Some physicist will correct me, but I think this is more of an accounting trick than a technological one: it serves to ask for more money.
I have no evidence, but my impression is, that aerogel is used to track the ageing of the decaying warheads, also providing data for the computed simulations to be more realistic. Such aerogel is readily available on the free (in the doublespeak sense) market.
The building it was originally made at in Y-12 (called facility 9404-11) was torn down in 2004. The nasty solvent (which metabolises in the body into cyanide) called acetonitrile, of which Wikipedia claims there is supposedly a world shortage as the sole remaining factory producing it was shut down in 2008 (to reduce pollution) for the Olympics.
I have no evidence, but my impression is, that aerogel is used to track the ageing of the decaying warheads, also providing data for the computed simulations to be more realistic. Such aerogel is readily available on the free (in the doublespeak sense) market.