The new terrorist movements are willing to use unlimited violence and cause massive casualties. Europe is both a target and a base for such terrorists.
Tactical nuclear weapons are difficult to obtain and deploy. Assembling a dirty bomb is less difficult but without state support still a bridge too far.
Nuclear power plants pose a serious security risk
Inside safety "failings" without disastrous results occur regularly.
There were zero attacks on operational nuclear plants and security failings to prevent attacks are obviously possible but uncommon.
Running people over, spraying them with gunfire, and blowing them up offers the package the media need, attacking a nuclear plant in not publicly accessible areas does not.
The threat environment has changed
Attacks with planes or rockets are unlikely to result in a significant radiation threat, the building structure of nuclear plants qualifies them per definition as hard target. However, using rockets or the like must then be aimed at the reactor’s spent fuel pool, waste material and/or (military) fuel facilities, which are a radiation hazard and less protected than the reactor core.
Outdated security has become the standard for commercially operated nuclear facilities, focused on prevention, not response. A determined, knowledgeable and well trained adversary can render large portions of Europe uninhabitable if successful.
Insider threat - Al-Qaeda members have worked at nuclear power plant sites. Even with the best criminal background checks in place, enemies can introduce informants or a saboteur to disable crucial equipment.
