Terrorists pose a threat the world over and security at nuclear sites is crucial. But there are far easier targets for terrorists to hit.
Nuclear power stations are not bombs waiting to go off. Their systems are entirely different from military weapons. Nuclear reactors are operated within robustly thick concrete containment vessels designed to withstand massive impact. In fact, in computer simulations an aircraft flown into a nuclear power station containment vessel at 500 mph evaporated, but the containment was largely undamaged.
For many reasons, including the difficulty of handling used nuclear fuel without specialist equipment, there has never been a terrorist attack on a nuclear installation.
Any reactor licensable in the West has a substantial containment structure and most also have a very robust pressure vessel and internal structures. Evaluations since 2001 suggest that power reactors would be well equipped to survive an impact of that kind without any significant radiological hazard locally. Civil waste and spent fuel storage is also robust and often below ground level.