From: Ron Baalke Wed, 24 Sep 1997 0:22:25 GMT To: astro@lists.mindspring.com Subject: [ASTRO] Cassini Plutonium For The Technically Minded Cassini Plutonium for the technically minded Jeff Cuzzi I'm sure we will all have friends and relatives asking us what's up with the Cassini Plutonium issue as launch approaches in early October. Allegations of risk have arisen due to Cassini's onboard RTG's (Radioisotope Thermal Generators) which derive electricity from decay of 72 lb (33kg) of Plutonium dioxide fuel. In anticipation, I wanted to provide some "derived from basic principles" satisfaction that the Cassini health threat is negligibly small even in the extremely small chance that anything does go wrong with the mission (either at launch or at flyby). The Cassini project has devoted more than ten million dollars to a thorough analysis of the problem, but the back-of-an-envelope analysis below is a little easier to grasp and serves as a calibration and sanity check. I am a Cassini scientist, and neither a health expert, nor a nuclear physicist. I do care about the health of the people of the world. I had several discussions with a physicist at the Nuclear Regulatory Agency (NRC) concerning decay rates and comparative relationships to health effects. I also had this reviewed by the President of the Health Physics Society, a 6500 member national organization (who has publicly stated that NASA has done a very good job and has, if anything, OVERestimated the health risks). For my initial health effect data I relied on Web sites maintained by the EPA and the Agency for Toxic Substances and Disease Registry (ATSDR; part of the Center for Disease Control - see references below); my NRC and Davis contacts confirmed these values and identified their primary source (FGR-11, 1988). I suspect anyone can reproduce the calculations below who can read a simple physics textbook and the World Wide Web. 238-Pu decays by alpha-particle emission (like the longer-lived weapons grade isotope 239-Pu, but 250x faster). The decay rate can be calculated from the half life (88 yrs) and the number of nuclei per gram, and is about 6E11 decays/sec/gm, defined as 17 Curie/gm. A Curie (Ci) of 238-Pu and a Ci of 239-Pu have the same radiation damage potential (they emit the same alpha particles). Because 238 decays faster, it has a higher Ci/g rating by the ratio of half lives (about 250). The convenient unit is pico-Curies (10^-12 Ci = pCi). Health standards are set by the International Commission on Radiological Protection (ICRP), and found in FGR-11 and the ATSDR web page. The conversion factors between radioactivity (Ci) and potential tissue damage in rem (Roentgen Equivalent Measure, or more often millirem (mrem = 10^-3 rem)) are from the FGR-11 (note 1). They can be derived from values on the ATSDR page as well. The ATSDR quoted Annual Limit on Intake (ALI) is 20000pCi/yr for "workers", and the corresponding dose limit is 5 rem/yr, giving a conversion factor of 0.25 mrem/pCi (note 1), in good agreement with the standard value of 0.29 mrem/pCi tabulated in FGR-11. Several expressions can be found for EPA-allowable levels of radioactivity. The ATSDR web page gives a mixture of recommended limits for the public and for "occuptional exposure" in rem, Annual Limits on Intake (ALI) in pCi/yr, and in Derived Air Concentration (DAC; pCi/m3) levels. These are generally consistent with a 10 times lower limit for the general public than for workers, but my NRC contact says the DAC's for the general public are maybe another 10 times smaller than can be inferred from this web page (probably factors for time off-job as fraction of 24 hr, etc). Also it appears that the 500 mrem annual limit for the public cited by ATSDR probably includes the unavoidable background level of 360 mrem/yr from Radon gas, cosmic rays, the dentist, etc. My NRC contact thinks this would be consistent with his knowledge of an ICRP recommendation for the public of no more than 100 mrem annually above the annual background. Presume a worst case scenario involving vaporization of ALL the Pu-238 that is in the RTG's. This 'astrophysical accuracy' calculation makes no allowance for removal of Pu into the ocean, by rainout, deposition onto uninhabited terrain, etc. The 72lb of Cassini fuel is actually nearly 30% oxygen and less active Pu isotopes, so is only 50 lb Pu-238 = 23 kg = 400,000 Ci (about 17 Ci/g). The volume of air in the Northern troposphere and stratosphere (which receive 99% of the Pu) = 2 pi X (10 + 40) X 6000^2 km3 = 10^19 m3. Dispersion of all this vaporized Pu in the northern atmosphere gives a radiation density of about 0.04 pCi/m3, comparable to the allowable DAC. The ATSDR numbers imply that you breathe air at about 0.1 liter/sec (plausible) so get 3000 m3/yr, or about 120pCi/yr. the conversion factor above (0.25) gives a 50 year dose of 30 mrem from each year of breathing this Plutonium - less than 10% of the annual background. You'd need to breathe it for 10 years just to get the equivalent of one year of natural radiation. Meanwhile, of course, it is being lost from the system so the real numbers are far smaller. And this is using ALL the Plutonium. Looked at another way, all the Pu settles out eventually, providing 2000 pCi/m2, probably over a few years. If a person has a cross section of 1 m2 and inhales ALL the fallout in this area, he gets a 500 mrem 50 year dose. This is still considerably smaller than the 18000 mrem we naturally receive over the same 50 year period. For comparison, 500 mrem total dose is about the same as one mammogram. Of course, most of this settling Pu misses people's noses and mouths, and if this amount of Plutonium were mixed into the top 1 mm of soil, it could be shipped as non-radioactive material. And this is using ALL the Plutonium. No credible indication has ever been found of increased health risk even to the many people who worked milling Pu in the hot and cold war days. The only documented health effects I have been able to find are on the ATSDR web site (see references). Dogs (apparently beagles) inhaled Plutonium at a rate of 1400 - 100,000 pCi per kg body mass in a day, and suffered lung damage, even cancer, depending on dose, after several months to years. Allowing for 20 kg body mass, these dog martyrs consumed, in one DAY, amounts which would be 14 to 1000 times the average person's share of the entire Cassini Pu load as overestimated above. The president of the Health Physics Society has himself done extensive research on mice that confirms these dog results. Vaporization of all the Plutonium is, of course, a gross overestimate. Forget (for a moment) the one-in-a-million probability that ANY kind of flyby mishap will even occur which leads to reentry and vaporization. Even if a mishap does occur, only a tiny fraction of the Pu is able to end up in people (this is the analogue of the fact that there are enough germs in one sneeze to give a billion people a cold - it's the distribution problem that stops this from happening). The Cassini project and its consultants have done exhaustive analyses of this problem. Atmospheric incineration and ground impact have both been considered. The RTG housing itself probably does come apart under entry heating, but the triple-protected modules (2 layers of carbon composite, and an iridium cladding on each Plutonium golf ball) are extremely durable, and designed to withstand atmospheric deceleration and heating. They hit the ground at terminal velocity - only 100-300 feet/second, or one-tenth the speed of a rifle bullet. Rifle bullets don't vaporize on impact. Neither do meteorites; they dig a little hole. So the units might dent the hood of your car pretty badly, or make a hole in your yard, but won't spray pulverized plutonium all over your house. All this has been tested. Factoring in these issues, the projects finds that the average expected dose (per person) is only 1 mrem over the entire 50 year lifespan of the at-risk population. Comparing this to the above upper limit of about 500 mrem/50 yr, one gets a distribution efficiency factor of about .002. If a sneeze had the same efficiency then each sneeze would give 2 million people a cold. So the project's distribution efficiency factor, which includes the difficulty of burning through the carbon-composite and Iridium cladding of the fuel, is hardly unreasonable and actually seems quite conservative. Given the low distribution efficiency, the "average" person receives practically no Pu at all. So what's all the fuss about? There is a very narrow range of "hot" particle sizes (about 6-10 micron radius) that is both large enough to have a significant radiation damage potential (in the range that damaged dogs' lungs) AND small enough to have any conceivable chance of being inhaled (but only a very, very small chance - see note 2). Because of the high density of the Pu (11 g/cm3), the aerodynamic radius is 11 times the actual radius. That is, cigarette smoke particles as large as 6-10 microns are inhalable with small probability (a percent or less), but Pu particles of the same size behave like 60-100 micron carbon grit. If ALL the Cassini Pu were in this 6-10 micron size range, there would be 5 E11 particles to distribute - "100 for each person" is what the critics might say. But in reality there are enormous reduction factors that must be considered. For instance, the fraction of Pu fuel that is actually vaporized is probably less than 10%. The fraction of all released particles that lie in the narrow hazardous size range is perhaps 1%. The fraction of Pu that ends up landing where people live (say, the 20 largest cities) is roughly their area fraction or say 0.0001. The fraction of these grit particles that are actually inhaled, because of their large aerodynamic size of about 100 microns, is also small - surely less than 0.01 (note 2). There is slop in these estimates, but they are plausible "delivery inefficiences" and lead to 500 inhaled "hazardous" particles worldwide, consistent with the Cassini project's far more careful estimate of 100 additional fatalities over a 50 year period. Recall that the probability of this happening in the first place is one in a million; another type of celestial mishap with the same probability, impact of a mile-wide asteroid, would kill over a billion people. Also recall that a billion people will die from cancer unrelated to Cassini during this same 50 years. The health hazard numbers are even smaller for a launch-related accident (even while it is perhaps 1000 times more "probable" at 1/1500 chance of Pu-release related to launch accident), because a far smaller amount of Pu is vaporized and fewer people are exposed. The RTG's have been exhaustively tested under conditions comparable to such accidents; their Carbon-Iridium protection scheme is incredibly robust. Overall, I think the above simple arguments make the more exhaustive analysis done by the Cassini project very easy to understand and accept. The health hazard due to Cassini Plutonium really is negligible. Statistics in the World Almanac verify that a person's risk of dying from Cassini is a million times smaller than his or her risk of a fatal auto accident while driving one mile. Notes: 1) For the cognoscenti, all doses given here are effective (whole body), equivalent (radiation type independent), committed (50-year) doses (unless specified as annual). This is necessary to compare different sources of radioactivity. There are factor-of-2 or 3 differences depending on how soluble the Plutonium is; the values on the web page are appropriate for "insoluble" Plutonium such as the Cassini ceramic form. The basic constants are thus the 50-year integrated effective (whole body) damage-causing dose in mrem from a certain quantity of radioactivity in pCi. 2) The human nose is 100% effective at filtering particles that are 10 microns or greater and 95% effective at filtering particles over 5 microns. These particles can then be excreted easily. The critical size for deposition in lung cells is 1-2 microns. Once inhaled, the material is subject to removal processes involving incoproration in mucous suspension and being swept out by the action of the cillia which line the portions of the lung which are exposed to air (Glasstone and Dolan 1977). References: FGR-11 (1988), or Federal Guidance Report-11: "Limiting values of radionuclide intake and air concentration and dose conversion factors for inhalation, submersion, and ingestion"; K. F. Eckerman et al, EPA Report EPA-520/1-88-020. This is based on standards developed by the International Commission on Radiological Protection, and is endorsed by the President of the US. Glasstone and Dolan (1977), Department of Defense Publication, "The Effect of Nuclear Weapons" ASTDR Web site: http://atsdr1.atsdr.cdc.gov:8080/ToxProfiles/phs9021.html. JPL Cassini Home Page: http://www.jpl.nasa.gov/cassini/ and http://www.jpl.nasa.gov/cassini/MoreInfo/rtginfo/riskframes.htm