November 29, 2005
Pluto/Plutonium

In case you wanted the facts about the risks from New Horizons' RTG, here's the Final Environmental Impact Statement.

But then, why let facts stand in the way of scaremongering?

ADDENDUM: I spent some time looking through the New Horizons EIS, and it makes for some interesting reading...

Volume I, Section 2 contains a technical summary of the mission and the RTG unit specifically. It also includes (at 2.3.1, page 2-18ff.) a description of alternative power sources considered. In a nutshell:

  • alternative radioisotopes are unfeasible -- only Sr-90 and CM-244 have equivalent radiothermal and mass properties, but both are gamma emitters which would necessitate extensive shielding on the vehicle and during ground handling, and there exist no production facilities for either material in the quantities that would be required for the RTG;
  • reducing the amount of plutonium used was feasible -- there exist a number of candidate technologies for conversion of heat to electricity at higher efficiency than the thermocouples currently used, but none of the technologies compatible with a (reduced) plutonium heat source would be ready in time for the mission; finally
  • solar power was considered and found to be wholly impractical -- sunlight intensity at Pluto (33AU) is less than 1/1000 that at Earth (1 AU), so a huge solar array (on the order of 1000 m2) would be required to power and heat the spacecraft, even with concentrators; the size of the solar array and its supporting structures and mechanisms would create additional problems for launch, attitude control, etc. that would render it unfeasible.

Chemical options (fuel cells, combustion, etc.) were not discussed, presumably because they too would be impractical due to mass, complexity, etc. Not sure what other "alternative power sources" that leaves, but I suspect that wind, geothermal, and biomass are even less promising candidates...

A summary of accident consequences (probabilities of adverse health effects and such) is at 2.4.3.2 (pg. 2-28ff.). The "1 in 300 chance of an accident leading to the release of plutonium" figure being thrown around is the "total probability of an accident resulting in a release across the entire mission" (pg. 2-30), and corresponds to a risk of 0.2 "health effects" ("statistically estimated additional latent cancer fatalities resulting from an exposure over a 50-year period to a release of radiocative material") in the total exposed population, local and global. Hardly the extinction of all life on Earth. It's interesting to note that that total risk isn't even the worst-case scenario presented in this section, it just has the "1-in-N" number with the highest fearmongering utility.

Section 3 describes the environment surrounding CCAFS, and the global atmospheric environment. Interestingly, at 3.2.5.2 (Pg. 3-24ff.) we find a summary of the sources and amounts of plutonium already present in the environment. The total amount of Pu-238 (the majority isotope in New Horizons' RTG) dispersed in the environment is estimated at 29,810 Ci. The RTG carries an "assumed inventory" of 132,500 Ci, but more likely to be in the range of 108,000-124,000 Ci...but...it carries its plutonium in small, armored ceramic pellets, making it exceedingly unlikely that any realistic accident could release sizeable portion of that total load (let alone all of it). (For perspective, the report helpfully points out that the Chernobyl accident of April 1986 released 100,000,000 Ci of various radionuclides into the environment.)

I'll have to get back to Section 4, Environmental Consequences tomorrow, as it's getting late. However, I did skim Volume II earlier today:

Appendix B provides general information on the behavior of plutonium in the environment and the human body. It is largely a summary of how plutonium interacts with the environment (in short, not very much), how it can enter the human body (inhalation is more problematic than ingestion), what it can do once there (irradiate lung tissue, and what little gets absorbed into the blood eventually ends up mainly in the skeleton and liver), and the mechanics of how the relevant form of radioactivity (alpha particle emissions) can cause damage at the cellular level. I was impressed to see an acknowledgement that the traditional "Linear, No-Threshold" model is increasingly questionable, and a discussion of alternative models of the relationship between dose and cancer probability.

Appendix C contains the "environmental justice" evaluation, in which (I kid you not) NASA evaluates the impacts on minorities if plutonium happens to be released in an accident.

Appendix D is the report's comic relief section, particularly Table D-3, Comments from Organizations and Individuals Submitted by Email. Ever wonder what happens to those letters Global Network, et al, implore their followers to send in during the public comment phase? Well, here they are in all their moonbat glory, along with dry, rational responses from NASA personnel who clearly have the patience of saints.

Posted by T.L. James on November 29, 2005 10:53 PM

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