text: Ali Asghar Zarean told state TV that Iran would sign an agreement with China to modify the reactor before doing so.
  On Monday, the semi-official Fars news agency cited unnamed sources as saying the reactor had been decommissioned.
  It would represent a final step towards the implementation of July's nuclear deal between Iran and world powers.
  Iran has agreed to limit its sensitive nuclear activities in return for the lifting of crippling sanctions.
  Iran has for years been building a heavy-water nuclear facility at Arak, the spent fuel from which would contain plutonium suitable for a nuclear bomb.
  The P5+1 group of world powers - the US, UK, France, China and Russia plus Germany - had originally wanted Arak dismantled because of the proliferation risk. But they eventually agreed that Iran could redesign the reactor so it would not produce any weapons-grade plutonium.
  Monday's report by Fars, which is close to the powerful Revolutionary Guards, said technicians had removed the core of the existing reactor and poured concrete into the cavity.
  In July, Iran agreed a landmark nuclear deal with six world powers to limit its sensitive nuclear activities for more than a decade in return for the lifting of crippling sanctions. The US is confident the agreement will prevent Iran from obtaining a nuclear weapon. Iran says it has the right to nuclear energy - and stresses that its nuclear programme is for peaceful purposes only.
  Iran will not see the UN, US and EU sanctions lifted until the global nuclear watchdog, the International Atomic Energy Agency (IAEA), certifies that it has fulfilled its commitments under the deal. The precise date of the so-called "implementation day" has not been determined. But Iran says it has met those commitments earlier than expected and last week US Secretary of State John Kerry declared: "We are days away from implementation."
  The sanctions have cost Iran more than more than $160bn (Â£102bn) in oil revenue since 2012 alone. Once they are lifted, the country will be able to resume selling oil on international markets and using the global financial system for trade. Iran has the fourth largest oil reserves in the world and the energy industry is braced for lower prices. Iran will also be able to access more than $100bn in assets frozen overseas.
  But on Tuesday, Mr Zarean insisted that Iran would not decommission the reactor until it had signed a deal with China regarding its redesign - something that is expected next week.
  "Definitely, we will not apply any physical change in this field until a final agreement is finalised," he said, without making reference to the Fars report directly.
  Mr Zarean also said that once the modifications were completed and Arak went online, Iran hoped to export excess heavy water produced there to the US for research purposes, via a third country.
  Iran estimates it will need about 6 of the 20 tonnes a year of heavy water it will produce for medical isotopes.
  Government spokesman Mohammad Baqer Nobakht separately announced on Tuesday that the sanctions on Iran would be lifted "in the coming days".
  The precise date when the nuclear deal is implemented and the sanctions are lifted by the UN, US and EU has not been determined and depends on the International Atomic Energy Agency (IAEA) certifying that Iran has fulfilled its commitments.
target: Iran's deputy nuclear chief has denied a report that the core of the Arak heavy-water reactor has been removed and filled with concrete.