the event did happen it did cause the mutation of people and animals but not like zombies they wont come and attack you its like an extra arm when babies are born the mutation was mostly from women who where affected to the radiation making their children have an extra arm or one eye or have something growing from their skin but they wont attack you their just like normal people
While it is true that the nuclear reactor at Chernobyl experienced a catastrophic meltdown and many people died, I have yet to see any documentation that nuclear fall-out mutants are inhabiting the area.
Chernobyl diaries the movie is based on real events (story) but the characters used are fictional. The story is based on something that actually happened like the 'The Texas Chainsaw Massacre' just to name one. The people used in the characters are just fictional.