Subject: Why would high grade aluminum be used in nuclear weapons?

About a week ago, I heard about the controversey involving the Russians 
exporting material to the Palestinians.  One of these exports was high grade 
aluminum in which the news announcer stated that it could be used with enriched 
uranium for nuclear bombs.  I'm not sure how combining uranium with aluminum 
could improve the materials effectiveness.  I would think that this aluminum 
could be used as a radiation shield for the bomb casings.  Yet, the announcer 
said that it could be combined with the uranium.  The problem I figured out was 
that uranium should be in pure form and hardly contain any other metal in order 
to be effective bomb material.  Am I missing something on the physics of 
weapons or was this just bad reporting?

Re: Why would high grade aluminum be used in nuclear weapons?