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Cassini Fly-By:
Will the "Ceramic" Plutonium Still Be a Cohesive Solid,
or Will Its Self-Irradiation Have Made It Very Fragile?

An Open Inquiry to NASA and DOE.
From John W. Gofman, M.D., Ph.D., October 23, 1997.

Part 1.   Reason for This Open Inquiry

          NASA and DOE have issued public assurances that, during the fly-by of Cassini on route to Saturn, even a burn-up accident would cause no health-hazard on Earth. Reason: The plutonium-dioxide on board is said to be a cohesive solid ("ceramic") which would never be able to fall-out as small inhalable particles of the size (less than 10 microns in diameter) which could deposit themselves in human lungs and cause lung cancer.

          Below are the calculations which indicate that, as a result of self-irradiation, every plutonium atom on the mission will have been subjected on the average to about 800 high-energy events, per year of decay before the fly-by.

          I do not know the evidence that Cassini's pellets of "ceramic" plutonium-dioxide will still be a cohesive solid after all that internal bombardment. Does "ceramic" plutonium really have much of a structure after all this energy-deposition? Is the evidence compelling that so much internal bombardment will not render the plutonium-dioxide on Cassini fragile and easily powdered?

          Probably NASA, DOE, or some of their consultants have already done the requisite experiments to provide this answer for plutonium-based Radio-isotope Thermo-electric Generators (RTGs) which have been radioactively decaying for well over a year. If the work has been done, who (by name) did it, and exactly where on the Internet (full URL) are the results available?

          If the results go in the direction of fine powder rather than "ceramic" cohesion, such results would seriously undermine current claims that a fly-by accident would have no health consequences on Earth.

Part 2.   The Calculations, Yielding 800 Energy-Events per Atom of Pu

          A gram of PuO2 is composed by weight of 238 parts Pu-238 + 32 parts O2. So the fraction of a gram of PuO2 which is Pu-238 is (238 / 270) = 0.88.

          So 1 gram of PuO2 has in it 0.88 gram of Pu-238.

          One mole of Pu02 has in it 6.02 x 10^23 atoms of Pu-238. note that we use the expression "10^23" to mean "ten raised to the power 23." By definition, one mole of any substance consists of 6.02 x 10^23 molecules of the substance.

          Plutonium-238 has a mass number of 238. So there are 238 grams per mole of Pu-238.

          Therefore, 0.88 gram of Pu-238 has (0.88g / 238g) moles x (6.02 x 10^23 atoms / mole) = 2.2 x 10^21 atoms of Pu-238.

          The Law of Radioactive Decay is that Fraction of Atoms Decaying per Unit Time = 0.693 / T1/2 . T1/2 is radioactive half-life. The radioactive half-life of Pu-238 is 88 years. Therefore:

          Fraction of Pu-238 atoms decaying / year = 0.693 / 88 = 0.00787.

          Per gram of PuO2, the number of Pu-238 atoms decaying in one year is the initial number times the fraction decaying per year:

          (2.2 x 10^21 atoms) x (0.00787) = 0.017314 x 10^21 atoms = 1.73 x 10^19 atoms.

          Each alpha-particle disintegration from Pu-238 deposits 5.45 million electron volts of energy within the RTG's plutonium-dioxide. We can simplify by saying 5 MeV per alpha emission.

          A powerful chemical bond is well under 50 electron volts, so each plutonium decay represents at least 100,000 "energy-events" which are capable of bond-breaking in the crystal lattice (or other bonding) in the RTG plutonium dioxide.

          How many energy-events, each of 50 ev, occur during the first year in a gram of PuO2? (Number of decays from Part 2b) x (100,000 energy-events per decay from Part 2c):

          (1.73 x 10^19 decays) x (1 x 10^5 events / decay) = 1.73 x 10^24 energy-events per gram of PuO2 during the first year of decay.

          As an approximation, we can say that the energy-deposits are evenly spaced throughout the PuO2. Using the approximation of uniformity, we can say that the average number of energy-events experienced per atom of Pu-238 is (number of energy-events from above) / (number of Pu-238 atoms present from Part 2a). Thus, per gram of PuO2:

          (1.73 x 10^24 energy-events) / (2.2 x 10^21 atoms) = 0.786 x 10^3 energy-events per atom of Pu-238, or approximately 800 per atom on the average during the first year, per gram of PuO2.

          In reality, some molecules of PuO2 will receive more of the decay-energy than others. In addition, the calculation above is very conservative, because it is limited to the first year of decay. When Cassini flies by Earth, its plutonium-238 load will have been decaying for more than one year.

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Committee for Nuclear Responsibility, Inc. (CNR)
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