South Korea's Nuclear Safety and Security Commission (NSSC) on 25 May approved the restart of unit 1 at the Wolsong NPP. The reactor was taken off line on 11 May because of damage to a rubber diaphragm in the valve that controls cooling system pressure. The NSSC said the damaged diaphragm was replaced and that it had ordered the plant's owner and operator, Korea Hydro and Nuclear Power, to take measures to complement its maintenance procedures. Wolsong 1, a 657MWe Candu 6 unit, began commercial operation in 1983. There are three other Candu 6 units at the site in Gyeongju. South Korea's 23 nuclear reactors supply about a third of its electricity.