The submarines are powered by a pressurised water reactor with highly enriched uranium fuel . The miniaturized version of the reactor was designed and built by the Bhabha Atomic Research Centre ( BARC ) at the Indira Gandhi Centre for Atomic Research ( IGCAR ) in Kalpakkam . It included a 42 @-@ metre ( 138 ft ) section of the submarine 's pressure hull containing the shielding tank with water and the reactor , a control room , as well as an auxiliary control room for monitoring safety parameters . The prototype reactor became critical on 11 November 2003 and was declared operational on 22 September 2006 . Successful operation of the prototype for three years enabled the production version of the reactor for Arihant . The reactor subsystems was tested at the Machinery Test Center in Visakhapatnam . Facilities for loading and replacing the fuel cores of the naval reactors in berthed submarines was also established .