Just my guess, but even in environmentalist movements many old people take leading roles, thats even more true when it comes to political parties (like european green parties). Many of them arent just "traumatized" by chernobyl, but were pretty much born as reaction to it, or on other nuclear power related issues. The predecessor lists of the german green party (one of the most radical ones against nuclear power among european environmentalist parties) spawned out of a discussion about a repository for nuclear waste and the difficulties when dealing with nuclear waste. Back then Nuclear was what fossils are today, if not even worse. You can add to that the cold war background, and their work as peace movements, first and foremost against nuclear weapons/arms race. Thats pretty much how anything "nuclear" is associated by these older environmentalists. Chernobyl and nuclear war.

2008 protest of german green party members. As mentioned the german movement against nuclear power is also the probably strongest in europe.

https://upload.wikimedia.org/wikipedia/commons/d/d0/Gr%C3%BCne_protests_against_nuclear_energy.jpg

Do you see all these students and young people commonly associated with the green party and spearheading their demonstrations? (although it should be said that there probably were some. its germany. You can find at least some anti nuclear power banners even in the current climate protests against fossil fuels in germany)