Last month, India launched its first nuclear submarine, the INS Arihant (Destroyer of Enemies). This came after over a decade of planning and construction. But now it's been revealed that the Arihant was launched without its nuclear reactor, which will not be ready for another year, or so. No one is saying for sure when the reactor will be ready. At that point, Arihant will have to go back into dry dock, be torn open, and have the reactor installed. Or parts of it, or just the fuel cores. It's unclear exactly what state the Arihants reactor is in. What is known is that the Arihant cannot move under its own power (as it has none), and apparently is not equipped with weapons. Other major systems may also be missing. So why was the Arihant launched at all? Probably because work on the sub had been going on for more than a decade, and it was becoming embarrassing to have nothing to show for all that effort.
Once the Arihant has a working reactor and weapons systems, it will undergo up to two years of testing and sea trials before being accepted for service. The Arihant is based on the Russian Charlie II sub, which it resembles. A leased Russian Akula II nuclear sub will basically serve as a training boat for India's new nuclear submarine force. Russia retired all its Charlie class subs in the early 1990s. India leased one from 1988-91, and gained a great deal of familiarity with it. The Charlie class had eight launch tubes, outside the pressure hull, for anti-ship missiles. The Arihant has vertical launch tubes, apparently large enough for the cruise missiles, but not any SLBM (Sea Launched Ballistic Missile) India has (like the Sagarika, which is too long to fit in a vertical silo on the Arihant.) Actually, the exact purpose of vertical launch tubes on the Arihant is unclear. The navy revealed very little detail on the new sub (which, until two years ago, the government refused to say anything about.) Access by photographers was restricted.
The new Indian SSN was long referred to as the ATV (Advanced Technology Vessel) class. There are apparently to be five boats in the class, assuming that the first one works well. The ATV is a 5,000 ton boat, and comparisons have also been made to the new Chinese 093 (Shang) class, which is a 6,000 ton boat that entered service two years ago, after more than a decade of construction. That was China's second class of SSNs. The first, the Han class, was a disaster.
India is trying to learn from Chinas mistakes. That's one reason the ATV project has been kept so secret. Another reason for the secrecy was that so much of the ATV project involved developing a compact, light water reactor technology that would fit in a submarine. This 85 MW reactor makes the Arihant underpowered by the standards of other SSNs, and the Indians give the Arihant's top speed as 55 kilometers an hour.
Once the Arihant class SSN is proven, a modified version can be built as a SSBN (ballistic missile carrying sub). This was how everyone else did it, including the Chinese. Get an SSN operational, then modify the design to include some SLBM launch tubes. But the Arihant already appears to have vertical launch tubes, but not of sufficient diameter and length to hold any of the SLBM (sea launched ballistic missiles) in service.