Effectively, they already do that. Although the United States doesn't launch reactors, we do on occasion launch radioisotope thermoelectric generators (RTGs). T

Effectively, they already do that. Although the United States doesn't launch reactors, we do on occasion launch radioisotope thermoelectric generators (RTGs). These use a core of sub-critical plutonium surrounded by thermocouples, which turn the heat into electricity. These are used for probes going to the outer solar system, where solar panels aren't effective enough. Anyway - there is certainly a concern with the plutonium in RTGs being dispersed by a launch failure. The engineering that goes into designing the protective system for RTGs is extensive; they each have their own miniature heat shield, and are surrounded by iridium and carbon blocks. Tests show that they can indeed survive the explosion of the launch vehicle.