Sellafield is a large site consisting of many facilities [1]. These include Magnox reactors [2] which were historically used to create weapons grade Plutonium and are much "messier" than typical commercial power reactors. The Sellafield site has been and is still home to a very large spent fuel reprocessing plant that handles spent fuel from numerous UK reactors. Reprocessing plants are an entirely different beast. And a lot of the activity at the Sellafield site is generally from older, messier technologies as well.

[1] https://en.wikipedia.org/wiki/Sellafield

[2] https://en.wikipedia.org/wiki/Magnox