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> I suspect the use of plutonium for RTGs in spacecraft is mainly because people already had a pipeline for extracting and concentrating plutonium for the weap

by qayxc 5y ago

> I suspect the use of plutonium for RTGs in spacecraft is mainly because people already had a pipeline for extracting and concentrating plutonium for the weapons industry. You suspect wrong. In order to be a decent RTG power source, an isotope needs to have: 1. Good power density, since the current energy conversion efficiencies are quite low 2. Good half-life, since it is aimed to operate the spacecraft as long as possible 3. Require little to no shielding, in order not to interfere with sensitive instruments aboard and also to protect humans (i.e. scientists and engineers working on the spacecraft) 4. High power/mass ratio, to minimize the propulsion requirements to launch it into space (which would be less of a factor in this context) Pu-238 is an ideal fuel due its very good performance in all of these characteristics. Nothing to do with nuclear weapons. The next best fuel would be Am-241, which is more readily available but is a more aggressive alpha-emitter and therefore performs poorer in (3). Strontium-90 has a lower power density and a significantly lower half-life. This makes it OK for terrestrial applications, where operation times of a decade are perfectly acceptable, but not ideal for interplanetary missions. Keep in mind that a lot of power capacity is lost during transit if we're talking about missions to Saturn and beyond. Cheap cost to LEO doesn't pull the outer planets any closer so probes will still take the better part of a decade to get to their destination. Overprovisioning is also not really an option, because excess heat needs to be radiated away, which is very difficult and costly to do in space.

ncmncm 5y ago
Strontium-90 has better power-density per unit mass than Pu, albeit with larger volume. Half-life is comparable. Safety is better: less gamma, no neutrons, no shielding needed. Overprovisioning is needed in every case, and excess power has to be dealt with the same way. Low cost to orbit means more fuel / more ∆V is practical to provide, thus much shorter transit times, translating to less up-front decay.
- qayxc 5y ago
  > Strontium-90 has better power-density per unit mass than Pu The power density of Sr-90 fuel is 15% lower by mass than Pu-238 - 0.46kW/kg thermal compared to 0.54kW/kg thermal. You must have been looking at the wrong figures. Since the Sr-90 fuel rod doesn't get as hot as a Pu-238 fuel rod of the same size, energy conversion efficiency drops significantly and you need up to 100% more Sr-90 fuel (and consequently more shielding as well) to get the same power. > Half-life is comparable. A factor of 3x is not really comparable (87.7 vs 28.7 years). > Safety is better: less gamma, no neutrons, no shielding needed. Where did you get that from? Strontium-90 and its daughter product Yttrium-90 emit high-energy beta particles and thus give off high energy bremsstrahlung which requires heavy shielding. The BUP-500 battery (the largest RTG ever built, designed to provide 500W electric after 5 years; 1.8m³ in size and 3.6 metric tons in weight) used a tungsten alloy for shielding; its predecessors used depleted uranium... Pu-238 on the other hand is pretty much exclusively an alpha emitter and has no short-lived daughter products (U-234 has a half-life of >200ka). > Overprovisioning is needed in every case, and excess power has to be dealt with the same way. Again, where did you get that from? A space mission is designed to a certain power spec and if you need your primary instruments for a 15 year mission, you plan your power such that after 15 years all primary instruments can still be powered. With Pu-238 that'd be about 75%-80% of the initial RTG capacity (some losses due to development time and efficiency losses from thermocouples are included). Due to its properties, Sr-90 would require about 3x as much fuel just to make up for its shorter half-life in addition to the up to 100% increase in required fuel mass from its lower temperature. Since it's a beta emitter, the additional radiation requires heavy shielding as well. So no, there's a pretty substantial difference there. > Low cost to orbit means more fuel / more ∆V is practical to provide, thus much shorter transit times, translating to less up-front decay. That's not how interplanetary missions work. Chemical rockets are incapable of providing enough delta-V to substantially shorten transit times to the outer solar system. If you want to do anything other than fly-by missions, more chemical fuel isn't going to help with that - you'd need nuclear or electric propulsion [1]. [1] https://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Pluto.pdf https://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Plu...
  - ncmncm 5y ago
    Wikipedia claims strontium-90 emits 0.95W/g. But it also says SrTiO3 emits only 0.256W/g, which is consistent with your figure. So, I am obliged to conclude Wikipedia's figure is off by 2x, undermining my argument. That said, if there were no Pu, Sr-90 would have to do. It has the advantage of being very cheap, as radiothermal isotopes go, and not requiring G7-military-grade security. Its bremsstralung radiation is only in X-ray range, so easily shielded, yielding heat. When it gets cheap to loft 150 tons to orbit, in a couple of years, the economic and timing verities of outer solar system transport will be up-ended. Cassini was lofted at 6 tons, and took 7 years to get to Saturn. Much of its cost was driven by limits on mass. Given 150 tons to work with, it is hard to imagine any detail of the mission remaining similar, except its rousing success. It certainly would not have needed 7 years to get to Saturn. It might not even have needed RTG power. A four-year transit to Pluto using FRC direct fusion propulsion is more appealing than RTG-powered ion drive, with the additional advantage of much less likelihood of wholesale civilizational collapse in the interim. But FRC fusion needs development work; its research budget is currently diverted to dead-end Tokamak projects.

by qayxc 5y ago

ncmncm 5y ago

Strontium-90 has better power-density per unit mass than Pu, albeit with larger volume. Half-life is comparable. Safety is better: less gamma, no neutrons, no shielding needed. Overprovisioning is needed in every case, and excess power has to be dealt with the same way. Low cost to orbit means more fuel / more ∆V is practical to provide, thus much shorter transit times, translating to less up-front decay.

qayxc 5y ago
> Strontium-90 has better power-density per unit mass than Pu The power density of Sr-90 fuel is 15% lower by mass than Pu-238 - 0.46kW/kg thermal compared to 0.54kW/kg thermal. You must have been looking at the wrong figures. Since the Sr-90 fuel rod doesn't get as hot as a Pu-238 fuel rod of the same size, energy conversion efficiency drops significantly and you need up to 100% more Sr-90 fuel (and consequently more shielding as well) to get the same power. > Half-life is comparable. A factor of 3x is not really comparable (87.7 vs 28.7 years). > Safety is better: less gamma, no neutrons, no shielding needed. Where did you get that from? Strontium-90 and its daughter product Yttrium-90 emit high-energy beta particles and thus give off high energy bremsstrahlung which requires heavy shielding. The BUP-500 battery (the largest RTG ever built, designed to provide 500W electric after 5 years; 1.8m³ in size and 3.6 metric tons in weight) used a tungsten alloy for shielding; its predecessors used depleted uranium... Pu-238 on the other hand is pretty much exclusively an alpha emitter and has no short-lived daughter products (U-234 has a half-life of >200ka). > Overprovisioning is needed in every case, and excess power has to be dealt with the same way. Again, where did you get that from? A space mission is designed to a certain power spec and if you need your primary instruments for a 15 year mission, you plan your power such that after 15 years all primary instruments can still be powered. With Pu-238 that'd be about 75%-80% of the initial RTG capacity (some losses due to development time and efficiency losses from thermocouples are included). Due to its properties, Sr-90 would require about 3x as much fuel just to make up for its shorter half-life in addition to the up to 100% increase in required fuel mass from its lower temperature. Since it's a beta emitter, the additional radiation requires heavy shielding as well. So no, there's a pretty substantial difference there. > Low cost to orbit means more fuel / more ∆V is practical to provide, thus much shorter transit times, translating to less up-front decay. That's not how interplanetary missions work. Chemical rockets are incapable of providing enough delta-V to substantially shorten transit times to the outer solar system. If you want to do anything other than fly-by missions, more chemical fuel isn't going to help with that - you'd need nuclear or electric propulsion [1]. [1] https://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Pluto.pdf https://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Plu...
- ncmncm 5y ago
  Wikipedia claims strontium-90 emits 0.95W/g. But it also says SrTiO3 emits only 0.256W/g, which is consistent with your figure. So, I am obliged to conclude Wikipedia's figure is off by 2x, undermining my argument. That said, if there were no Pu, Sr-90 would have to do. It has the advantage of being very cheap, as radiothermal isotopes go, and not requiring G7-military-grade security. Its bremsstralung radiation is only in X-ray range, so easily shielded, yielding heat. When it gets cheap to loft 150 tons to orbit, in a couple of years, the economic and timing verities of outer solar system transport will be up-ended. Cassini was lofted at 6 tons, and took 7 years to get to Saturn. Much of its cost was driven by limits on mass. Given 150 tons to work with, it is hard to imagine any detail of the mission remaining similar, except its rousing success. It certainly would not have needed 7 years to get to Saturn. It might not even have needed RTG power. A four-year transit to Pluto using FRC direct fusion propulsion is more appealing than RTG-powered ion drive, with the additional advantage of much less likelihood of wholesale civilizational collapse in the interim. But FRC fusion needs development work; its research budget is currently diverted to dead-end Tokamak projects.