Iran's nuclear chief said that in October fuel was removed from the country's sole nuclear reactor because debris had been left behind during its construction.
The Sunday report by several Iranian newspapers quotes Fereidoun Abbasi as saying that bolts and welding material left inside the Russian-built Bushehr reactor had led to abnormal readings during operation.
Abbasi added that fuel removal and temporary plant shutdown are part of normal operating procedures.
On Wednesday, Iran said the power plant is ready to resume operations after refueling. The plant reached its full 1,000-megawatt capacity in September and is undergoing final technical assessments before Iranian engineers take over full responsibility from a Russian team in early 2013.
The West suspects Iran of trying to develop nuclear arms. Iran denies the accusations, and its first plant near the Gulf city of Bushehr is a symbol of what it says are its purely peaceful nuclear ambitions.
The UN nuclear agency said in a confidential Nov. 16 report that fuel assemblies — bundles of fuel rods loaded into the reactor — had been transferred from the reactor core to a spent-fuel pond in October, but it gave no reason.
It was the second time in less than two years that fuel has been unloaded from the reactor, which Russian state nuclear corporation Rosatom said in late August had been powered up to full capacity.
Rosatom subsidiary NIAEP, which runs the project, said in October that Bushehr would be formally "handed over for use" to Iran in March 2013. Officials had expected this to happen by the end of 2012.
In a written reply to questions, NIAEP said "unscheduled additional tests and checks as well as repair work on equipment and systems" had been done at the plant. NIAEP said that the reactor would be at full capacity again in late December and that the additional work was behind the delayed transfer of responsibility to Iranian operators.
It has said that dozens of Russian specialists will remain at the plant after the handover, which will be a milestone for the project, which was started by Germany's Siemens before the 1979 Islamic Revolution that toppled the U.S.-backed shah.
U.S. concerns that the Bushehr project could help Tehran develop a nuclear weapon were eased by a requirement that Iran return spent fuel to Russia.
Weapons-grade plutonium can be extracted from spent fuel.
Bushehr is not considered a major proliferation risk by Western states, whose fears are focused on sites where Iran has defied global pressure and Russia-approved UN sanctions by enriching uranium beyond levels needed to fuel power plants.
But Western officials have voiced concern about unloading fuel, saying it raised questions about safety and potentially about proliferation if the fuel were misused. They have also criticized Iran for the secrecy surrounding the operation.
Asked on Thursday about fuel removal from the plant, Iran's ambassador to the International Atomic Energy Agency said the country was determined to make sure safety was guaranteed after the plant is turned over to Iranian operators.
"We are going to make sure that every item, every part of it is safe before we receive (it) because it will be our obligation afterward," Ambassador Ali Asghar Soltanieh told reporters at the UN nuclear agency in Vienna.