 The submarines are powered by a <unk> water reactor with highly enriched uranium fuel . The <unk> version of the reactor was designed and built by the <unk> Atomic Research Centre ( <unk> ) at the Indira Gandhi Centre for Atomic Research ( <unk> ) in <unk> . 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 <unk> were tested at the Machinery Test Center in <unk> . Facilities for loading and replacing the fuel cores of the naval reactors in berthed submarines were also established . 
