Complex sentence: Two reactors were eventually built at Chalk River; the small ZEEP, which went critical on 5 September 1945, and the larger NRX, which followed on 21 July 1947, which for a time was the most powerful research reactor in the world.
Split into simpler sentences: Two reactors were eventually built at Chalk River; the small ZEEP, which went critical on 5 September 1945. The larger NRX that followed on 21 July 1947 was for a time was the most powerful research reactor in the world.