Spanish nuclear technicians have got a screw loose - forcing an entire plant to stay off-line.
The plants owners, Union Fenosa, deny there is any risk
The screw has vanished from a tool being used in a refuelling operation at the Zorita plant east of Madrid.
Two days of intensive searching have failed to find the 2.5cm-diameter (one-inch) screw - forcing the plant to stay shut until further notice.
Spain's Nuclear Safety Council says it can't be sure when the plant will be allowed back on stream.
"We don't know when we'll let them restart," said council spokesman Jose Francisco Morales.
"We're waiting for a report that will tell us that the screw is in a safe place, that it won't move and that it won't cause any
Production at the 35-year-old nuclear plant had been due to resume Monday.
The search involved dismantling the nuclear core with remote-controlled machines in a sealed cell, and using cameras to peer into inaccessible areas.
The plant's owners, Union Fenosa called the disappearance of the screw a "hypothetical loss" and said it presented no risk.
The screw was too big to have fallen into the nuclear fuel core, it said.
But environmental groups, which already believed the plant was unsafe, say the detailed nature of the search proves the significance of the lost screw.
"They would hardly go to the trouble of such a complex
search if it weren't important," said Greenpeace spokesman
Greenpeace claims that if the screw were loose in the nuclear core or in the
high-pressure vessel containing it, it could cause a serious accident.
But a safety council spokesman said the screw was unlikely to pose any risk.
The reactor is due to be dismantled in 2006.