Date: 5/22/97
Hi!
Regarding our ongoing discussion about the RTGs:
I've posted about 15 pages from the June, 1995 "FINAL" ENVIRONMENTAL IMPACT STATEMENT FOR THE CASSINI MISSION at my web site. These pages refer to the structure of the nuclear components for the Cassini mission and the NASA-published test results and various percentages and so forth. Here is the URL of the index to the pages, which are GIF images:
http://www.animatedsoftware.com/cassini/feis1995/index.htm
While it is certainly not enough information to draw final conclusions, I would like to briefly go over these pages.
I think a review of them will bring us ahead on the RTG question somewhat -- I do wish I had found (or re-investigated, to more be exact) these things sooner! In any event, I'm sure you'll want to see them.
Here's a quick Glossary of Terms:
RTG=
Radioisotope Thermoelectric Generator. (Sore thumbs on 3 sides of
probe)
GPHS=
General Purpose Heat Source (Rectangular boxes)
GIS=
Graphite Impact Shell (A Graphitic (carbon-carbon composite) material (whatever that
means))
There are 3 RTGs, each with 18 GPHS units, each GPHS has two Pu 238 pellets [which are each encased in a GIS].
Here is a sentence from page 4-49:
"The variations in the reentry conditions that these 54 GPHS modules experience is predicted to result in a range of fuel end states, including damaged and undamaged GPHS aeroshell modules, GISs, fuel chunks, and fuel particles and vapor."
The way I read the above sentence is: They expect a rainbow of effects. All the way down to Earth. Continuing where I just left off, they say:
"Based on the best available information, evaluations determined that these fuel end states were possible for both shallow and steep reentry angles."
Next the pages cover four different effects in more detail.
FIRST: Intact GPHS Modules":
"...For the shallow and steep VVEJGA and VEEGA reentry cases studied, an average of 34 (steep reentries) to 49 (shallow reentries) percent of the GPHS modules from the 3 RTGs are expected to survive reentry intact." If the ones that survive reentry land on a hard surface, the "assumed release fraction" upon impact is 25%!
Looked at from the other side, that means that fully 51% to 66% of the GPHS's are EXPECTED to be damaged BEFORE they reach Earth, and 25% of those that make it to Earth will still crash hard enough to release fuel if they land on anything hard (or come in a little faster than NASA expects?).
SECOND: INTACT BUT DAMAGED GPHS MODULES WITH INTACT GISs
These are expected to constitute about 10 or 11 percent of the total. A TOTAL release of fuel from these GISs is EXPECTED if they impact a hard surface. And if they land on soil, a 25% release is expected, on average.
THIRD: INTACT GISs:
"For the reentry cases studied, an average of 7.3 (shallow reentries) and 23 percent (steep reentries) of the GISs are expected to be released from the GPHS modules at high altitude and to survive reentry." If the intact GISs make it to Earth and strike rock, a "total release of the fuel from the fueled clads is assumed."
FOURTH: FUEL PARTICLE AND VAPOR:
"For all the reentry cases studied, about 32 to 34 percent of the fuel from the three RTGs is expected to be released at high altitude."
From 20% (for steep reentries) to 66% ("very shallow reentries") of the released fuel will be in respirable particles (page 4-51).
-----------------------------------------
So I think, that the debate here is much more an issue of HOW MUCH than of IF. It seems to me that NASA and DOE are fully admitting that significant numbers of these plutonium pods will breach. I think NASA should modify its repeated claims such as the following from Mary Beth Murrill to Frederica Russell (as shown in an article at my web site):
"To answer your question, be assured that RTGs are designed to contain their plutonium-238 in the event of a launch or reentry accident."
Here is the URL of the full text of Ms Murrill's letter, and a response: http://www.animatedsoftware.com/cassini/murrill1.htm
So the question you and I face here, seems to be this: is it 32% to 34%, or 100%? And then, how much damage can the 5 to 15 pounds or so of inhalable Pu 238 that even NASA expects to be created in a reentry accident really do? And another question is: Now that we have some "real" NASA numbers, what degree of confidence does NASA actually have in these numbers?
All of the arguments regarding the strength of the packaging, must also somehow account for what I consider to be a horrible failure rate for 72.3 pounds of PU 238 -- 32% to 34% at high altitude, and who knows what rate as it streams closer to Earth (I can't find that figure in NASA's documentation), and then any impact releases that might occur from hitting a hard surface such as rock, or ice I would assume, and so on. I believe we are talking about the size of a dragon, not whether a dragon exists or not.
If the high-strength cladding is EXPECTED to fail 32% to 34% of the time if a reentry occurs, why shouldn't I wonder just how sure we are that it won't be 100% considering that Cassini will be the fastest object mankind has ever propelled [past Earth in a flyby manuever]? I think NASA's margin of error is probably enormous here.
Thank you again for all your correspondence.
Sincerely,
Russell D. Hoffman
P.S.: This is definitely a DOE question as well as a NASA question: On page 2-12 of the Final EIS, it reads:
"The U.S. Department of Energy (DOE) would provide the RTGs and RHUs and would retain title to them at all times."
cc: Beverly Cook of the DOE, other interested parties...
rhoffman@animatedsoftware.com