Re: Plutonium and nuclear weapons

["Scott D. Kniffin" <Scott.D.Kniffin.1@gsfc.nasa.gov>]

Mr. Cohen requested some help in his mass of Pu from nuclear weapons tests.
 I hope this proves usefull:

The definition for kilotons in a thermonuclear explosive refers to the
equivalent mass of TNT (trinitro toluene) needed to produce the same
effect.  The yield of a nuclear weapon is extreemly subjective.  It depends
greatly on the purpose of the bomb and the military objective desired.  For
100 pounds bomb mass (nuclear material + bomb casing), the yield can be
adjusted from as low as 1kiloTon to somewhat greater than 200kT.  The
actual mass of the nuclear material is dependent on the isotope used.  For
a >99% enriched U-235 weapon, the minimum bare critical mass for a nuclear
weapon is around (If I remember this exercise correctly) 17kg.  For a
Pu-239 weapon it's about 7 or 8kg.  Size wise, that's about softball size
for the U and a little bigger than a baseball for Pu.  (You can see why the
SNM in bombs are called "pits".)  The energy created in an explosion is
directly related to the mass of material around the bomb; espically the
iron casing.  The majority of the thermal energy is created when the bomb's
hypervelocity fission fragments convert the bomb casing and several
thousand pounds of air (yes, tons) into plasma in times less than 1
microsecond.  I can go on and on, but I'll try to wrap this up.  The yield
is adjusted by several things, the major factors being: mass of the bomb
casing, the configuration of the detonator (precision explosive or neutron
flash generator), and the purity of the fissionable element.  Depending on
wether you want there to be nothing left (yield >90%) or there to be only
moderate dammage but no one can live there for a very long time (yield
60%>y>50%) you set the bomb up differently.  US policy (from my very
limited knowledge of it) is that the idea is to get the highest yield
possible for the minimum ammount of material.  The former Soviets were not
so picky.  Most of our modern bombs are probably in the >92% yield range.
I have rather exact numbers somewhere for the earlier bombs but I'm at work
and don't have that refernece with me.  (The Effects of Nucelar Weapons by
Glasstone(?) from the GPO back in the 60's.)  They were quite high, seeing
as people do live in Hiroshima and Nagasaki today.  Bikini Atol is a
different beast all together. Last I heard the blast site was still quite
uninhabitable.  Nukes and saltwater really don't mix.  
In summary, yes there has been a lot of Pu vaporized into the atmosphere
and it's very safe to say that it is on the order of several hundred pounds.  

Oh, incidently, the Pu in the Cassini RTGs is PuO2, not pure metal.


At 10:42 08/27/97 -0500, you wrote:
>
>
>
> --I noticed these numbers in UNSCEAR 1982, but they do not agree
>with the numbers I gave in my message -- about 3.6 million Curies of
>Pu239 evaporated in bomb tests. This is seven times the amount in the 
>Cassini probe, rather than 1/2 the amount as Otto states.I don't see how
>my calculation can be substantially in error, unless I am going "off
>the deep end". Can someone help me with this, by confirming my calculation
>or by showing where I went wrong? [My calculation is 150,000 Ktons of
>fission with 1.7 Ktons/lb = 90,000 lb of Pu used. 80% of this not consumed
>but evaporated = 72,000 lb. Bomb Pu is .11 Curie/gram x 454 gram/lb x
>72,000 lb = 3,600,000 Curies.]
>
>Bernard L. Cohen
>Physics Dept.
>University of Pittsburgh
>Pittsburgh, PA 15260
>Tel: (412)624-9245
>Fax: (412)624-9163
>e-mail: blc+@pitt.edu
>
The opinions expressed here are my own.  They do not necessarialy represent
the views of Unisys Corporation or NASA.  This information has not been
reviewed by my employer or supervisor.  

Scott.D.Kniffin.1@gsfc.nasa.gov
RSO, Unisys Corp. @ Lanham, MD
CHO, Radiation Effects Facility, GSFC, NASA, Greenbelt, MD