I think he means that once the fuel rods are in or near the reactor, the danger of bombing becomes greater due to possible radioactive contamination.
My point is that this assessment is incorrect. The fuel can be sitting in the reactor, but, as long as the reactor has not sustained a chain reaction, the amount of radioactivity is minimal.
When the reactor starts producing power, the radioactive inventory will increase, but, until then, the danger of wide-spread contamination is nil.
Besides, you want to disable the plant? Take out the turbine building with a few well-placed bombs. Unlike the reactor containment building (which is several feet of reinforced concrete and is designed to keep bad things outside from getting in and bad things inside from getting out), the turbine building typically has sheet metal exterior walls. Geez, some plants in the USofA, particularly in hurricane-prone areas, don’t even have walls of any sort.
Turbines take a long time to replace, particularly if the turbine building has been collapsed.
If the plant doesn’t have a turbine, then it cannot produce power (pesky thing, that law of conservation of energy).
Alternatively, take out the switch-yard. If you can’t send the power off site, then you can’t generate it.
I suspect that Ambassador Bolton knows these things.