0806 GMT February 17, 2020
Concrete mixer trucks poured their slurry into the prepared base of the second reactor as journalists watched in Bushehr, some 700 kilometers (440 miles) south from Iran's capital, Tehran. Bushehr's working reactor stood behind it.
Officials say the new reactor, and a third planned to be built, will each add over 1,000 megawatts to Iran's power grid. It is being built with the help of Russia, which helped put Bushehr's first reactor online in 2011.
Bushehr is fueled by uranium produced in Russia, not Iran, and is monitored by the United Nations' International Atomic Energy Agency. However, Iran began 4.5% enrichment in part to supply Bushehr despite the 2015 nuclear deal with world powers limiting it to 3.67%.
The start of construction comes as a US pressure campaign of sanctions that blocks Tehran from selling its crude oil abroad. Those sanctions took effect after US President Donald Trump unilaterally withdrew America from the accord in May 2018, lighting the fuse for tensions in the Middle East.
"It was not us who started breaking commitments, it was them who did not keep to their commitments and cannot accept the nuclear deal as a one-way roadmap," said Ali Akbar Salehi, the head of the Atomic Energy Organization of Iran.
The deal was designed to limit Iran’s enrichment program in exchange for the lifting of international and unilateral US sanctions.
Iran long has maintained its nuclear program is for peaceful purposes,
Salehi, speaking to reporters, praised the plant's operations.
"The security of this power plant has been provided by the armed forces and its safety has been endorsed by international institutions," he said.
Salehi also offered Iran's help to neighboring Arab nations, noting the United Arab Emirates' nuclear power plant in Barakah had seen years of delays.
Iran resumed uranium enrichment at its Fordo plant south of Tehran Thursday in a new step back from its commitments under the nuclear deal.
The resumption of enrichment at Fordo is Iran's fourth step away from the agreement. Iran took the first step in May a year after the US pulled out of deal.
Iran is now enriching uranium to 4.5 percent, exceeding the 3.67 percent limit set by the 2015 deal but less than the 20 percent level it had previously operated.
AP and AFP contributed to this story.