"The failures that Iran has been reproached for are minor, and are only in the order of the gramme or milligramme, while in the past some countries had problems with larger quantities of plutonium," Iran's representative to the IAEA, Ali Akbar Salehi, was quoted as saying by state television.
"Failures are a normal thing, and the report of last year (by the IAEA) stated failures by 50 states," he added.
The UN nuclear watchdog said in a report Monday that it had so far found no evidence Iran was trying to develop nuclear weapons, but the agency was also not ready to certify that Tehran's atomic programme was exclusively peaceful.
The IAEA reported a series of breaches by Iran of international nuclear monitoring agreements, including the secret production of plutonium at the Tehran Nuclear Research Center "and subsequent plutonium separation experiments" between 1988 and 1992.
Also listed as infringements were Iran's enrichment of uranium and the import of certain nuclear materials.
Salehi said these failures only concerned "experiments in laboratories which we should have declared to the agency".
"Given that these failures correspond to the past, corrective measures have been taken and therefore this matter is closed," he asserted.
"And taking into account all the information now in the hands of the agency (given to the IAEA by Iran), it is clear that Iran had failed on several occasions and for a long period to meet its safeguard commitments" set out in the nuclear Non-Proliferation Treaty (NPT), he added.
Salehi said the plutonium was "experimental" and designed for use in medication for hospitals.
"When you produce these medicines, a very small quantity of plutonium is also produced, regardless of the manufacturer's intentions. It's a secondary effect," he said.
The IAEA report, which is to be submitted to a meeting next week of the agency's 35-nation board of governors, said the IAEA was still investigating the possibility that Iran is hiding an atomic weapons programme.
The IAEA's executive board of governors could declare Iran in non-compliance with the NPT, which could lead to UN sanctions. But some diplomats said the country may escape a non-compliance ruling as it has over the past month yielded to key IAEA demands.
Crucially, the IAEA report said that until October, Iran's cooperation had been "limited and reactive" but "since that time Iran has shown active cooperation and openness."
The IAEA in September had asked Iran to do three main things ahead of the November 20 meeting: fully disclose its nuclear programme, agree to tougher inspections and suspend the enrichment of uranium that could be used to make an atomic bomb.
Iran told foreign ministers from Britain, France and Germany it would cooperate when the three diplomats visited Tehran on October 21 to break the deadlock.
Tehran then promptly handed the IAEA a full declaration of its nuclear activities, and on Monday handed the IAEA a letter agreeing to tougher inspections of its nuclear program.
It also informed the agency it was suspending the enrichment of uranium.
Hassan Rowhani, who as Iran's top national security official handles the country's nuclear affairs, told state television on his return from Moscow that the IAEA report had vindicated Tehran.
"The report shows that Iran is cooperating fully with the agency ... Iran has not violated the NPT," he said.