Add punctuation to the following sentence: This is a historic event It is the first time a Western nuclear power plant has been closed as a precautionary measure that is before a serious accident occurs Barsebck is only 20 km from Copenhagen capital of Denmark
"This is a historic event. It is the first time a Western nuclear power plant has been closed as a precautionary measure, that is, before a serious accident occurs. Barseback is only 20 km from Copenhagen, capital of Denmark.