MADRID, Sept 23 (Reuters) - Spain's 1,000 megawatt Asco I nuclear plant began to step down power output on Tuesday with a view to disconnecting from the grid later in the day to fix an oil leak, a statement from the plant's operators said.
That will reduce to six out of eight the number of Spain's normally working nuclear power stations.
The plant's operators said it would be reconnected to the grid once a leak in an oil line to a safety valve had been repaired, but gave no further details.
By 1330 GMT, Spain's nuclear plants were providing 6,058 MW, according to data from national grid operator REE, which compares with about 7,300 MW when all eight are running normally.
The Vandellos II plant -- which has the same operators as Asco I -- has been off line since one of its generators caught fire on Aug. 24.
Drops in supply from nuclear plants tend to force up the wholesale price of power for prompt delivery, although this also depends on the availability of wind and hydroelectric power.
Power traders said that bids for delivery on Thursday had risen on the over-the-counter market since news of the forthcoming halt broke, and that offers had been withdrawn.
Endesa-owned Asco I, which is near the northeastern port of Tarragona, faces a hefty fine after the nuclear watchdog pressed for the government to apply sanctions over inadequate handling of an earlier incident involving a radioactive leak.
Spain's nuclear industry has an uncertain future as operating permits for seven of the ageing plants expire between 2009 and 2011, or within the mandate of the recently re-elected Socialist government.
The government has vowed not to build any new nuclear plants, but has yet to rule out extending the working lives of those which are still in service.
Earlier this month, the Nuclear Safety Council (CSN) told the nuclear industry that renewing operating permits would depend on how they implemented tighter safety procedures spurred by a rash of unscheduled stoppages.