Americium 241 RTG

lambda0 · Newbie

Americium 241 RTG

GoogleNaut · Guru

I haven't heard af Am-241 being used in RTG's but it is currently used in smoke detectors to provide a source of ionization for the detection circuit.

From my CRC Handbook (2004-2005) I see that Am-241 has a half life of T1/2=432.7 years; and a alpha primary decay mode with 5.443 MeV (12.8%) and 5.486 MeV (85.2%.) So we can average the alpha decay energy to be about 5.371 MeV. The gamma-rays are of neglible energies and in any case, will probably penetrate enough to escape, so their energy contribution will be neglected here.

Given that the half-life is 432.7 years, this means that any given sample of Am-241 will have half of the original isotope in 432.7 years.

So, picking a nice round number, we will start with one mole of Am-241:

Our goal is to "build a unit bridge" to go from moles to watts of decay power:
Looking up Avagadro's Number: NA=6.02214*10^23 per mole. This is the number of atoms in a single mole of a substance, and is the same regardless of what that substance is.

[(1 mole Am241) * (6.02214*10^23 1/mole) * (1 decay/2 atoms) * (5.371 MeV/decay) * (1.602*10^-13 J/MeV)] / (432.7 yr * 31.557*10^6 sec/yr) =

(crunching the numbers...) : 18.97 Watts/mole

If 1 mole of Am-241 is about 241.06 g/mole, this works out to a specific decay power of about:

7.87*10^-2 Watts/gram. A 1 kilogram block of AM-241 would give off about 78.7 W, so 1 metric ton would give about 78.7 KW of heat. Interestingly enough, a 10kg sphere of this stuff would radiate almost 800 W of power (I guess notbody told that kid in the silly movie "Manhattan Project" about this little problem!)

O.K., trouble is, if I am not mistaken, Am-241 is also a fissile isotope, which means if you get enough of it together, you can get a fission chain reaction. The complexity of engineering needed to avoid that is going to be comporable to just building a fission reactor that uses the stuff for fuel in the first place. And if you were going to do that, why not just build a 'conventional' fission reactor using 'conventional' nuclear fuels like enriched uranium?

-- Edited by GoogleNaut at 19:00, 2007-04-25

lambda0 · Newbie

Hello

Am-242 is fissile, but I didn't know that it was also the case with Am-241. Am-242 has very different properties, and could even be used for propulsion, in a nuclear thermal engine, I am not sure that it's possible with Am-241.
The interest in using Am-241 for planetary RTG is the duration life of such a generator, compared to Pu-238, and it is also a nuclear waste that may be available in greater quantities than Pu-238.
We don't know what to do with our nuclear wastes, Am-241 is one of the most difficult to store (because of the heat generated by the decay !), so let's use those wastes to build RTGs (that also emits much less radiations than a true fission-based nuclear generator) for planetary exploration.

GoogleNaut · Guru

Interestingly enough, there is a growing percentage of nuclear advocates who promote the idea of waste reprocessing to extract the unburned 'actinides,' viewing them not as waste but as fuel. They argue that seperating fission products from unburned actinides will greatly enhance our fuel stocks, promote security by burning up fissile materials where they can do some good, and reduce the longevity of storage of spent fuels or nuclear wastes. As Jaro has pointed out to me several times, the actinides are mostly responsible for the need for long term storage because of their relative longevity. Extracting them from the fission product wastes allows them to be recycled, and burned; and the fission products--considered the "high level" shortlived waste, can then be stored for a much more modest few centuries. Centuries versus hundreds of millenia is a much easier engineering task--and this will not only help us out, but future generations as well.

Andrew · Senior Member

I recall that there was a space probe or two that used Am.

GoogleNaut · Guru

I don't think so--atleast not for a power source.

I think Sojourner may have used a Curium or Cerium source for it's rock composition analysis experiment--I don't think it used Americium. I know that it used several small Pu-238 heat units to prevent the batteries from freezing at night. Same with the two Mars rovers Spirit and Opportunity.

Andrew · Senior Member

No, I'm pretty sure there was a probe or two using Am-fueled RTGs, and I'm pretty sure that there is a reason why it wasn't used in later probes, if it is such an ideal fuel for RTGs.
Then again, I may only remember an analysis of possible fuels, which raises the question again, why don't they use it? Plu-283 is difficult to produce, while Am-241 is nuclear waste, so I think there is something wrong.

Is there a nuclear engineer specialising in aeronautics in the house?

10kBq Jaro · Senior Member

GoogleNaut wrote:
O.K., trouble is, if I am not mistaken, Am-241 is also a fissile isotope, which means if you get enough of it together, you can get a fission chain reaction. The complexity of engineering needed to avoid that is going to be comporable to just building a fission reactor that uses the stuff for fuel in the first place. And if you were going to do that, why not just build a 'conventional' fission reactor using 'conventional' nuclear fuels like enriched uranium?

Ty is right -- the fast neutron fission x-section of Am241 is about the same as Pu239, for neutron energies from 1MeV to 5MeV.

But as you can see in this graph, Pu238 (green trace), which is the standard RTG material, is slightly higher than either Pu239 (black) or Am2421 (red) :

The important thing is that the larger quantity of Am241 required to produce a similar amount of heat as Pu238, makes the criticality control issue more difficult for the former.

Andrew · Senior Member

And what about Am-242?

10kBq Jaro · Senior Member

There is nothing remarkable about Am-242, other than the curious fact that the ground state of this isotope has a much shorter half-life (16 h) than the m-isomer, Am-242m (152 years).
Am-242m is VERY remarkable in its huge thermal neutron fission x-section, by far the highest of any known isotope. Published tables usually list it as about 7000 barns, but of course that varies, depending on the exact neutron energy within the spectrum, as shown here (Am-242m is the green trace) :

Unfortunately, there is very little Am-242m to be found anywhere.
It could potentially be produced by neutron irradiation of Am-241, which is available in large quantities, but this tends to yield mostly Am-242 and Am-242f, which we don't want, or it causes Am-241 to fission (the "n,f" reaction).
The Am-242m yield could be maximized by filtering the neutrons to a window in the energy spectrum, as shown here :

Andrew · Senior Member

I'd thought there be some kind of problem like this, and my guess was rarity. 16 hours is too short for any practical probe mission.

Also, what does the "m" and "f" stand for after the atomic element? That it has more or less neutrons then protons?

10kBq Jaro · Senior Member

Andrew wrote:
what does the "m" and "f" stand for after the atomic element? That it has more or less neutrons then protons?

No, the isomers of an isotope have an energy state different from the ground state.
Thus Am242m and Am242f are the m and f excited nuclear isomers of Am242, which is the ground state of the same isotope.

The chemical equivalent is atoms whose electrons have been energized to a higher energy state, and subsequently fluoresce, when they drop back down to the ground state. The light they emit is characteristic of the difference in energy between the excited state and the ground state.

Same thing for nuclear isomers, except that the emitted photon is usually a gamma ray, instead of visible or infrared light, because anything happening in the nucleus of atoms (ie. involving changing proton or neutron energy states) is generally millions of times more energetic than chemical reactions involving electrons.....

See also http://en.wikipedia.org/wiki/Nuclear_isomer

lambda0 · Newbie

Andrew wrote:

Then again, I may only remember an analysis of possible fuels, which raises the question again, why don't they use it? Plu-283 is difficult to produce, while Am-241 is nuclear waste, so I think there is something wrong.

The period of Am-241 is about 430 years, against 90 years for Pu-238, which means that for the same power, a Am-241 RTG will be at least 5 times heavier, too much for a space probe. The shielding is also heavier.
So I don't think that a Am-241 RTG is interesting for space probes, a better application would be planetary energy source.

About Am-242 : the application is very different, there has been some proposals for a nuclear thermal engine that uses Am-242, with very high performances (the fission fragments engine, studied by Carlo Rubbia).

GoogleNaut · Guru

Stricly speaking, Pu-238 is also processed from spent fuel, I believe. It's just a little harder to get.

10kBq Jaro · Senior Member

GoogleNaut wrote:

Stricly speaking, Pu-238 is also processed from spent fuel, I believe. It's just a little harder to get.

If Pu-238 were to be processed from ordinary power plant spent fuel, it would require isotopic separation, because most of the Pu in SNF is comprised of the heavier isotopes, from 239 to 242.

In fact, Pu-238 for RTGs is produced in a two-step process that completely avoids isotopic separation:
First you irradiate U-235 to produce Np-237 (this takes 2 neutrons + a beta decay).
Then you separate out the Np chemically.
Further irradiation of the pure Np, plus a beta decay, yields pure Pu-238.

A good source for Np is spent fuel from NAVY reactors, since these use HEU.
Even without processing for Np, a good yield of Pu-238 can be obtained this way, although it is not pure.

Otherwise, specially made HEU irradiation "targets" must be used in research/ isotope production reactors, to get the necessary Np.

GoogleNaut · Guru

O.K., that makes sense to me. And since there isn't really a whole lot of spent HEU naval reactor fuel these days, that means Np-237 is fairly scarce. And so Pu-238 ends up being pretty darn expensive. As I recall, most of it came from former Soviet Union stocks...

rpjacks · Newbie

Perhaps my reasoning is skewd but couldn't Americium 241 be used for low output terrestrial RTG's?

The shielding for Am241 is only 18mm and even if each RTG only produced 400W of power, if more output were needed a second or even multiple RTG's could be used together to create the desired output.

Americium being a long halflife nuclear waste, it is desirable to utilize in a useful safe manner. I do understand that there are limitations to what can be utilized as a power source, but considering the ease of use as a terrestrial RTG and the relatively low risk when the mass is kept below the fissile range the exploration of this use should be considered.

Recent articles have provided another use of the element, in a direct power output battery. My view of this battery is that the Americium is being used to produce power, however it is spent in approximately 80 days and therefore is almost constantly being replaced. If this is the only application that becomes viable for space exploration then I think the more stable terrestrial application to be a superior endeavor.