The secondary circuit is responsible for transferring the heat from the first circuit to cooling towers and finally to the outside environment.

The launching of the secondary circuit does not violate the 2015 nuclear deal, known as the JCPOA.

As part of the deal, China and the UK are currently cooperating with Iran in the redesigning and modernization of the Arak Reactor.

The new design will be fueled by low enriched Uranium, which renders less than 1 kg of plutonium each year; 1 tenth of the amount reached at the old reactor.

The ultimate product of the reactor is a range of radio-isotopes, which would spare Iran the need to spend hundreds of thousands of dollars on importing them for use in medical and industrial areas.

While Iran is closely cooperating with the remaining signatories to the nuclear deal in technical areas, it is complaining about the European parties' inaction when it comes to the economic side of the deal and the removal of the US-led sanctions against Iran.

The country has so far taken four retaliatory steps to reduce its obligations to the JCPOA in response to the US's unilateral withdrawal from the deal last year and the EU's dilly dallying in keeping its side of the bargain.

The steps included an increase in the grade and supply of enriched Uranium.

Iran says the project of Arak Heavy Water Reactor is an exception with respect to the steps it is taking to reduce its JCPOA commitments. Meanwhile, it has not ruled out a possible fifth step due in two weeks if the EU continues to shrug off its obligations under the nuclear deal.