The bigger problem is proliferation. All nuclear power plants produce plutonium from fuel stocks, which goes straight to the waste. Plutonium can be chemically separated from the nuclear waste, which is not quite easy, but much much easier than isotope separation. Thankfully, a single neutron capture transforms Pu-239 to Pu-240, which is unusable in nuclear weapons (if it wasn't the case, human civilization would have ended decades ago), and it is (or was) nearly impossible to separate these two isotopes. To obtain relative pure Pu-239, fuel assemblies or special U-238 rods need to be extracted early in the campaign, which is observable (unless a reactor is specifically designed for that). Unfortunately, modern laser isotope separation methods (spin chemistry, magnetic isotope effects) make it possible to separate Pu-239 from Pu-240. Most of the details are classified, but secrets can work only for a limited time.
A plane crash attack is no different than any other type of accident; it can be integrated in the incident response plan (much less dangerous than tsunami). This is not the big problem.
Up until the 1990s it was generally accepted that the greatest risk to humanity was not the slow problems of global warning but the near-instant destruction of an exchange of the thousands of ICBM-delivered nuclear warheads possessed by the two-and-a-half superpowers. These weapons depended on a supply of plutonium. Therefore many of the earlier reactors (eg Windscale/Sellafield) were built in a tremendous hurry to ensure enough plutonium.
Conversely, on the other side, environmentalists focused on cutting off the supply of plutonium to weapons. Recall that Greenpeace got started campaigning against nuclear bomb testing. This attracted hostility from governments, up to and including the French DGSE sinking the Rainbow Warrior and murdering the photographer who was on board. This has affected the subsequent debate because there is no trust.
The nuclear trust issue affects which countries are allowed to have nuclear reactors (see Iran, Iraq).
In the 2000s the big question was terrorism: would nuclear plants be vulnerable to attack, including crashed planes, and how could they be secured? Meanwhile a trillion dollars was spent on a largely pointless war, proving which kinds of threats are taken seriously and which aren't.