Using Strontium as an Alternative RTG Fuel

[NuclearNut]

Well I was reading the americium RTG thread, and then I came up with an idea. Why not instead use something that is composes nearly 5 percent of "fresh" nuclear waste? What I am referring to is strontium 90. After looking at it I identified that it does indeed have a lower energy density, is a very reactive element, and has a shorter half-life than Pu-238, but Sr-90 does seem to have the advantage of not needing specialized production facilities, ie. it can be extracted from existing nuclear waste stocks. It also was developed by the Soviet Union when the Soviet Union existed, so there is already some data on the matter.

Anyway, what say you the denizens of the KSP forums about my (unoriginal) idea?

[PB666]

Dangerous to handle. half life is 28 years, which offers a reasonable power material. OToh, its very corrosive in the metalic form best used as a phosphate.

[NuclearNut]

Well, yes, it is a alkaline earth metal, and Sr-90 is radioactive, so yes, you do have a radioactive material that flames on contact with the air from the heat it produces, however that overall is not terribly dangerous. Not something you would want to be messing with in your home, but in small quantities and in a specialized reprocessing facility, probably one of the least nasty things there.

Now, you mentioned phosphate, wouldn't strontium oxide be more efficient as a solution, adding less mass for the same quantity of strontium, seeing that both are mostly inert.

[_Augustus_]

Good idea! Plutonium RTG's on probes always last way longer than they're needed to, anyway. If the Uranus Orbiter can't use the last 1 or 2 MMRTG's then it should just have strontium or americium ones.

[lajoswinkler]

There's no need to mention metallic strontium. LOL

Highly radioactive, chemically very reactive elements in their elemental forms are just a scientific curiosity. They are always used as compounds inert towards our atmosphere.

Strontium is chemically similar to calcium so it's neat to use it as a phosphate.

A phosphate of strontium will behave similarly to Ca₃(PO₄)₂, which is the main ingredient of the inorganic part of our bones. Its aqueous solubility is very low, with its K_sp being 1*10^-31.

To compare, K_sp (Ca₃(PO₄)₂) = 2.07*10^-33 which is two orders of magnitude less soluble, but at these exponents, it doesn't really matter. Both are practically insoluble.

⁹⁰Sr decays into ⁹⁰Y and it decays into ⁹⁰Zr. Their phosphates also have very similar solubility in water.

That being said, strontium-90 is already being used in RTGs for special applications.

[fredinno]

There's no need to mention metallic strontium. LOL

Highly radioactive, chemically very reactive elements in their elemental forms are just a scientific curiosity. They are always used as compounds inert towards our atmosphere.

Strontium is chemically similar to calcium so it's neat to use it as a phosphate.

http://image.wistatutor.com/content/feed/tvcs/periodic_table.GIF

A phosphate of strontium will behave similarly to Ca₃(PO₄)₂, which is the main ingredient of the inorganic part of our bones. Its aqueous solubility is very low, with its K_sp being 1*10^-31.

To compare, K_sp (Ca₃(PO₄)₂) = 2.07*10^-33 which is two orders of magnitude less soluble, but at these exponents, it doesn't really matter. Both are practically insoluble.

⁹⁰Sr decays into ⁹⁰Y and it decays into ⁹⁰Zr. Their phosphates also have very similar solubility in water.

That being said, strontium-90 is already being used in RTGs for special applications.

Where?

What about radiation sheilding requirements?

[cryogen]

Strontium is problematic because it's difficult to shield. Beta radiation itself doesn't go far but, when it stops, the impact creates secondary x-rays (bremsstrahlung) that are a lot more penetrating. It doesn't help that ⁹⁰Sr's betas are very energetic -- 2.28 MeV from its daughter ⁹⁰Y.

NASA appears (?) to consider it impractical.

There are lots of (heavily-shielded) ⁹⁰Sr generators, but none in space (as far as Google knows). The US tested a prototype (SNAP-17A) intended for space satellites -- this before solar photovoltaics were practical -- but it didn't fly. I can't find information about how they planned to shield it.

Strontium-90 is produced as a fission product during reactor operation along with ⁸⁹Sr, which has a half-life of 50.5 days, and other strontium isotopes (including the stable isotopes ⁸⁶Sr and ⁸⁸Sr). Yttrium-90, the decay daughter in secular equilibrium with ⁹⁰Sr, emits a very high energy beta particle. The intense high energy beta activities of ⁸⁹Sr and ⁹⁰Y, as well as the softer beta activity of ⁹⁰Sr, result in high-energy bremsstrahlung radiation for which heavy shielding is required. Unless the strontium has aged sufficiently, the 50.5-day ⁸⁹Sr also contributes a substantial fraction of the heat of the isotopic mixture.

ORNL-IIC-36, "Strontium-90 heat sources" (1971)

(pdf, 144pp.): http://web.ornl.gov/info/reports/1971/3445605716035.pdf

Two other radioisotopes possible for RTGs are the oxides of strontium-90 (Sr-90) and curium-244 (Cm-244). Sr-90 emits gamma radiation and Cm-244 emits both gamma and neutron radiation. PuO₂ emits much less gamma and neutron radiation than Sr-90 and Cm-244. Because gamma and neutron radiation are more penetrating than the alpha particles emitted by Pu-238, extensive shielding (not required with PuO₂) would be required during production and handling, as well as onboard the spacecraft to protect sensitive components. In addition, extensive development and safety testing would also be required, and production facilities for sufficient quantities of these radioisotopes are not available. Therefore, Sr-90 and Cm-244 oxides cannot be considered as feasible isotopic heat sources for the New Horizons spacecraft's power system.

"Draft Environmental Impact Statement for the New Horizons Mission" (2005)

(pdf, 195pp.) http://pluto.jhuapl.edu/Mission/Spacecraft/docs/NH_DEIS_Full.pdf

In order to provide a longer-lived radioisotope fuel, Strontium-90 (Sr-90), an abundant fission product with a 28.6-year half-life, was recovered from defense wastes at Hanford. A very stable and insoluble fuel form, strontium-titanate, was developed and widely used in terrestrial power systems. Because Sr-90 and its daughter Yttrium-90 emit high-energy beta particles, they give off significant bremsstrahlung radiation and require heavy shielding. However, shield mass is not as critical for most terrestrial power systems as it is for space power applications.

[...] The amount of Pu-238 that could be produced has always been a limiting factor in its use in space missions. Therefore, several other radioisotopes have been thoroughly evaluated for space use over the years. Sr-90 and Po-210 fuels were considered for use in higher powered military satellite constellations for which there were insufficient quantities of Pu-238 available. These programs were cancelled before they were completed, so these fuels were never used in space by the U.S.

[...] At that time, however, the radioisotope fuel of choice, Plutonium-238 (Pu-238), was unavailable due to AEC restrictions, and APL refused to use beta-decaying Strontium-90 because of the excessive weight associated with its necessary shielding. The AEC eventually acquiesced and agreed to provide the Pu-238 fuel. The SNAP-3 was converted from use of Po-210 to Pu-238, and acquired the new designation, SNAP-3B.

(pdf, 39pp.) http://large.stanford.edu/courses/2013/ph241/jiang1/docs/schmidt.pdf

ORNL-TM-3382

(pdf, 346pp.) http://web.ornl.gov/info/reports/1971/3445606041903.pdf

Edited September 24, 2015 by cryogen

[lajoswinkler]

Where?

What about radiation sheilding requirements?

Remote, polar sea lighthouses and buoys, often the ones established by the Soviet Union. Quite a bit of those are abandoned.

Strontium-90 source gives off plenty of penetrating gamma and beta radiation, and those betas, when they hit a metal like aluminium, produce a burst of röntgen rays.

Edited September 24, 2015 by lajoswinkler

[PB666]

There's no need to mention metallic strontium. LOL

Highly radioactive, chemically very reactive elements in their elemental forms are just a scientific curiosity. They are always used as compounds inert towards our atmosphere.

Strontium is chemically similar to calcium so it's neat to use it as a phosphate.

http://image.wistatutor.com/content/feed/tvcs/periodic_table.GIF

A phosphate of strontium will behave similarly to Ca₃(PO₄)₂, which is the main ingredient of the inorganic part of our bones. Its aqueous solubility is very low, with its K_sp being 1*10^-31.

To compare, K_sp (Ca₃(PO₄)₂) = 2.07*10^-33 which is two orders of magnitude less soluble, but at these exponents, it doesn't really matter. Both are practically insoluble.

⁹⁰Sr decays into ⁹⁰Y and it decays into ⁹⁰Zr. Their phosphates also have very similar solubility in water.

That being said, strontium-90 is already being used in RTGs for special applications.

And you can precipitate quantitatively Sr2+ by adding it to a 0.66 molar equivalents of Na₃PO₄ and and heating it in an autoclave.

So that you can take strontium waste, basically add hydrochloric acid to dissociate it forming dissolve SrCl₂ then neutralize this with something ammonium, then precipitate the Strontium phosphate with with Na3PO4 by heating under advanced pressure, centrifuge the pellet and dry completely. Mix thoroughly with powder potassium bromide and you can make a solid. Wrap the thermocouple, around the solid cylinder and insert into a b/gamma sheild such lead embedded ceramic or heavily leaded glass, hole for electrode at one end that would be filled with a plastic lead embedded material. A leaded ceramic lid with a hole for the other electrode, insert the wire, and fuse the lead to the device with clamps.And there you have it, a completely less dangerous way of making electricity than a solar panel could otherwise make more safely. Note you will probably need a copper core alongside the electrodes to manage heat inside the unit.

You really don't care about the solubility of Y and Zr once you have made your solid, since in the above designed the casing in made of inert ceramics (having been to Nagasaki A-bomb museum, those ceramics will discolor a little, but otherwise hold up well to the conditions of a nuclear blast). You might want to ceramic coat the thermocouple to protect its chemical composition and extend its life.

Now who is going to volunteer to dissociate, neutralize, precipitate& autoclave, dry, KBr embed and sheild. Think before you do this we best invent suitable robots first.

[lajoswinkler]

And you can precipitate quantitatively Sr2+ by adding it to a 0.66 molar equivalents of Na₃PO₄ and and heating it in an autoclave.

So that you can take strontium waste, basically add hydrochloric acid to dissociate it forming dissolve SrCl₂ then neutralize this with something ammonium, then precipitate the Strontium phosphate with with Na3PO4 by heating under advanced pressure, centrifuge the pellet and dry completely. Mix thoroughly with powder potassium bromide and you can make a solid. Wrap the thermocouple, around the solid cylinder and insert into a b/gamma sheild such lead embedded ceramic or heavily leaded glass, hole for electrode at one end that would be filled with a plastic lead embedded material. A leaded ceramic lid with a hole for the other electrode, insert the wire, and fuse the lead to the device with clamps.And there you have it, a completely less dangerous way of making electricity than a solar panel could otherwise make more safely. Note you will probably need a copper core alongside the electrodes to manage heat inside the unit.

You really don't care about the solubility of Y and Zr once you have made your solid, since in the above designed the casing in made of inert ceramics (having been to Nagasaki A-bomb museum, those ceramics will discolor a little, but otherwise hold up well to the conditions of a nuclear blast). You might want to ceramic coat the thermocouple to protect its chemical composition and extend its life.

Now who is going to volunteer to dissociate, neutralize, precipitate& autoclave, dry, KBr embed and sheild. Think before you do this we best invent suitable robots first.

Why do you think this method would work? There's number of flaws in it. What's "strontium waste"? If you mean spent fission fuel, that thing has a great deal of periodic table inside and requires much more procedure steps than described here.

Not to mention that macroscopic amounts of Sr-90 are abominably dangerous. To approach a small analytical crucible with few grammes of its phosphate sitting on a desk would mean almost certain death.

I'd do it, but not without this.

[SciMan]

Why do we keep considering that beta radiation to be such a problem?

Sr90 decays by emitting beta radiation, so use Betavoltaics along with the standard thermocouples for increased efficiency.

Radiation shielding can be accomplished by using Depleted Uranium (U238).

A lead shield needs to be 5x thicker than a U238 shield, for the same level of protection.

U238 is therefore a better choice on a shielding/mass basis, because it's 5x as effective but only ~40% more dense.

As I see it, the problems stop once the thing's in a stable high orbit or on an escape trajectory, anyways. Unless of course it's being used on a manned spacecraft.

If that process could be sped up it would be much safer (insert Sr90 RTG during fueling of the launch rocket, for example).

On the other hand, I agree with not wanting to work with the stuff unless I have access to a Hot Cell.

Edited September 25, 2015 by SciMan

[passinglurker]

Good idea! Plutonium RTG's on probes always last way longer than they're needed to, anyway. If the Uranus Orbiter can't use the last 1 or 2 MMRTG's then it should just have strontium or americium ones.

a radioisotopes output scales down linearly over the course of it half life until it is essentially halved (hence the term half life) it will then scale down to half of that over its next half life cycle so on and so forth until nuclear physics things I'm not qualified to explain happen.

This means that mission planers need to plan for this drop in power which over the ~20 optimistically expected life of a deep space probe could drop by ~15% (give or take ~5% cause I'm not an expert) using pu238. This effect would be even worse with su90 making it only suitable for shorter missions which then makes it impractical because any nuclear power supply period would be so expensive you'd be better off buying multiple redundant solar probes. Nuclear power is only ideal for long life missions very far away from the sun su90 isn't suitable for this.

am241 on the other hand suffers negligible power drop over a 20 year probe life because its half life is ~400 years and like su90 comes from more common waste its even already used to make smoke alarms making it at the very least a "better than nothing" option.

I vote we get Ahmed to do it and bring it to school...

This is just the worst and represents either very poor taste or poor choice of words on your part sir. A kid shouldn't be ostracize and paraded through his school in hand cuffs for the sake of an overbearing security theater because he was being clever, creative, inventive, having an electronics hobby, etc... no matter peoples paranoia.

[justidutch]

… This is just the worst and represents either very poor taste or poor choice of words on your part sir. A kid shouldn't be ostracize and paraded through his school in hand cuffs for the sake of an overbearing security theater because he was being clever, creative, inventive, having an electronics hobby, etc... no matter peoples paranoia.

I apologize, and have since deleted that post. I actually was trying to be funny, I guess I must have misunderstood the post I was replying to. I know the kid is clever, creative and inventive, which is why I thought he would be able to build whatever device is being talked about here. My bad.