BTW the best explanation is that 2-2,5 tons missile cannot have nuclear source of energy. And who said that is has 2 tons? or Tu-160 can have 6 in revolver launcher?
Uranium and lead are very dense materials that don't take up much room... the MTOW of the Blackjack is 45 tons in the current upgraded model, so the limit is volume and not weight... and why would you put it on a bomber?
It can fly all the way to any target on earth with a much smaller RCS...
A 2KT nuclear bomb can be fitted into an artillery shell 152mm wide and 40kgs in weight... to be perfectly honest, I really don't think a nuclear turbojet would need to be much bigger or heavier than that... but with a 2-2.5 ton cruise missile you wont need about 1.8 tons of fuel, so even if the nuclear propulsion is 1.5 tons, it would still be 300kgs lighter than a normal turbojet powered version of the same missile... except with unlimited range...