In France too, and they have an efficient way of solving the “what if there are terrorists among the visitors” problem: the groups are escorted by armed members of the French special forces (there's an on-site platoon of them on every nuclear site to act as a rapid reaction force).
I did too. I also visited Hinckley Point as a teenager, but for that we could just turn up whereas for last year's trip we had to book well in advance and submit ID information for vetting.