GENEVA, Dec 23 (Reuters)

The secondary circuit of Iran's Arak heavy water nuclear reactor will become operational on Monday, the country's semi-official Mehr news agency reported.

The starting of the secondary circuit will not violate restrictions placed on Iran's nuclear program under a 2015 deal with world powers, but shows Iran is continuing work to develop the reactor amid renewed tensions with the United States.

As part of the deal, the core of the Arak reactor was removed and filled with concrete to make it unusable, but Iran was able to continue to produce heavy water.

"The first circuit is tasked with removing heat from the heart of the reactor, and the secondary circuit is responsible for transferring the heat from the first circuit to cooling towers and finally to the outside environment," Mehr reported.

U.S. President Donald Trump withdrew from the nuclear deal last year and reimposed sanctions on Iran, spiking tensions between the two countries.

Iranian officials have repeatedly said the Islamic Republic will continue to reduce its commitments to the deal until European parties to the pact protect Iran's economy from U.S. penalties, but have said the reduction of commitments can be reversed.

Iran has the capacity to produce up to 25 tonnes of heavy water per year, Ali Asghar Zarean, a special assistant to the chief of Iran's Atomic Energy Organization, was quoted as saying in October by the semi-official Tasnim news agency, noting that the Islamic Republic currently produces 20 tonnes of heavy water annually, which is exported to other countries.

Heavy water can be employed in reactors to produce plutonium, a fuel used in nuclear warheads.

Despite having nuclear technology, Iran has never pursued building or using nuclear weapons, which its religion forbids, the country's highest political authority, Ayatollah Ali Khamenei, said in October.