Say the block was adequately heat shielded so that it could survive the launch and reentry. Would the force of impact be enough to trigger a fission reaction?
Also, I thought the fallout had a relatively short half life, right...
This idea stems from the Pascal B nuclear test, which placed a 900kg steel plate over a shaft in where a nuclear explosive was detonated: http://en.wikipedia.org/wiki/Operation_Plumbbob#Propulsion_of_steel_plate_cap
The plate shot up at an estimated 66km/s, or 6x escape velocity. Say you...