Technology

First, nuclear fuel production is one of the most highly regulated and monitored processes mankind has ever developed. Second, if by some astronomically small chance enough of the fuel is faulty, there are multiple other safety layers to keep a runaway reaction from occurring. These are built into the design of the reactor itself and don't need human intervention. Third, if the reactor were destroyed, by anything, all it would accomplish is scattering the fuel pellets, effectively extinguishing any reaction. After that it's a fairly straightforward job to gather the scattered fuel during cleanup. These reactors don't need a water source like the old ones do so once the fuel lands somewhere it's not going to be washed away and become difficult to trace. Fourth, reactors are built seriously tough. If something hits one hard enough to cause anything more than an emergency shutdown then you've got much bigger problems to worry about.