CIRUS is a 40 MW-thermal heavy-water-moderated, light-water-cooled, natural-uranium fueled reactor of the same design as the Canadian NRX reactor. Canada provided financial assistance for the construction of the reactor; heavy water was supplied by the United States. The reactor first became critical in July 1960 and fully operational - in 1963. In 1997-2003 it underwent an extended refurbishment. The reactor was used to produce plutonium for the first India's nuclear test in 1974. IPFM estimates that CIRUS produced from 165 to 270 kg of plutonium (this includes plutonium in the fuel still to be reprocessed).