"With two out of three units already operational, we are currently able to produce heavy water with 15 percent," said the plant's research and development chief, Manoushehr Madadi, the state news agency IRNA reported.

"In a month's time and with operation of the fourth unit, we will be able to produce heavy water with 99.8 percent," he said.

"By then we will be able to produce 16 tons from the current eight tons and therefore the country will no longer need to import heavy water," he said. The plant itself "was constructed and operated by Iranians without outside help."

Heavy water is used in nuclear reactors.

Britain, France and Germany, which have been in negotiation with Iran, have called for Tehran to suspend uranium enrichment activities and the construction of the Arak plant.

Iran returned Wednesday to talks in Vienna with Europe's three heavyweight countries aimed at resolving a long-running dispute over its nuclear program, which Washington charges is being used to develop a nuclear bomb.

Iran was to respond to an offer by the Europeans that would enable it to avoid potential UN sanctions and receive nuclear technology by indefinitely suspending uranium enrichment.

The offer was made at talks in the Austrian capital on October 21.

Under the European offer, Iran would receive valuable nuclear technology, including a light-water research reactor which would produce less fissionable material than the heavy-water reactor Tehran wants to build.

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