Uranium powered RTGs?

[Der Anfang]

I don't get why it is that NASA and other space agencies decide to use plutonium? Especially for the Voyager program, since plutonium-238 has a very short half life compared to Uranium (or even thorium). Also plutonium is rare. So what's the deal? Why use plutonium when you could use something far more common and much more longer lasting for a space craft?

 

And yes I know the difference between an RTG and a nuclear reactor. RTGs use the heat created from the decay of the radioactive elements. 

Edited December 8, 2017 by Der Anfang

[NSEP]

I don't know chemistry/nuclear physics but i think its because Plutonium produces more energy. Just a quick geuss, correct me if im wrong.

[Hannu2]

Heating power of long living isotopes are negligible. The more atoms decay during time the more heat it releases.

Pu238 is technically relatively easy to make in nuclear reactor and there is well known technology to handle it and pack it in suitable form so that it is not very dangerous if it fall accidentally on Earth.

It would be possible to use fission reaction of U235 or some plutonium isotopes to produce much larger power and get high power ion drives and many interesting applications, but unfortunately, it is politically impossible to make and launch nuclear reactors because irrational anti-nuclear attitudes are so common with people and politicians.

[peadar1987]

Yeah, RTGs aren't nuclear reactors, they're just a block of material that decays and releases heat as it does so. You need a completely different material for an RTG than for a reactor.

[DDE]

The longer the halflife, the less radioactive the isotope is and hence the lower its specific power. You’d need a hilariously big pile of the relatively short-lived artificial U-233, never mind U-235, to produce meaningful voltage.

[Bornholio]

The half-life of uranium-238 is about 4.5 billion years, uranium-235 about 700 million years, and uranium-234 about 25 thousand years.  All too slow

Pu-239 has a half-life of 24,100 years and Pu-241's half-life is 14.4 years. Pu-239 is energetic but slow, pu-241 is fast but not energetic enough

Good materials for RTG's each has a high energy output per decay event and a reasonable half life

https://en.wikipedia.org/wiki/Plutonium-238 87.7 yrs

https://en.wikipedia.org/wiki/Strontium-90 28.2 yrs

https://en.wikipedia.org/wiki/Isotopes_of_polonium#Polonium-210 138 days

https://en.wikipedia.org/wiki/Americium-241 432yrs

[Exploro]

I'd like to point out it's not just the longevity of the isotope that affects how long an RTG can provide adequate power; it's also the life span of the thermocouples one has to consider. These components degrade over time; reducing how much heat energy is converted into electricity.

[Nuke]

5 hours ago, DDE said:

The longer the halflife, the less radioactive the isotope is and hence the lower its specific power. You’d need a hilariously big pile of the relatively short-lived artificial U-233, never mind U-235, to produce meaningful voltage.

ignoring that the thermocouples are very inefficient.

[PB666]

4 hours ago, Exploro said:

I'd like to point out it's not just the longevity of the isotope that affects how long an RTG can provide adequate power; it's also the life span of the thermocouples one has to consider. These components degrade over time; reducing how much heat energy is converted into electricity.

IOW there is no reason to have an isotope that has a half-life of 25,000 years when your probe will be out of communication range in 100 years and your thermocouple and the computer electronics on board died long before that.

In space weight is everything, if you have to add 100000 times more uranium to get the same localized heat as 1 volume of polonium-210 for shorter spaceflight. For longer spaceflight you want something that generates hot alpha particles (they are insulatable by paper thin shielding). https://en.wikipedia.org/wiki/Plutonium#Power_and_heat_source. ²³⁸Pu has a power density of 570W/kg having a half-life of 87.4 years, whereas the shortest , and is a natural product of uranium decay. ²³²U is a synthetic product and difficult to obtain has a halflife of 69 years. THe closest natural isotope is ²³⁵U which has a half life of 700,000,000 years meaning you would need millions of times more Uranium, a specific type of heat retaining insulation to generate. MORE IMPORTANTLY, at this weight of ²³⁵U you surpass the critical mass (52Kg), this is a very bad thing for space-craft . . . .well not spacecraft specifically. . . but the poor unfortunate soul that constructed the RTG that blew up half the town.

Long story short

²¹⁰Po- heat your feet in your soyuz moon going space craft.
²³⁸Pu - run your experiments in voyager (HL = 89 years)
²³⁵U - Make'm big'm bomb or or create Chernobyl frankenwolves.(critical mass is low but half-life is high)
²³⁸U - A slow degrading isotope of Uranium goes by the tradename DU (depleted uranium) good for blowing the crap out of any tanks that you don't happen to like.

Note: fast breeder reactors can generate ²³⁹Pu from ²³⁸U but this is irrelevant in space since the mass required to initiate and control the reaction are two great to make them practical.

 

 

Edited November 28, 2017 by PB666

[KerikBalm]

Assuming the same energy per decay event, a longer half-life means a lower power output. If they store the same total energy, and one releases half of it in 100 years, and the other releases half its energy in 1,000,000,000 years, you can imagine that the power output of the one that releases half its energy in 1,000,000,000 years is very very very low.

You don't want a super long halflife just as you don't want a super short halflife. You want the halflife to be only several times longer (not hundreds, not several *orders of magnitude* longer) than the mission duration so that power output is relatively stable over the mission duration, while minimizing the mass of the RTG.

Of course, at a certain point, a nuclear reactor is the way to go, but that would only be for some pretty high power demands.

[softweir]

The main reason for using Pu238 is its very low emission of ionising radiation for the amount of heat it produces. This leads to several advantages: it is relatively safe to process and handle; though not completely safe because (like all the really heavy metals) it is highly toxic, the radiation it does produce won't cause the thermocouple elements to degrade, and its radiation won't interfere with sensitive scientific instruments. (It produces alpha and beta radiation which can both be shielded with very thin layers of material.) Furthermore, the energy produced is directly proportional to the mass of Pu238 used, unlike Uranium where - to get a usable amount of energy - you must have a mass very nearly close to the critical mass: less than that and you get almost no energy, more than that and your RTG melts!

[YNM]

Yeah, I guess it has to do with the way things decay.

Pu-238 have lots of α decay in it's decay path. U-235 have some β decay. U-238 would be... too unreactive.

As long as you're utilizing the heat, you want lots of α decay. Maybe we could have γ-ray PVs ?

Edited December 1, 2017 by YNM

[wumpus]

On 11/28/2017 at 2:09 PM, PB666 said:

IOW there is no reason to have an isotope that has a half-life of 25,000 years when your probe will be out of communication range in 100 years and your thermocouple and the computer electronics on board died long before that.

A thermocouple is a pair (or more) of twisted wires of two different metals.  Of all the bits of a 100year+ spacecraft, that would have to be the easiest to design (of course,  I'd look into peltier devices for better efficiency, but don't expect them to last all that long).  The computer electronics I don't have much hope surviving more than a century.

I'd assume that any such device (Uranium powered thermal generator) would involve some type of criticality.  The obvious choice is for a capture burn decades or centuries after the initial burn, and to send messages back to  Earth when you got  there.  Thus it wouldn't be a RTG but some sort of ultra-simplified reactor (the original "Chicago Pile" had no cooling and produced nearly no heating/power).  The reason for going with the reactor is to not require an isotope that lasts the entire journey.

A better solution would be to use Americanium₂₄₁ this is a possible replacement to Pu₂₃₈ for those (EU and others who simply can't get the stuff, also Pu tends to alarm the general public).  It only puts out  1/4 the power  per gram, but a 432 year half life makes it almost certain to outlast any other component on the spacecraft (I think we will lose contact with Voyager due to running out of propellant to keep the antenna aligned, it should be a bit over 3/4 power in the RTG).

[PB666]

30 minutes ago, wumpus said:

A thermocouple is a pair (or more) of twisted wires of two different metals.  Of all the bits of a 100year+ spacecraft, that would have to be the easiest to design (of course,  I'd look into peltier devices for better efficiency, but don't expect them to last all that long).  The computer electronics I don't have much hope surviving more than a century.

I'd assume that any such device (Uranium powered thermal generator) would involve some type of criticality.  The obvious choice is for a capture burn decades or centuries after the initial burn, and to send messages back to  Earth when you got  there.  Thus it wouldn't be a RTG but some sort of ultra-simplified reactor (the original "Chicago Pile" had no cooling and produced nearly no heating/power).  The reason for going with the reactor is to not require an isotope that lasts the entire journey.

A better solution would be to use Americanium₂₄₁ this is a possible replacement to Pu₂₃₈ for those (EU and others who simply can't get the stuff, also Pu tends to alarm the general public).  It only puts out  1/4 the power  per gram, but a 432 year half life makes it almost certain to outlast any other component on the spacecraft (I think we will lose contact with Voyager due to running out of propellant to keep the antenna aligned, it should be a bit over 3/4 power in the RTG).

The electronics were made with 1960-1970 level of durability. Made in USA and overengineered.

By criticality you mean pulse subcriticality, allow the material to approach the prompt critical limit for a few seconds every hour just enough to create isotopes for heat generation. Again if you use a layered thermocouple design you would never get more than 30% of the heat energy from the reactor, the rest is waste. So as the units power output drops so that 70% of output < Wattage of radiant energy release at the desired radiator temperature you would pulse the reactor briefly and then let it idle. You never want a dry reactor to approach criticality, most of the metals have very low heat retention capability and will rapidly heat to disintegration temperature.

If you want to know more about Uranium styled reactors you need to read up on the Russian designs, they are the leading experts on these types of reactors in space.

Plutonium is very dangerous to work with, period, it can be rendered safe, but the biggest problem is that someone needs to work with really dangerous materials to get it into the safe state. I used to be a safety officer and we worked with more dangerous materials, suffice it to say whatever you can't imagine that might go wrong has the unfortunate problem of being the thing that happens. 

Here is the basic problem. We tend to think of cooling as something that has to be done to keep reactor cool. Cooling of a reactor is equivalent in function to turbocharging a rocket engine. The reason for this is that power is generated along the temperature potentials. If we removed steam from the equation then the best method of heat transfer and work generation is lost. So that cooling moves power from the reactor and to the surfaces that can generate power. If we remove all cooling you drop the output capacity of Uranium based reactors by magnitudes. The only thing that really limits the ability to produce more power in a fission electric steam generation system is that as you increase cooling capacity and thus power production your reactor coil inches closer and closer to prompt criticality and your margin of safety begins to disappear.

As per chernobyl the single moment that caused everything to 'uncontrolled' was the building up of resident steam on the uranium pellets. IOW parts of the reactor could not get rid of steam fast enough. In water steam conversion steam always builds to a point and is released and so heat flow needs to stay below the point were steam becomes static.