Message-Id: <v01540b12b059de0ad2e4@[192.174.2.173]>
Date: Thu, 2 Oct 1997 15:29:06 -0800
To: pinhole@exploratorium.edu
From: pauld@exploratorium.edu (Paul Doherty)
Subject: Re: Cassini, more Nasa answers
Here are more NASA answers to Cassini questions.
from
http://www-a.jpl.nasa.gov/cassini/MoreInfo/spacepwr.html
The original article shows the construction of the RTG units.
Paul D
February 1996
Cassini's electrical power source - Radioisotope Thermoelectric Generators
(RTGs) - have provided
electrical power for some of the U.S. space program's greatest successes,
including the Apollo lunar landings and the
Viking Landers that searched for life on Mars. RTGs made possible NASA's
celebrated Voyager explorations of
Jupiter, Saturn, Uranus and Neptune, as well as the Pioneer missions to
Jupiter and Saturn. RTG power sources are
enabling the Galileo mission to Jupiter and the international Ulysses
mission studying the Sun's polar regions.
Extensive studies conducted by NASA's Jet Propulsion Laboratory (JPL) have
shown that NASA's Cassini mission,
given its science objectives, available launch systems, travel time to its
destination and Saturn's extreme distance from
the Sun, requires RTGs.
What Are RTGs?
RTGs are lightweight, compact spacecraft power systems that are
extraordinarily reliable. RTGs are not nuclear
reactors and have no moving parts. They use neither fission nor fusion
processes to produce energy. Instead, they
provide power through the natural radioactive decay of plutonium (mostly
Pu-238, a non-weaponsgrade isotope). The
heat generated by this natural process is changed into electricity by
solid-state thermoelectric converters. RTGs enable
spacecraft to operate at significant distances from the Sun or in other
areas where solar power systems would be
infeasible. They remain unmatched for power output, reliability and
durability by any other power source for missions
to the outer solar system.
Safety Design
The United States has an outstanding record of safety in using RTGs on 23
missions over the past three decades.
While RTGs have never caused a spacecraft failure on any of these missions,
they have been on-board three missions
which experienced malfunctions for other reasons. In all cases, the RTGs
performed as designed.
More than 30 years have been invested in the engineering, safety analysis
and testing of RTGs. Safety features are
incorporated into the RTG's design, and extensive testing has demonstrated
that they can withstand physical conditions
more severe than those expected from most accidents.
First, the fuel is in the heat-resistant, ceramic form of plutonium
dioxide, which reduces its chance of vaporizing in fire
or reentry environments. This ceramic-form fuel is also highly insoluble,
has a low chemical reactivity, and primarily
fractures into large, non-respirable particles and chunks. These
characteristics help to mitigate the potential health
effects from accidents involving the release of this fuel.
Second, the fuel is divided among 18 small, independent modular units, each
with its own heat shield and impact shell.
This design reduces the chances of fuel release in an accident because all
modules would not be equally impacted in an
accident.
Third, multiple layers of protective materials, including iridium capsules
and high-strength graphite blocks, are used to
protect the fuel and prevent its accidental release. Iridium is a metal
that has a very high melting point and is strong,
corrosion resistant and chemically compatible with plutonium dioxide. These
characteristics make iridium useful for
protecting and containing each fuel pellet. Graphite is used because it is
lightweight and highly heat-resistant.
Potential RTG accidents are sometimes mistakenly equated with accidents at
nuclear power plants. It is completely
inaccurate to associate an RTG accident with Chernobyl or any other past
radiation accident involving fission. RTGs
do not use either a fusion or fission process and could never explode like
a nuclear bomb under any accident scenario.
Neither could an accident involving an RTG create the acute radiation
sickness similar to that associated with nuclear
explosions.
NASA places the highest priority on assuring the safe use of plutonium in
space. Thorough and detailed safety
analyses are conducted prior to launching NASA spacecraft with RTGs, and
many prudent steps are taken to reduce
the risks involved in NASA missions using RTGs. In addition to NASA's
internal safety requirements and reviews,
missions that carry nuclear material also undergo an extensive safety
review involving detailed verification testing and
analysis. Further, an independent safety evaluation of the Cassini mission
will be performed as part of the nuclear
launch safety approval process by an Interagency Nuclear Safety Review
Panel (INSRP), which is supported by
experts from government, industry and academia.
Radiation Hazards of Plutonium-238
Isotopes of plutonium such as Pu-238 characteristically give off
short-range alpha particles, helium nuclei that usually
travel no more than about three inches in air. While the fuel is contained
within its iridium capsule, the alpha radiation
does not present a hazard, and the external dose resulting from the low
levels of gamma and neutron radiation
associated with the plutonium dioxide RTG fuel generally would not
represent a significant health hazard, either.
External alpha radiation would be stopped by clothing, an outer layer of
unbroken skin, or even a sheet of paper. The
point at which Pu-238 can become a health hazard is when it is deposited
into the body.
If an individual were to inhale plutonium dioxide particles of a
sufficiently small size to be deposited and retained in
proximity to living lung tissues, the alpha radiation could alter or kill
nearby living cells. Over years or decades, the
altered cells could become cancerous and form tumors in the lung.
Additionally, some of this material could be
dissolved in body fluids and transferred by the blood to be deposited in
other organs, generally the liver and skeleton,
with similar potential consequences. The ceramic form of plutonium used in
RTGs, however, is not likely to shatter
into fine particles that could be readily inhaled. Other exposure pathways,
such as ingestion, contribute far less to
health effects.
The ceramic form of plutonium dioxide fuel also has low solubility in
water, so its migration in ground water and
potential for uptake by plants is limited. The actual proportion of
plutonium released from an RTG that could enter the
food chain, if any, is small.
A common misconception is that a small amount of plutonium, such as one
pound, if evenly distributed over the entire
world, could induce lung cancer in every person on Earth. While plutonium
can alter or kill living cells if deposited
directly onto sensitive human tissue, the important point is that it must
be in a form that enables environmental
transport and intake by humans. Research has demonstrated that the
mechanisms of plutonium dispersion into and
transport through the environment, and hence in to humans, are extremely
inefficient.