The removal of the core of the nearly completed Arak reactor is a key step before crippling international sanctions on Iran are lifted.
The work must still be verified by outside experts.
Under the deal reached last summer, the heavy-water reactor is to be re-engineered so that it produces only minute amounts of plutonium, like enriched uranium a potential pathway to nuclear arms.
That involves exchanging the core and other major modifications.
"About an hour ago, our job was finished," he said.
State TV reported that the holes left after the core was removed have been filled with concrete.
International inspectors will now verify the job and will send their report to the International Atomic Energy Agency, Kamalvandi said.
The core itself would be kept as a symbol of Iran's nuclear achievements, he added.
Under the deal, reached last July, Iran was required to ship out most of its stockpile of enriched uranium, a material that can be used to make bombs, and take apart thousands of the centrifuges that enrich the material.
Most of the nuclear restrictions last 10 or 15 years.
While Iran accomplished all of the biggest tasks in recent weeks, several technical issues remained.
Officials had spoken of lingering questions related to Iran's uranium and plutonium programs, including over the properties of certain centrifuges.
Iran would be permitted to maintain for research purposes.
Iran has insisted it needs the Arak heavy water reactor for production of medical isotopes. It denies it had any intention to build nuclear weapons.
Iran's Fars news agency, which is close to the country's hard-liners, on Monday reported that technicians had already dismantled the core of the Arak reactor and filled it with concrete.
Iranian hard-liners are opposed to the nuclear deal.
They argue that the so-called "disabling" of Arak is a slap on Iran's face and see it as evidence that moderate President Hassan Rouhani has made too many concessions to the West.
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