Re: cassini flyby

[Steve Frey <sfreyohp@SLAC.Stanford.EDU>]

"Radsafer Ralph" wrote:

>Apollo 13 unfortunately had some problems and 
fortunately returned to earth.  The LEM reentered and 
burned up.  The Snap reentered and no evidence of 
released Pu was discovered. Therefore the reentry 
design worked<

Although obviously not intended as part of the mission 
plan for Apollo 13, a substantial quantity of Pu-238 
did indeed reenter the atmosphere aboard the Lunar 
Module (LM). To date, no evidence of harm to the 
residents of earth from that Pu-238 "release" event 
has been discerned.

Moreover, the reentry conditions were quite 
conservative.

First,  the Pu-238 inventory aboard Apollo 13 was 
several hundred thousand curies, which is essentially 
equivalent to the Pu-238 inventory borne by Cassini. 

Second, the Pu-238 was "encapsulated" by only 
modest measures. The Pu-238 was in the form of an 
unsealed plutunium oxide cylindircal rod about two 
feet long and two inches in diameter in length (if I 
recall correctly), carried on the lower stage of the LM 
in a lightweight tube. (The rod was to be withdrawn 
by hand on the surface of the moon and inserted 
into a coffee-table-sized power station to serve as 
a RTG-type source of electricity for various 
instruments to be left behind.) 

Third, the rod had a surface temperature of hundreds 
of degrees farenheit (F). So the brittle ceramic rod 
material was already under substantial heat "stress", 
producing a conditions more conducive to potential 
breakup of the rod.

Fourth, the reentry speed of the rod into the atmosphere 
was extreme. The LM, having no reentry protection at all, 
was exposed to the full force of a 25,000 mile-per-hour 
collision with the upper atmosphere. The resulting heat 
of friction reached several thousand degrees F, further 
adding to the heat of the rod. Combined with powerful 
physical and sonic shock waves inherent with high-speed 
reentries, the potential for disintegration of the Pu-238 
into micron-sized, readily-dispersible and respirable 
particles likely has never been higher with any potential 
Pu-238 reentry scenario to date. The approach speed 
of the better Pu-238-protected Cassini probe to Earth 
essentially was no more extreme than that for the 
reentry of Apollo 13.  

Despite the relative lack of safeguards compared to 
Cassini, there has been no increase in stochastic 
health effects worldwide known to be affiliated with 
external exposure to, or intake of, the plutonium of 
Apollo 13. Nearly thirty years have passed since 
Apollo 13 occurred. If any health problems, immediate 
or latent, were going to express themselves from it, 
their patterns should have emerged for detection by 
now. 

Some years after Apollo 13, a Russian oceanic radar   
spy satellite bearing over a hundred pounds of on-board 
nuclear reactor fuel fell out of orbit and broke up over 
Canada. The path of detectible radioactive material 
stretched for many miles on the ground, with parts found 
reading in the rems-per-hour range. There was some 
plutonium present in the core in that device, too. 
Although the speed of reentry was lower than that of 
Apollo 13, no evidence exists that any of that plutonium 
migrated beyond the debris path, nor has any evidence 
surfaced of harmful health consequences from any 
potential intake of the radiological contents of the 
satellite.

So there have already been at least a couple of 
plutonium reentry events into the atmosphere. No proof 
of any increases in lung cancer have been found from 
those events.

Is it reasonable to start drawing some 
conclusions, based on the plutonium reentry 
experiences so far, about the adequacy of the 
engineering controls associated with Cassini? 
Seems reasonable to me.

 
   
******************************************************
Steve Frey, Head
Operational Health Physics (OHP) Department
Stanford Linear Accelerator Center (SLAC)
Phone:(650) 926-3839 (office),
      (650) 926-3030 (fax),
E-mail address: sfreyohp@SLAC.Stanford.EDU
Any thoughts expressed here are not meant 
to speak for SLAC or any other party in any 
capacity unless so stated.
*******************************************************
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