The first answer is by Jane Prettyman (openmind@rain.org), editor of The Real News Page and was written September 21st, 1997. The second answer is by Russell Hoffman (rhoffman@animatedsoftware.com) and was written September 23rd, 1997.
The original document was passed to me (Hoffman) by several people. The source closest to the "original" source appears to be the copy used here, from Jim Spellman, forwarded to him by Louis Friedman, who I am guessing got it from the source, Jeff Cuzzi. For the answer by Prettyman, chosen sections were repeated and placed "bold italics". In the answer by Hoffman the entire document was reprinted with comments inserted in italics. Where paragraphs or sentences are broken apart we have added double dashes ("--") to distinquish that fact. The entire uncut pro-nuclear Cassini document as it was received is reprinted below the two answers.
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.
Have you directed your thoughts to Dr. Michio Kaku (MKaku@aol.com)?
From the prespective of a dumb-bunny non-rocket scientist, it seems to me that calculating the release pattern of Cassini's plutonium in an inadvertant fly-by re-entry accident in terms of uniform dispersal of the plutonium throughout the atmosphere of the entire Northern hemisphere (why only the Northern hemsiphere?) is disingenuous since the re-entry is likely to occur some WHERE as a general "footprint" area somewhat more focused than the entire Northern hemisphere, such as the vicinity of the midAtlantic or the vicinity of the mid Pacific or the vicinity of New England or the vicinity of Southern California or the vicinity of Alaska and the Bering Straits, or the vicinity of central China, or the vicinity of Atlanta GA or New York City or London or Scotland or Tokyo or New Delhi or wherever. In this scenario, the concentration of respirable plutonium is likely to be somewhat higher than your diluted estimates and could have higher impact depending upon the density of population. No?
the 18000 mrem we naturally receive
May I ask if the "natural background" (into which a full release of Cassini's Pu 238 contents would be statistically insignificant) includes the already released radioactive material from Russian and American nuclear spacecraft, from atomic testing, and from nuclear reactors worldwide?
May I ask why you express no concern whatsoever about a Cassini plutonium release having a *cumulative* effect in adding to the already released plutonium from Russian and American accidents? Have you no concern that the modeling of US policy in using nuclear power in space gives permission for every nuclear nation to do the same, with much less safety-mindedness pehaps than the US, such as China, Russia, France, India? Have you no concern about potential future radiation releases from projects launched by those countries?
I had several discussions with a physicist at the Nuclear Regulatory Commission (NRC) concerning decay rates and comparative relationships to health effects.
I am not impressed by any conversations you have had in the inside circle of agreement among NASA, DoE, DoD, EPA, NRC and nuclear industry contractors. Cassini has not been sufficiently peer-reviewed by indepenent and neutral scientists. Have you or your colleagues debated in public with Dr. Michio Kaku, with Dr. Horst Poehler, with Dr. Karl Z. Morgan, with Dr. John Gofman, with physicist Kosta Tsipis of MIT? Blind spots are not being covered: such simple things as the effect of winds in spreading a plutonium release.
This is why we need fully informed Congressional hearings with testimony from all sides, to educate the public and give our elected representatives a full set of information upon which they can impartially decide for us this perilous questionof plutonium on Cassini and all future plutonium launches.
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).
Good for him. He should have a serious chat with his fellow Health Physicist, Dr. Karl Z. Morgan (423-482-9814), former head of health physics at Oak Ridge National Laboratory who strongly disagrees with your "President of the Health Physics Society."
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.
Are you being disingenuous by bringing in "vaporization on impact" on hard surfaces? Vaporization has only been mentioned, so far as I know, in connection with an inadvertant re-entry. Nobody I know ever said the RTG's would "vaporize on impact" although it is not known exactly what would happen to the Pu on impact on a "hard surface." We only know for sure that according to NASA's own statement in the FEIS, plutonium will be released. Mere disperal of plutonium if it falls on a rocky area might not produce respirable particles (or it may), but it would certainly make a nasty and still deadly mess (in other health effects besides respiration) if it occurred on land or in a populated area, potentially costing billions of dollars to clean up.
All this has been tested.
No, it has not been tested, not with full-scale experimentation. If so, please advise me the citations where I can find these full-scale re-enacted studies of RTG's containing 72.3 pounds of plutonium re-entering the atmosphere and disintegrating, or not; hitting hard surfaces, or not; dispersing or vaporizing, or not.
What sort of Cassini scientist are you? What part of it are you working on?
Please read Dr. Kaku's critique of NASA's fatality estimates and write a response to it. (http://www.rain.org/~openmind/kaku1.htm). That would be interesting to read.
Thanks, Jane
Also, note that the discussion of solar efficiencies versus RTG (nuclear) efficiencies always ignores an interesting aspect, which is if you combine NASA's unquestionable ability to run the same experiments with less power requirements than the current experiments use if given time, money, and a directive to do so, with the equally reasonable proposition that RTG thermocouple devices will probably become more efficient as well if development money continues to go in that direction, and one has to wonder if in a decade or so, not a long time in space exploration planning, NASA might be able to run the entire scientific projects for the Cassini mission with a phenomenally LESS amount of plutonium? What if the amount of plutonium NASA needed to get the 745 watts of electricity was just 1/100th of what the current requirement is? It is a reasonable question to ask! In fact, RTGs do not look like efficient heat source transfer systems at all!
Rather than have the thermocouples outside the containment system, what if they were butted right up against the plutonium? Has this concept been tried? Just asking. Solar would be a better solution even if Cassini could fly on 1/1000th of the plutonium it currently uses.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
From: WSpaceport@aol.com
Date: Wed, 17 Sep 1997 21:18:12 -0400 (EDT)
To: nss-discuss@nss.org, rhoffman@animatedsoftware.com
Subject: FWD: Plutonium primer -- FYI
From: Jim Spellman - National Space Society/Western Spaceport Chapter
Here's some very valuable information that I just received via Lou Friedman, executive director of the Planetary Society in Pasadena. I'm only the messenger, so don't waste bandwidth questioning me if you don't understand it (or in some cases, don't care to believe it).
~JS~
**********************************************
Subj: Plutonium primer -- FYI
Date: 97-09-17 10:57:38 EDT
From: tps.ldf@mars.planetary.org (Louis Friedman)
To: WSpaceport@aol.com, NSSNYC@aol.com
Date: Tue, 16 Sep 1997 22:14:46 -0700
From: cuzzi@cosmic.arc.nasa.gov (Jeff Cuzzi)
Subject: Plutonium primer
Cassini Plutonium for the technically minded
by 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 a 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.
A million dollars is about .07% of the total expenditure on Cassini so far, about .03% of the projected total, and perhaps just .003% or even .0003% of the potential cost to humanity of an accident, plus the "human toll". That's not a lot of study for the risk associated, especially considering all the questions left unanswered by this great expenditure and "thorough" analyses. Also, too bad they didn't do the modern technologically achievable thing and put the data on the World Wide Web so all could see it and so that it was not necessary to spend hours and hours scanning portions of it in for people to read...
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 Commission (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).
Ooh, I'm scared. Such brilliant men against me. Such brilliant men as an unamed NRC physicist whose livelihood depends on public acceptance of low level nuclear pollution appearing to be harmless. The President of the Health Physics Society is Dr. Otto Raabe. I have seen Dr. Raabe's statements and responded to one, a letter published in USA Today. Dr. Raabe is relying on either NASA containment, or NASA PR assurances of a worldwide very even dispersal to make his claims. Neither assurance is reasonable. On the one hand, NASA assurances of strict containment conflict with NASA's own data on same, and are argued against by good, respected scientists on other grounds as well. On the other hand, show us the study that proves that so-called low level plutonium 238 radiation really is safe. Show us the statistical analysis of millions of people that would be necessary to prove that statement. Show us the careful research that should have been done on atomic weapons testing fallout, but wasn't. In fact if you show us the books on human exposure during weapons testing you will have to show us TWO books because the government kept TWO books, one for the soldiers to see, and one for real. The real books are generally missing, destroyed or still classified. Gofman and others have made a strong case that vaporized plutonium 238 is an incredible hazard to human health. When pressed, Dr. Raabe always falls back on NASA containment to save the day. Not the reality of what 72+ pounds of plutonium dioxide can do if spread into the atmosphere above a major population center. And NASA, of course, never discusses it either. As if it can't happen.
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.
24,131 (half-life of Pu 239, see
June 1995 EIS, page 2-18)
/ 87.75 (half-life of Pu 238, see
June 1995 EIS, page 2-18)
= 274.99715 (not 250).
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 "occupational 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).
You have let rest a potential order of magnitude error. Typical NASA scientist. Also, it is confusing what exactly you are saying is the relevance of the various factors might be.
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. --
You are including manmade sources with "natural" background radiation, which is a criminally unscientific way to explain that a little more won't hurt you. This is proven philosophically as follows: If we always include both "natural " and "manmade" radiation effects, then as we creep slowly towards more and more pollution and an eventual overload and threshold of survivability, then as this happens EACH NEW RELEASE seems, as reported as a percent of "natural and manmade background radiation" to be a SMALLER AND LESS SIGNIFICANT additional amount! But that is not correct! As we approach a hopelessly polluted threshold, each release of the same magnitude in absolute amounts becomes MORE significant as a real and potential killer. So the whole numbering system presented in this statement is wrong, wrong, wrong.
-- 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.
That number is too high. Your unnamed NRC contact is clearly in the business of promoting "safe" nuclear technology and has a financial and professional interest in making "safe" recommendations rather than realizing the mess we are making of this sorry planet. You also have a financial and professional interest in Cassini as you have noted.
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.--
But what it DOES allow for is an absurdly even dispersion.
--The 72lb of Cassini fuel is actually nearly 30% oxygen and less active Pu isotopes,--
11.852% oxygen according to NASA June 1995 EIS page 2-18. The other isotopes of plutonium are serious hazards in their own right as are many of their daughter elements.
-- 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.---
That calculation right there is nuts, nuts, nuts, because it assumes even dispersal of the vapors. The world just doesn't work like that. Many people will get vastly greater doses -- it's the luck of the draw. If life did work like that and it was all evenly distributed, you would still have a science problem you need to answer, because the safety of such low levels of plutonium 238 vapors has hardly been proven and in fact incredibly minute quantities of plutonium are a known serious carcinogen... It is funny that in this entire article no attempt is made to express just how much Pu 238 in dioxide form is enough to kill, say, 50% of the people who receive a given dose. Also the Pu 239 portion (about 12%) will stay in the environment for many millennia. I wonder if any human can truly comprehend such time frames and what effects pollution we create today will have over those sorts of periods.
--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.--
That sounds like a horrific dose to me, but then, you don't see the millions of people around the world who die of cancer from unknown causes each year, do you? You don't know why they die, so it couldn't possibly be from low level radiation exposure, could it (including "natural background radiation")?
--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.
And spreading it unrealistically thin over average population densities, with healthy people for victims.
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 a silly calculation. Your lungs filter far more air than just the column that stands above your cross section, as you seemed to have realized above, but have now forgotten.
--This is still considerably smaller than the 18000 mrem we naturally receive over the same 50 year period.
What kills so many people of cancer? If you don't know, how dare you risk increasing the levels of a serious known carcinogen by so much? If we, the anti-nuclear Cassini people are wrong but win the debate (somehow), then in a few years a non-nuclear Cassini type mission will fly anyway. If you and your absurd calculations and "proofs" are wrong and God, for whatever reason, who (according to 'a rather famous scientist') "does not throw dice", must let the devil have his due this time (or next time, or the next, if this policy continues) then we are all as a species in for a heep of hurt. What NASA is doing is not scientific, it is "gambling with the devil" as Mark Elsis has put it, testing God, if you prefer, or simply gambling innocent lives against random occurrences beyond their control. The space debris that recently came within 500 yards of MIR can hit Cassini on its way out or on its way back, and so can 100's of thousands of other lethal pieces, and NASA cannot track or even see all the debris that can destroy Cassini. In fact they don't see 100 potentially catastrophic pieces for every one that they track. It's a gamble, a crap-shoot. It's Russian roulette. It's not science in the public good.
For comparison, 500 mrem total dose is about the same as one mammogram.--
Mammograms are not risk-free by any means, but breast cancer is considered a far more serious threat since it affects about one in seven women.
--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.
Ship it where?
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.--
Cite your references. Define "credible" and give us some examples of scientific indications you do not consider "credible".
--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.
My understanding is that from these experiments, it could be extrapolated that the human equivalent carcinogenic dose of Pu 239 from inhalation is about 27 millionths of a gram, and for Pu 238 because of the Curie conversion factors you discuss above, it would be about .1 millionths of a gram. The dogs died.
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.--
14 times the average person's share is certainly a reasonable amount to expect some people to get, probably some unlucky few will even get 20 or 30 times the "average" dose that a worldwide population receives. It's the luck of the draw. In this case we are talking about drawing unlucky breaths.
--The president of the Health Physics Society has himself done extensive research on mice that confirms these dog results.
Interesting statement, since I cannot conceive of how he could have studied the effects of extremely low levels of radiation on his mice. Perhaps he tried larger doses and extrapolated down? If so, what were the doses and what were the results? What does Dr. Raabe consider to be a lethal or carcinogenic dose of Plutonium 238 from his mice experiments?
Vaporization of all the Plutonium is, of course, a gross overestimate.--
NOT NECESSARILY.
--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.--
One in one million is NASA's number and is disputed by scientists and NASA's own past failures with such numbers, for example in estimating the Shuttle failure rates before Challenger, and also in estimating the SNAP-9A rocket's failure rate in 1964.
--Even if a mishap does occur, only a tiny fraction of the Pu is able to end up in people (this is the analog 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).
There is no reasonable analogy here. Your body's own defensive mechanisms will kill 99.999% (or whatever) of a sneezes' germs. But your body does not have any known defense against alpha radiation pollution. It's not just a distribution problem.
The Cassini project and its consultants have done exhaustive analyses of this problem.--
I'd call them coverups of the problem, but I guess you're entitled to use the milder term "analyses".
--Atmospheric incineration and ground impact have both been considered. The RTG housing itself probably does come apart under entry heating,--
If you read the NASA documentation you would not say "probably", you would say "In a flyby reenty the RTG housing is expected to melt away releasing the 54 GPHS units within. Some of these will smash into each other or into other components of the 12 ton spacecraft as it breaks apart. This will damage an UNKNOWN percentage of the units (3%? 33%? More?). Each GPHS unit that is damaged in this way is expected to release it's full content of plutonium at high altitude, which will be vaporized, over a pound each." THAT would be consistent with NASA's own documentation.
--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.
When all goes well. According to NASA's June 1995 EIS, page 4-51, 33% of the time all will not go well. (They changed it to about 3% in the June 1997 EIS, for who-kows-what-reason.) And that's just the failures at high altitude. Then there is the problem of what makes it to Earth and smashes on rock or rocket scientist's hard heads. That part could be "pulverized" to some extent, perhaps run over by a tank, blown up by a land mine (even after they are banned), or lofted into the air in a conflagration of some sort. Hit by an asteriod. A mere .01% of the plutonium payload being "pulverized" one way or another is hundreds of thousands of lethal doses. NASA is guaranteeing to contain ALL THAT PLUTONIUM? Hardly! NASA and NASA's scientists do NOT take responsibility for an accident! Instead they rely on a strange piece of inappropriate legislation called the Price-Anderson Act. Plutonium is lethal in what are CLEARLY 'unbelievably small portions'.
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.
"Spray pulverized plutonium"? Plutonium particles will be released, available for later suspension in fires, for ingestion in food, for all the creatures great and small who inhabit this planet under our seemingly uncaring care. And we are just talking about the stuff that makes it to Earth quickly. The rest of it will settle slowly over a period of decades.
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.--
A simplistic number that does not cover the true risks to people, millions of whom will get two, three, four, and even ten times your "average" dose. Again, you average too much, to the point of an unrealistic value. It does not reflect all possible realities.
--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.
A pointless comparision, as noted above.
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.
I disagree, and others disagree, and the burden should be on NASA (and you) to prove such statements, but they (and you) cannot.
Given the low distribution efficiency, the "average" person receive practically no Pu at all. So what's all the fuss about?--
You just don't get it? There is no "average" person! The doses will not be spread out to an "average", low amount. We are all individuals who want to LIVE and not be ASSAULTED by your hundreds of billions of lethal doses of Plutonium 238 poison in an environment filled with nearly 6 billion people in it!!!
--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).
And look at NASA's own estimates to calculate how many hundreds of millions or billions of these "hot" particles are EXPECTED to be released in a flyby reentry accident! Furthermore the assumption that sub 6-micron particles of Plutonium 238 do not have "significant radiation damage potential" is based on what? Even if billions of people inhale such particles?
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%.--
There you go again, switching horses in mid stream. We are trying to decide what the true hazards are in a "worst case" scenario and it is absolutely reasonable to recognize that in some accident scenarios, ALL of the plutonium can be released and much of that will be vaporized into sub 10-micron respirable particles.
--The fraction of all released particles that lie in the narrow hazardous size range is perhaps 1%.--
See NASA's June 1995 EIS, page 4-51: 20% to 66% of that which is incinerated in the upper atmosphere will be in "respirable particles" which NASA defines as less than 10 microns in size.
--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.--
Here is your most faulty mistake. The question is, what happens if it does come down where it shouldn't? Is NASA prepared for that? Will NASA pay? Will the individual scientists such as yourself take personal financial responsibility for what you are risking? Why not?
--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 inefficiencies" 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.
If all the things happen according to your happy plan for a safe Cassini accident, great. But this is not the only possible result, nor even a plausible one.
Recall that the probability of this happening in the first place is one in a million;--
NO. Recall instead that NASA's claims it is, but NASA has not been very accurate in such predictions in the past.
--another type of celestial mishap with the same probability, impact of a mile-wide asteroid, would kill over a billion people.--
So what? We can't stop an asteriod. Even with an interplanetary nuclear arsenal we couldn't stop such an asteriod and could well make things worse by trying. But we CAN stop Cassini and should.
--Also recall that a billion people will die from cancer unrelated to Cassini during this same 50 years.
Right, and so how will you find a million additional deaths if Cassini crashes, or even 10 million? As always, you will hide your deaths in the numbers game and claim, as so many pro-nuclear apologists have done before you with weapons testing and other nuclear disasters, that "no one died."
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.
It still relies ultimately on LUCK to assure that the worst cannot happen, and luck favors the well prepared, not the cavalier risk taker. What about a Full Stack Intact Impact (FSII)? What might get released then?
Overall, I think the above simple arguments make the more exhaustive analysis done by the Cassini project very easy to understand and accept.--
NASA's analyses are simplistic. NASA did an exhaustive "cooking" of the numbers. It is understood but not accepted. But your analyses does seem like a good explanation of where NASA's (ill-) logic lies...
-- The health hazard due to Cassini Plutonium really is negligible.
Would you be willing to inhale it yourself? EXACTLY how big a dose?
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.
Show me the World Almanac estimate of the Cassini risk. But okay, what's your point other than to show that you cannot transfer from thinking about yourself ("a person") to thinking about a global exposure? 50,000 people die each year from auto accidents. At least the rest of us all got somewhere for the risk we each took. Cassini benefits very few at a horrendous risk and expense. The potential world gain in knowledge if the mission succeeds through all it's phases will NEVER be able to compensate the unnecessary risk being undemocratically foisted on an unsuspecting and unappreciative public.
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.--
Some air is drawn in through the mouth... Besides, we pass a lot of air through our filters (our lungs) in a lifetime. Probably a greater volume than in the entire Vehicle Assembly Building, for example. (I'll go out on a limb and guess several times that volume in a lifetime of breathing.)
--These particles can then be excreted easily.
I think, since we are talking about inhalation, the author does not mean excretion but rather, expectoration. In any event the phrase "in most cases" needs to be here, then a description of what happens to people the millions of times (or hundreds of thousands of times, or what?) that, for some reason, plutonium particles cannot be properly excreted or expectorated.
The critical size for deposition in lung cells is 1-2 microns.
Once inhaled, the material is subject to removal processes involving incorporation in mucous suspension and being swept out by the action of the cilia which line the portions of the lung which are exposed to air (Glasstone and Dolan 1977).
But we are talking about a global dose. These removal processes are imperfect, especially for smokers, and there are other problems such as infants being more susceptible to radiation damage than adults. Reducing the average life expectancy of the world population is a "stupid" thing to do, because most of us grow smarter as we grow older. If we are not given a chance to grow older, we many of us may never have a chance to get smart either...
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 United States.
Glasstone and Dolan (1977), Department of Defense Publication, "The Effect of Nuclear Weapons"
I wish to note for the record that Louis Friedman was quite upset with me for using peer reviewed references from the 1970's (see Newsletters). I won't repeat his mistake, but I will question whether DoD publications are "peer reviewed" in the eyes of the general public...
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
Here's some very valuable information that I just received via Lou Friedman, executive director of the Planetary Society in Pasadena. I'm only the messager, so don't waste bandwidth questioning me if you don't understand it (or in some cases, don't care to believe it).
~JS~
**********************************************
Subj: Plutonium primer -- FYI
Date: 97-09-17 10:57:38 EDT
From: tps.ldf@mars.planetary.org (Louis Friedman)
To: WSpaceport@aol.com, NSSNYC@aol.com
Date: Tue, 16 Sep 1997 22:14:46 -0700
From: cuzzi@cosmic.arc.nasa.gov (Jeff Cuzzi)
Subject: Plutonium primer
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 a 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 Commission (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 United States.
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
rhoffman@animatedsoftware.com
