Space, plutonium and risk

["Sandy Perle" <sandyfl@ix.netcom.com>]

The article below was published in the Christian Science Monitor, and 
all in all, it appears to be well written. It does discuss potential 
risks due to exposure if the source would fall to earth, comparing 
the levels to comparable background radiation. I don't see the 
typical scare tactics seen in many other articles. Hope the article 
is informative of what the general public is reading that can and 
does affect our profession in so many ways.
-----------------------------------------

BOSTON -- Mindful of the risks to using nuclear power sources in 
space, planners for NASA's upcoming Cassini mission say they are
working to keep those risks to a minimum. 
 Their approach acknowledges that `things like Mars 96 happen, so  you
have to be prepared to deal with that reality,` says Richard
Spehalski, senior program manager for the Cassini project at Caltech's
Jet Propulsion Laboratory in Pasadena, Calif. `That's reflected in the
design of the spacecraft, the design of the mission, and in the design
of contingency plans.` 
 The nuclear power sources used, known as radioisotope  
thermoelectric generators (RTGs), use plutonium 238 for heat. Unlike a
nuclear reactor, whose heat comes from chain reactions, an RTG uses
heat from plutonium's natural decay. Solid-state devices turn the heat
into electricity. Smaller amounts in special containers are used to
keep a craft's electronics warm enough to operate. The solar-powered
Sojourner, a tiny rover launched earlier this month on the Mars
Pathfinder mission, has several of these plutonium heaters on board. 
 Alternatives, such as fuel cells, batteries, and solar panels,  exist
for producing electricity in space. But for Cassini, mission planners
say, they fail to meet power, weight, or reliability requirements. The
European Space Agency has developed in the lab a lighter,
more-efficient solar cell. But in a memo last August, ESA officials
acknowledged that too little is known currently about the cells to
make them a viable alternative. 
 Plutonium 238, while nothing to trifle with, is less radioactive 
than its nuclear-weapons cousin. The danger to humans comes mainly
from inhaling tiny particles of plutonium dust, according to
physicists, as well as NASA's environmental impact statement. 
 In the 1960s, the first RTGs were designed to break up on reentry - 
and did so. When studies found radiation after the breakups, designers
shifted gears. From the high-temperature ceramic fuel pellets to heat
and impact shields, components aim to keep the fuel pellets intact and
shielded during the heat of reentry and aftershock of landing from
Earth orbit. The design aims to keep the plutonium from being released
into the environment. If it is, the ceramic pellets are designed to
break apart in chunks too big to inhale. 
 Much of the concern surrounding Cassini centers on its  
gravity-assisted fly-by of Earth on its way to Saturn. According to
NASA's environmental impact statement, if plutonium is released during
an inadvertent reentry, up to 5 billion people could be affected. But
the statement adds that the radiation dose a person might receive
would be far lower than what they get from natural sources. Any
adverse health effects, it estimates, would likely be
indistinguishable from other, more-common occurrences. 
 Mission planners have designed the orbital fly-by to reduce the 
likelihood of an unplanned reentry. NASA estimates the risk at about 1
in 1 million. Some dispute the figure based on how dramatically NASA
revised its risk estimates for the space shuttle after the Challenger
disaster in 1986. 
 On missions involving nuclear materials, the White House gives  final
approval to proceed. That call typically is made a month before the
scheduled launch, after an interagency panel submits its final safety
analysis, according to Mr. Spehalski. 
 Such missions also fall under United Nations scrutiny. The world 
body adopted a set of principles on the use of nuclear power in outer
space in 1992. The UN principles acknowledge the usefulness of
nuclear-power sources in some space applications, outline sometimes
contradictory provisions on radiation protection, and sketch out
procedures in case a nuclear power source reenters the atmosphere. 
 If the Mars 96 debacle is any indication, bugs still need to be 
worked out of the system. Initial projections of where the debris
would fall were wrong. Eyewitnesses saw what is now widely believed to
be pieces of the craft burn up over Chile and Bolivia. The Bolivians,
who have asked for US assistance in searching for plutonium-bearing
debris, have complained that Russia has not been helpful. Moscow still
claims the debris fell into the ocean. 

Sandy Perle
Director, Technical Operations
ICN Dosimetry Division
Office: (800) 548-5100 Ext. 2306 
Fax: (714) 668-3149

E-Mail: sandyfl@ix.netcom.com    

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