"There is nothing preventing us from agreeing ... we will agree," he told reporters while in Vienna for a general conference of the International Atomic Energy Agency (IAEA).
In answer to concern about Iran getting its hands on radioactive material to use in weapons programs, Moscow has insisted that Iran send back to Russia spent fuel to be used at the Bushehr reactor it is helping Tehran build.
Western nations fear Tehran may use spent fuel from the plant to make atomic bombs.
Iran has in turn demanded that Moscow pay for the return of this fuel, something Moscow refuses, as it would be paying for fuel it has itself supplied.
Rumyantsev did not say when or under what financial conditions an agreement would be reached.
But he said that "today (Tuesday) the Iranians said two advisors to the president of Iran, the heads of their nuclear agency, will come to Moscow," to discuss techncial details.
Russia has called on Iran to abide by the IAEA's deadline for Iran to prove by October 31 that it is not secretly developing atomic weapons.
But while supporting this anti-proliferation push, Russia is still moving ahead with the Bushehr project, for which it has an 800 million dollarmillion euro) contract that means jobs for Russian workers.
The plant was originally due to go online by the end of 2005, but the project has been held up as Russia and Iran negotiated a separate agreement that would oblige Tehran to return the plant's spent fuel to Russia for storage.
Asked whether he thought Iran was trying to get the bomb, Rumyantsev said: "All scientific research conducted in Iran can be used for nuclear energy purposes as well as possibly for nuclear military technology."
"Any basic study has always dual purpose," he said.