The National Council of Resistance of Iran (NCRI), which has in the past provided accurate information on hidden Iranian nuclear activities, said Tehran had used front companies to obtain the substance, a hydrogen isotope known as tritium.
"The regime has tried to smuggle it in from South Korea," said Ali Safavi, a senior NCRI official, told a news conference in Brussels, citing what he called high-placed unnamed sources in the Iranian leadership.
Safavi said he understood that Iran’s attempts to acquire the substance had been successful, but gave no further details.
Tritium has many civilian uses such as in luminous paint and in testing the safety of drugs but can be combined with another hydrogen isotope known as deuterium to act as a "booster" in nuclear bombs. It is subject to export controls.
"Tritium and deuterium together increase the explosive power of a bomb tenfold. This is essential for producing a smaller size of nuclear bomb," said Safavi, adding that it had informed the U.N.’s nuclear watchdog, the IAEA, of its allegations.
The IAEA declined to comment.
South Korea ranks sixth in output of nuclear power in the world, producing about 40 percent of its electric power at its 20 nuclear plants.
The West suspects Iran is taking steps toward building atomic arms. Tehran denies this and says its nuclear program is for peaceful purposes only.
The NCRI, which is listed by the State Department as a terrorist organization, revealed in August 2002 the existence of the Arak heavy water facility and a massive underground uranium enrichment plant at Natanz.
Its new allegations come days after Britain, France and Germany, prompted by Tehran’s resumption of some nuclear work, canceled talks with Tehran aimed at encouraging it to halt activities in return for economic and other incentives.
Safavi also repeated allegations by the NCRI that Iran had already purchased deuterium from abroad. It is not illegal for Iran to buy deuterium, but it should be reported to the IAEA.
The NCRI said last week that Iran was forging ahead with a separate program that could produce plutonium for nuclear weapons at Arak, 240 km (150 miles) south of Tehran.
Safavi said the Iranian leadership was working on the assumption that the plant, which Tehran says is based around a 40 megawatt research reactor, would be ready by 2007.