FACTBOX-Spain's nuclear power plants

July 2 (Reuters) - Spain said on Thursday its oldest nuclear power station could stay open for another four years in the Socialist government's first test of its electoral pledge to phase out nuclear energy.

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Permits to operate another six of Spain's eight remaining nuclear plants are up for renewal in the next two years.

Following are some key facts about Spain's eight nuclear power stations.

* They have a total installed capacity, or the maximum power they are designed to generate, of 7,773 megawatts.

* They account for 8.1 percent of Spain's total installed capacity, but produced 19.1 percent of its electricity in May. * 10-year operating permits for six of the plants are due for renewal in 2010 and 2011.

* Spain's Socialist government promised to gradually replace nuclear plants with renewable energy in its successful re-election campaign last year, but did not rule out extending their working lives.

* Garona's original 40-year life span will expire in 2010, as measured from when it was first licensed. It has been in commercial operation since May 1971.

* The other plants will turn 40 between 2020 and 2027.

* Nuclear plants are Spain's third-largest source of power behind gas-fired plants and renewable energy.

* Spain is the world's third largest producer of wind power, with more 16,000 MW installed, and the second in solar, with more than 3,200 MW.

* The plants are owned in varying proportions by utilities Iberdrola (IBE.MC), Endesa (ELE.MC), Union Fenosa (UNF.MC) and Hidroantabrico (EDP.LS).

Sources: Spanish national grid operator REE (REE.MC), Nuclear Safety Council (CSN), Spanish Socialist Party (PSOE). (Compiled by Martin Roberts; Editing by Keiron Henderson)