Complex sentence: It was to be opened on May 1,1986, in time for the May Day celebrations (decorations for this event are still in place in Pripyat today) but the plans were interrupted when on April 26 the Chernobyl disaster occurred a few kilometers away.
Split into simpler sentences: It was to be opened on May 1,1986, in time for the May Day celebrations (decorations for this event are still in place in Pripyat today). The plans were interrupted when on April 26 the Chernobyl disaster occurred a few kilometers away.