Concerning Pu-238 Thermoelectric Generators for the Cassini Mission
Otto G. Raabe, Ph.D., CHP
University of California, Davis
Contrary to some outlandish claims of horrible consequences associated with
the use of plutonium-238 Radioisotope Thermoelectric Generator (RTG) power
supplies in the Cassini probe to Saturn scheduled for October by NASA,
there is actually little risk to anyone from accidental releases of
plutonium. One of the outspoken critics of NASA is a New York Physics
Professor, Michio Kaku, whose special area of research interest has been
Einstein's equations. He has no background in toxicology, health physics,
risk analysis, or any related field. He is telling the press that any
accident involving Cassini will probably involve major loss of life,
widespread cancer induction, and considerable property damage. He predicts
the end of NASA and U.S. space research if there is an accident.
In order to pose a significant risk to people from alpha radiation,
plutonium must be deposit within the human body by being breathed as
finely-divided airborne particles. Plutonium outside the body or swallowed
into the gastrointestinal tract poses little risk. Therefore, the important
issues relate to the ways in which plutonium could be converted to airborne
micron-sized airborne particles in an accident.
The RTG's are inert radioactive batteries that provide electricity
thermo-ionic materials utilizing the natural heat energy associated with
radioactive decay by alpha emission. The rigorous engineering safety design
of the RTG precludes serious release of inhalable plutonium from these
devices even in a catastrophic accident. Then plutonium is in the form of
high-fired ceramic pellets of plutonium dioxide. The plutonium is 80%
Pu-238. The pellets are wrapped in a protective layer of iridium and
protected by three layers of inert, heat-resistance composite graphitic
materials. If the rocket blows up during blastoff, contamination, if any,
would be quite localized, and the inhalable fraction would be very small.
After the launch, the Cassini vehicle is scheduled to travel to venus,
orbit venus to pick up that planets momentum, and swing back close to the
earth (500 miles) to get han extra boost for its trip to Saturn. Accidental
re-entry into the earth's atmosphere during this fly-by could lead to the
release and dispersion into the atmosphere of up to about 6% of the
plutonium-238 contained within the RTG power supplies. The resulting
radiation exposure, if any,to people would be too small to result in any
meaningful risk.
Since the first nuclear weapons test in July 1945, approximately 360,000
curies of plutonium-239/240 have been released into the atmosphere of the
earth. In addition, 17,000 curies of plutonium-238 were released into the
atmosphere in April 1964 as a result of the high-altitude burn-up of an
early-model satellite power source. Because of these releases, every person
living on the earth today has small amounts of plutonium in lung, liver and
bones, but the radiation dose received from these traces of plutonium is a
small fraction of natural background ionizing radiation exposures. There is
no known or expected risk from with this worldwide dispersion of plutonium
in the environment.
The Cassini probe will utilize about 393,000 curies of plutonium-238.
Although quite unlikely, even if the whole vehicle vaporizes in the
atmosphere during the fly-by, the total plutonium released into the
environment would only be about doubled. Subsequently, people on earth
might inhale trace amounts of Pu-238 leading to an average annual effective
dose equivalent from alpha radiation of about one millirem or less
depending on the rate of clearance from the atmosphere. We each receive
about this amount of ionizing radiation exposure every day of every year
from natural background sources and this is about the same dose of alpha
radiation that people have received from past exposures to plutonium
released to the atmosphere. There is no known or expected risk associated
with such small radiation exposures.
Otto G. Raabe
9/5/97
Davis, CA
*****************************************************
Prof. Otto G. Raabe, Ph.D., CHP
[President, Health Physics Society, 1997-1998]
Institute of Toxicology & Environmental Health (ITEH)
(Street address: Old Davis Road)
University of California, Davis, CA 95616
Phone: 916-752-7754 FAX: 916-758-6140
E-Mail: ograabe@ucdavis.edu
******************************************************