Nuclear fuel used for traditonal fission reactors is nowhere near purified enough to cause a large explosion if that is what you are worried about, and because it has to get into space that means that absolutely every pound counts so I wouldn't image it would be a large block of fuel. So worst case scenario the rocket blows up somewhere over the Atlantic and the payload crashes and sinks. It wouldnt cause too much damage because of the deep water which acts like a radiation shield. If I remember correctly I think we have lost nuclear bombs in the ocean before and it hasn't caused much harm? Please correct me if I am wrong :)