What INS Arihant may look like, with its reactor reportedly going critical in August 2013, four years after Arihant was launched (in 2009).
-India is very slowly building a nuclear (propelled) submarine capability as one aspect of its great power aspirations and as a counter to China's more developed nuclear submarine program. Many in Pakistan who follow naval matters believe Pakistan should also build nuclear submarines. However the cost of such an endeavour and low strategic utility of such submarines for Pakistan are two arguments against building nuclear submarines.
Even nuclear submarines in the Arabian Sea around Pakistan's landmass are highly vulnerable to air power, MRBMs and conventional submarines. Hence Pakistan's geography provides little advantage for nuclear submarines. Pakistan's AIP capability for two of its Agosta subs is highly efficient for anti-shipping, anti-submarine and missile land attack missions against India and other neighbouring states.
Meanwhile the reactor of India's locally built nuclear propelled submarine INS Arihant has reportedly gone critical - meaning it started working - today.
See http://indiatoday.intoday.in/story/arihant-reactor-goes-critical-submarine-to-start-sea-trials/1/298902.html . It will be several years until Arihant goes into service ie. becomes operational.
A news item on Arihant's criticality.
India's other nuclear submarine, INS Chakra ("II") - built by the Russians.
Here are some details on Arihant including the all important weapons fit http://gentleseas.blogspot.com.au/2009/08/arihant-indias-first-homebuilt-nuclear.html .