I assume you mean nuclear plants. The first series were the Magnox design which used natural metallic uranium in magnesium alloy sheaths, graphite moderator, and CO2 gas coolant. These are now obsolete and only one or two stations are still operating. The development of these was the AGR which used oxide fuel in stainless steel sheaths, graphite moderator, CO2 gas coolant. These could operate at much higher coolant outlet temperature and so are more efficient, but to keep the graphite at acceptable temperature a very complicated arrangement of re-entrant gas flow is required. There are 14 of these reactors operating. There is also one PWR reactor operating.