The US airforce played with the idea of nuclear propulsion for aircraft in the fifties or early sixties with a plan to build a version of the massive Convair B36 witha reactor driven power plant. I suppose if you are starting out with ten engines there is an incentive to try and rationalise things even if it involves something like lead which is hardly an ideal component for any aviation application .
I am not sure if the link will work but type' B36' in Google and you will find sooner or later
http://www.cowtown.net/proweb/nb36h.htm
With fuel prices and todays technology who knows whether the idea might get brought back -can hardly use wind or wave power on an aircraft can you
PB