Spent Fuel and Possible Bomb Material

[Myung Chul Jo <mjo@scs.unr.edu>]

This city where I live is having a few activities against proposed passage
of spent fuel from research reactors around the world (and maybe US Navy)
passing through the city.  The returned spent fuel would arrive at
Concord, CA and will be headed to a western state via Nevada.  An
anti-nuclear group called "Citizen Alert" claims that the shipment have
equivalent material to make 60 nuclear bombs dropped in Hiroshima.

Could anyone tell me how this type of group derive these numbers in
general?  I do not know any info about volume or weight of the spent fuel 
being shipped.

What is the critical mass for U-235 and Pu-239?  20 pounds is coming
to my mind somehow?  Is this a classified information?

What fraction of the spent fuel would be U-235?  I assume these fuels are
highly enriched with U-235.  

This is only my opinion...
Myung Chul Jo <mjo@scs.unr.edu>
(702) 784-4540(voice)
(702) 784-4553(fax)