Complex sentence: The plant was originally rated at 210 MWe, but the danger of significant oxidation of mild steel components by the high temperature carbon dioxide coolant required a reduction in operating temperature from 390 to 360 Celsius which reduced power from 210 MWe to 160 MWe.
Split into simpler sentences: The plant was originally rated at 210 MWe, but the danger of significant oxidation of mild steel components by the high temperature carbon dioxide coolant required a reduction in operating temperature and power output. Operating temperature was reduced from 390 to 360 Celsius which reduced power from 210 MWe to 160 MWe.