Complex sentence: The first research reactor was built in 1953, scaled up in 1957 and 1960 (referred to as R-1, R-2, and R-3), and finally deactivated in 1973 to make way for the PULSTAR reactor.
Split into simpler sentences: The first research reactor was completed in 1953; it was scaled up in 1957 and 1960 (referred to as R-1, R-2, and R-3). It was deactivated in 1973 to make way for the PULSTAR reactor.