After the explosion at reactor No. 4, the remaining three reactors at the power plant continued to operate, with reactor 3 working until 2000. How did people worked there? What security measures were taken?
While you're waiting for fresh answers from qualified experts, you may be interested in some previous threads on this topic.
After the Chernobyl incident and the explosion of reactor No. 4, reactors 1, 2, and 3 continued to operate well into the 1990s, how is this possible? with a detailed response by /u/restricteddata.
According to Wikipedia, the other 3 reactors at the Chernobyl power plant continued operating and producing electricity for many years after the accident, with the last reactor closing only in 2000 (!). Who worked there? Where did they live? How did they get to work every day? with multiple detailed responses by /u/Minardi-Man.