As someone said above, the colleges need to put on activities for all kinds of people and not everyone wants to go to nightclubs. But you'll be able to go out if you want to - sometimes colleges book out nightclubs/bars and your freshers reps will take you there. They like to take you to quite a few different ones in the week so you can get a feeling of where you like most. Like, for example, after your "mentor formal" people will probably go out to nightclubs. And at the end of the day even if nights out aren't on the official activity sheet, they can't exactly prevent you from going out if you want to.
Honestly don't worry! I'm not really into going out, but I know people who were very apprehensive about coming to Durham, and they've been able to fit in and find places to go. It's not the biggest place for nightlife (as it's quite a small city), but there's definitely stuff to do, and if you want things on a bigger scale, you can always get the train to Newcastle.