Reasons to be cranky about plutonium-238

[struggle4progress]

<snip> The U.S. government has decided it needs more plutonium-238 because current stocks will be gone by 2010. It has "promised" the substance will not be used for military purposes, but won't say what it will use it for. That's a secret. <snip>

With fears about terrorism, creation of space weapons, contamination, and the admission that plutonium 238 could be used in a "dirty bomb" (one that disperses radioactive material with conventional explosives)—drama was inevitable.

The hearing was remarkable in that not one person even hinted concern that plutonium production could hurt the valley's property values. It was unlike other local hearings in which everything from hotels and traffic to affordable housing have been held up as major threats to life here. The 200-member audience of primarily working residents clearly had greater dangers in mind.

The hearing was also remarkable in what the DOE spokesman conveyed. The proposal to manufacture plutonium-238 came off as a done deal. The government has already decided it will manufacture 238 and listed INL as its preferred site. It looks to be simply going through the public-hearing motions before a final decision is issued. <snip>

http://www.mtexpress.com/index2.php?issue_date=07-27-2005&ID=2005104416

[Spinzonner]

1. They only want to make glow-in-the-dark watches

[punpirate]

2. Pu 238 isn't really suitable for weapons...

... so my guess is that this is going for thermal power supplies for satellites. There's a new round of so-called "low-cost" spy satellites under development now, and that may be part of the cost structure--nuclear power supplies.

[Dogmudgeon]

3. That would be a LOT of satellites

I think those thermal reactors use 5-50 pounds of material, of which Pu238 is a smallish part. Perhaps a lot of reactors are being used by the military.

The secrecy isn't helping any. Isn't this government capable of doing anything without secrecy?

--p!

[punpirate]

4. This is the header paragraph of a news release from INL...

... issued last week:

"Based on nearly 40 years of operating experience and a well-documented record of continuous upgrades, Idaho National Laboratory scientists, engineers and safety professionals are confident the Advanced Test Reactor and other INL site facilities can safely support the proposed consolidation of nuclear operations related to the production of Plutonium-238 for space batteries and national security."

So, definitely, satellites and/or deep space missions. But, the "national security" bit has got me a bit perplexed. The upcoming batch of spy satellites was contracted out as twenty-eight or thirty, I think, with the possibility of more. Solar cells might be cheaper (except for all the deployment gear), but less reliable with all the junk in low orbit now, so that might account for some of it.

Maybe there are plans for some that operate at full capacity for extended periods in the dark?

The DOE put out a request for "expression of interest" to commercial operators in 1999:

"DOE's existing inventory of Pu-238 available for space
missions (approximately 9 kgs purchased primarily from Russia) will be
exhausted by about 2004. Though additional firm missions cannot be
specified at this time, some future space missions will require
Pu-238-fueled radioisotope power systems over the planning horizon of the next 20 to 25 years. A production rate of 2-5 kgs of Pu-238 per year would be sufficient to meet these projected long-term requirements."

They are talking about making 330 pounds. Thirty spy satellites? The Cassini battery was 70 kg total. 5 kg Pu 238 necessary per satellite?

Cheers.

[Dogmudgeon]

5. I'm short on hard data ...

A lot of it depends on the power requirements of planned spacecraft and the amount of power a given amount of Pu238 can generate.

The advantage to using non-solar energy systems, aside from reduced exposure to micrometeorites, is that they are easy to keep hidden. Paint the satellites black and give them a small profile. It might also just be easier to design spacecraft with batteries than with solar power collectors.

As for the Cassini battery, I'm not certain whether that 70 kg was the mass of the Pu238, or a total of isotope, matrix, container and electrical assemblies.

--p!

[punpirate]

6. The source with the 70 kg figure is likely wrong...

... according to Atomic Insights:

The only planned use of RTGs in the US space program in the near term is the unmanned, 1997 Cassini mission to explore Saturn. The Cassini spacecraft will be powered by three General Purpose Heat Source Radioisotope Thermal Generators (GPHS RTGs) each designed to provide 276 W of electrical power at the beginning of the mission.

The GPHS RTGs planned for use on Cassini are nearly identical in design to those used on Voyager 1 and 2 and Lincoln Experimental Satellites (LES) 8 and 9. These RTGs have been in continuous operation for 21 years (LES 8/9) and 20 years (Voyager 1/2). Nearly all of the degradation that these devices have experienced during their lives is due to the decay of the plutonium heat source.

The three RTGs that will be on the Cassini spacecraft each weigh 56 kilograms, have a diameter of 42.2 cm and a length of 114 cm. Like the SNAP-27 that served as a power source on the moon during the Apollo mission, the GPHS RTG consists of a cylindrical fuel supply surrounded by rings of thermocouples. Again, there are cooling fins attached to the cold shoes of the thermocouples.

Plutonium-238 is the fuel source. The fuel elements are modularized with each module producing approximately 250 watts of thermal power. The fuel modules are encased in a heat and impact resistant shell designed to prevent any postulated vehicle accident from releasing plutonium. Sufficient testing has been done to show that the modules will remain intact, even in a Challenger type launch explosion or a reentry accident.

This is the description of the power pack used on the lunar lander and for outside experiments:

SNAP-27 Characteristics

The SNAP-27 power supply weighed about 20 kilograms, was 46 cm long and 40.6 cm in diameter. It consisted of a central fuel capsule surrounded by concentric rings of thermocouples. Outside of the thermocouples was a set of fins to provide for heat rejection from the cold side of the thermocouple.

Each of the SNAP devices produced approximately 75 W of electrical power at 30 VDC. The energy source for each device was a rod of plutonium-238 weighing approximately 2.5 kilograms and providing a thermal power of approximately 1250 W.

Even though the only radiation from Pu-238 is alpha particles which require little shielding, it is necessary to use thick gloves when handling a 2.5 kilogram rod of Pu-238. The surface temperature will reach about 500 degrees C because of the energy being released by radioactive decay. After ten years of continuous power output, a Pu-238 based RTG will still produce 92% of its initial power.

One measure of performance that is often used for chemical storage batteries is the amp-hour. A modern battery might have a capacity of 1.5 amp-hrs/kg. The SNAP-27 power supplies demonstrated the ability to provide more than 4380 amp-hrs/kg during the four years that their performance was monitored. Similar RTGs have produced 24,000 amp-hrs/kg during a 20 year operating life and are still going strong.

Use of, say, Peltier-effect chips instead of thermocouples might raise the efficiency quite a bit, reducing the amount necessary, although engineering for those temperatures might be a bit of a trick. :) Or, they could be used on the outside of the container, or on the heat sinks on the cold end of the thermocouples in a kind of co-generation scheme.

Cheers.

[NNadir]

7. Here's real big reasons to make lots of Pu-238.

The formation Pu-238 is a natural consequence or recycling uranium from nuclear power plants. This has certain advantages.

There is only one serious risk associated with the use of nuclear power and even though it is already tiny, further minimization is desirable. I am speaking of course of the risk of weapons diversion. Pu-238 puts out considerable heat and of course, alpha radiation. Inclusion of Pu-238 in nuclear fuel therefore makes the manufacture of nuclear weapons from reactor grade plutonium even more problematic than it already is. This is because the heat and radiation reduce the stability of the chemical explosives on which nuclear weapons depend.

The radiation also makes nuclear weapons readily detectable and less easy to transport and handle.

The neutron flux from spontaneous fission in Pu-238 (also a property of Pu-240) lowers potential bomb yield and increases the difficulty of safe reliable assembly.

The inclusion of an isotope of lower weight than weapons grade Pu-239 with isotopes of higher weight (ie. 240, 241, and 242) complicates any attempt to effect isotopic separation.

The PU-238 acts effectively as a "burnable poison" in reactors at the same time that it is a fertile nucleus. This increases fuel burn-up, lengthens fuel lifetime, and reduces the volumes and mass of spent fuel. (Typical burn-ups today are in the range of 30,000 MW-days/ton, but there are fuel strategies that can raise this figure to more than 100,000 MW-days/ton.)

The PU-238 complicates any attempt at unauthorized isolation of plutonium as the high radioactivity limits chemical separations to very high tech equipment of the sort not available to poor countries.

It is also worth noting that the formation of Pu-238 necessarily leads to the destruction of Np-237. Np-237 is pretty much a worthless isotope, except in certain types of fast reactors. In addition, Neptunium, unlike the other actinides has a number of oxidation states of reasonable solubility. As the isotope is extremely long lived (half life 2,144,000 years). By contrast Pu-238 decays with a half life of around 87 years, decaying into the useful fertile isotope U-234. Moreover Pu has few soluble compounds.

Finally the use of Pu-238 to generate energy increases the total energy yield available from uranium. Pu-238 puts out about half a watt of energy per gram without fissioning. The decay product is itself a potential nuclear fuel after neutron capture. Indeed the use of U-234 may allow humanity the ability to eliminate or greatly curtail the use of isotopic separation plants. Such plants are always available for diversion to weapons purposes.

I expect that if humanity survives long enough to fully embrace nuclear energy (which is almost certainly the only way humanity will survive), Pu-238 will be a very important and widely produced isotope.

Even without space applications (for which in certain cases isotopes like Cm-244 or Sr-90 may be superior particularly in missions that will have short lifetimes), Pu-238 should hold an important place in nuclear technology.