A view of the Arak heavy-water project
A view of the Arak heavy-water project Reuters

The U.S. State Department confirmed Thursday that concrete has been poured into the core of Iran's Arak heavy water nuclear reactor, a move critical to the implementation of the nuclear agreement between Iran and world powers.

Quoted by Reuters, State Department spokesman John Kirby confirmed the concrete has been poured into the reactor's central vessel.

On Wednesday, Secretary of State John Kerry said the core of the reactor had been removed and was to be filled with concrete and destroyed.

In addition to removing the core of the Arak reactor, Iran has also sent a major shipment of low-enriched uranium materials to Russia as part of its obligations under the deal, a fact which Kerry recently hailed as “significant progress”.

The status of the Arak reactor was one of the sticking points in nuclear negotiations between Iran and the six world powers.

Satellite images from several years ago showed that the Arak plant was operational, raising fears that it is trying to produce plutonium for a nuclear bomb. Iran invited inspectors to visit the plant a year later, shortly after an interim agreement was signed with world powers.

Iran’s recent moves mean that “Implementation Day”, or the day when the United States lifts its sanctions against the Islamic Republic, could be around the corner.

This comes despite a December 2 report from the International Atomic Energy Agency (IAEA) which concluded that Iran made a "coordinated" effort to develop nuclear weapons in the past, although the efforts apparently ended at an early stage.

The UN watchdog also recently released a report which determined that Iran had violated the terms of its nuclear deal with the West by increasing its stockpile of low-enriched uranium by 460.2 kilograms.

Earlier it was also revealed that Iran had stopped dismantling its centrifuges at the Natanz and Fordow uranium enrichment plants, breaching the nuclear deal that calls for the dismantling.