dreams NASA has celebrated a shipment of half a kilo of plutonium oxide by the US Department of Energy, the largest since US production of plutonium-238 was restarted just over a decade ago. Plutonium-238 (Pu-238) is essential for NASA missions using Radioisotope Power Systems (RPS). RPS makes use of the natural decay of Pu-238 to …
RTGs basically use thermocouples to convert the heat produced by radioactive decay to electricity (and plutonium generates a *lot* of heat - I lump of plutonium will happily glow red hot from decay heat), however this process is very inefficient - the MRTGs installed on the Curiosity and Perseverance rovers generate about 110W of energy from 2,000W of heat - an thermal conversion efficiency of a tad over 5% (the waste heat is simply radiated away). I'm guessing that NASA are trying to find ways of improving the thermal conversion efficiency - this would obviously mean that they could use smaller RTGs to generate the same power (a big weight saving since RTGs are heavy) but it would also make the problem of dumping the waste heat that much simpler.
The terms RPS (Radioisotope Power System) and RTG (Radioisotope Thermoelectric Generator) were both used in the article. I was half wondering if that implied there might be some form of RPS that isn't an RTG, i.e. uses some more efficient method of converting the heat, but from a quick Google, NASA only seems to be talking about RTGs. I wonder if there's a reason is for this particular instance of acronym proliferation.
I was wondering exactly the same. The NASA document linked to near the end of the article seems to imply that RPS is a generic term and RTG is a subset - a particular type of RPS.
The NASA document goes on to mention several different types of RTG, such as MMRTG (in the Reg article), GPHS-RTG (General Purpose Heat Source RTG) and "Next Gen" RTG, which is an enhancement of an existing GPHS-RTG.
Other than that, I'm none the wiser and not bothered looking harder. If anyone else does please post here.
Yes, there are other ways. The advantage of the RTG is that the Thermo-electric effect it uses means that it's basically a solid-state system. It has no moving parts to generate it's power thus it's pretty much maintenance free, can be highly vibration and impact resistant (handy when you're planning to launch it on a rocket) and can be completely sealed (handy when you plan to land it on extremely dusty places like Mars or the moon).
The downside is that the Seebeck effect is most efficient at certain temperature differentials. Get the hot side too hot and you're not getting any more power/voltage out, get it too cool (or the cool side too hot) and you also lose output below a certain threshold. Even though there may be sufficient heat to use for something.
Other options to generate power from heat could be basically any other form of heat-engine, be it a Stirling cycle or Stirling cycle, organic Rankine cycle, use of thermogalvanic effects or whatever else you can think of. It may be possible to optimise some of these to work over a wider range of temperatures of the plutonium as it decays, producing more power as it is still very hot but also being able to still utilize more of the residual heat as it starts decaying and it's thermal output drops.
even with an 88 year half life, it is NOT that long when you think about weight and size.
In My Bombastic Opinion, an equal mass of nuclear waste, consisting of specific well known elements, might initially give you more heat and last just as long. The desired elements could theoretically be removed and isolated from the rest of the nuclear waste by chemical processes, then be formed into an appropriately sized thermal generator.
However with plutonium 238 (not 239) I expect you could still call it 'nuclear waste' since it cannot be used as bomb/reactor fuel. Yet we do not have very much, but lots of the other waste. So there ya go.
thermocouple efficiency is another thing. You need a high hot:cold temp difference. Peltier devices are commonly available, though. The tech could use some improvements I'd guess.
The great thing about plutonium is that it's fairly safe to handle and very predictable. It's long half life and predictable decay rate is also a good thing. Spent nuclear fuel has a LOT of different and nasty stuff in it. Pu-238 is a basically pure alpha emitter, which is easily blocked. Spent fuel will have a lot of other elements emitting Beta and Gamma radiation, requiring thick, heavy shielding and lots of precautions for safe handling before launch. Apart from that even if you manage to separate out all the other trans-uranics and decay products (not easy at all), a lot of the plutonium in spent fuel will be Pu-240, which isn't really suitable for an RTG as it doesn't really produce much heat as it has a much much longer half-life.
The approximately 88-year half life of Plutonium 238 is a certainly major limiting factor on the useful life of an RTG, but not the only factor. Helium gas is produced by the fuel decay (when a Plutonium 238 atom becomes Uranium 234 by emitting an alpha particle - a Helium nucleus). This tends to reduce the electric output because Helium gas has a relatively high thermal conductivity, which (slightly) reduces the temperature difference between the hot and cold ends of the thermoelectric conversion elements within the RTG. The lower the temperature difference, the less electricity is generated via the Seebeck effect. See: https://en.wikipedia.org/wiki/Thermoelectric_effect#Seebeck_effect
To counteract the buildup of Helium, some RTGs (in particular the SNAP-19 Viking RTG) had a reservoir of mostly Argon gas atop the RTG housing that would very slowly permeate through a Viton O ring into the RTG housing. Argon's lower thermal conductivity helped to maintain a higher thermal gradient across the thermoelectric couples.
Another life-limiting factor is degradation of the thermoelectric elements due to continuous exposure to high heat. The materials tend to very gradually sublime, which reduces their mass and thus the resulting magnitude of the Seebeck effect.
I did a quick google, Wikipedia states that in 1977 the RTGs were generating a total of 470 watts. As you say, Pu-238 has a half-life of 88 ish years, so would lose 0.79% of their power output each year. If that reduction was consistent then today they’ would be knocking out 299ish watts.
The actual figure is lower because the thermocouples that convert heat to electricity also degrade over time, in 2011 for instance, the Voyagers were generating 270 watts, rather than 343 watts.
As of Feb this year, Voyager 1 with it's 4 working instruments only has 8.3 watts to spare. Voyager 2 with it's 5 instruments only has 3.6 watts to spare. To keep comms up NASA will have to either turn another instrument off on each, or come up with clever ideas in typical NASA fashion (along the lines of how they kept the rovers going) to somehow ration the power between all working devices, outside of the original design.
Maybe if a more efficient RTG design had been available in the 70s, the Voyagers could have been kept in service say an extra decade, before reaching the point we are at now where NASA are having to power instruments down. Hopefully in the last 47 years, we've made some progress in power conversion efficiency that can be applied to future devices.
"Hopefully in the last 47 years, we've made some progress in power conversion efficiency that can be applied to future devices."
We do, but the temptation with better power sources is to use more of it. See the improvement in laptop batteries and power management, the end result being not a laptop that run for two or three days, but smaller, thinner laptops with the same battery life :-)
I do suppose there's an argument that being able to run 10 instruments rather than 4 or 5 after nearly 5 decades is preferable. No one expected that nearly 50 years later, the Voyagers are still discovering new science.
Yes, but the phones have bigger, brighter displays, louder audio, bigger memories and loads of internal funtions. But as a phone they are pathetic. My Nokia 3000 is better at text and phone and still lasts all week on one charge with a battery the size of a matchbox and half as thick. (the battery, not the phone)
The core of a nuclear weapon will steadily decay over time until at some point it won't go pop but just "fizzle" - the term for a nuclear detonation that really doesn't no nuclear.
The US is going through a process of recycling the warhead material and refurbishing warheads with new material from reactors hence the reason plutonium manufacturing is increasing. The UK, France, Russia, China etc will all have the same problem.
NASA certainly does deal with project requirements that few in the world have ever dreamed of, and does so on a near-daily basis. I could never see Pu being viable for an Earth-based power requirement, yet it is pretty much ideal for space exploration and longevity.
I can't say that I'm comfortable with having resumed nuclear weapon fuel production in order to produce the Pu, though. I have no doubt there are ways to intentionally degrade that nuclear weapons grade output sufficiently that it is useless for weapons.
The last thing this world needs is more nuclear weapons.
The Register 
Biting the hand that feeds IT
Copyright. All rights reserved © 1998–2024
