Given the apparent lack of such a hatch on Arihant (reports India's Economic Times, Jan 12, 2018) and likelihood of sensors to avoid major accidents the official "hatch left open" explanation seems suspicious. Maybe instead, sabotage by a disgruntled or bribed crewman or technician? Even Chinese or Pakistani agent involvement?
India's Visakhapatnam east coast naval base. Naval vessels can be seen halfway up the harbour. This is to the right of what looks like the submarine repair and SLBM loading shed that INS Arihant will need to use, or is already under repair at.
Repairing, or if need be, replacing Arihant's reactor is likely to be a Billion dollar exercise. Involving Arihant being placed in drydock, moved to Visakhapatnam repair shed then being vertically cut open, basically in half.
The Arihant accident not only involves major repair costs but delays India's SSBN and broader indigenous nuclear submarine technical modification/learning program. It also delays training of the officers and crew who will transition to a full size Aridhaman SSBN around 2020(?).
Compounding the problems is subsequent damage to India's other nuclear submarine, INS Chakra, in early October 2017. Russian built Chakra (ex Nerpa) has been 10 year leased to India. Chakra is used by India as a training platform and technology transfer (including its nuclear reactor) asset.
In fact these negative impacts (repair cost, technical improvements and training program) of the Arihant accident may have delayed Aridhaman's launch by a year or two.