Iran told atomic inspectors this week that it had run into a serious problem at a newly completed nuclear reactor that was supposed to start feeding electricity into the national grid this month, raising questions about whether the trouble was sabotage, a startup problem, or possibly the beginning of the project’s end.

In a report on Friday, the International Atomic Energy Agency said Iran told inspectors on Wednesday that it was planning to unload nuclear fuel from its Bushehr reactor  the sign of a major upset. For years, Tehran has hailed the reactor as a showcase of its peaceful nuclear intentions and its imminent startup as a sign of quickening progress.

But nuclear experts said the giant reactor, Iran’s first nuclear power plant, now threatens to become a major embarrassment, as engineers remove 163 fuel rods from its core.

Iran gave no reason for the unexpected fuel unloading, but it has previously admitted that the Stuxnet computer worm infected the Bushehr reactor. On Friday, computer experts debated whether Stuxnet was responsible for the surprising development.