Actually, they did cut open the reactor building. CANDU is a unique type of nuclear reactor in that it uses unenriched natural uranium. It uses a heavy water loop, which is heated by the reactor core (heavy water acts as the moderator). The heat from the heavy water loop is transferred to a light water loop in a steam generator. The generated steam is sent to another building where a turbine turns it into electricity. They replaced those steam generators, which are housed in the reactor building.

https://en.wikipedia.org/wiki/CANDU_reactor