Doesn't sound good this morning, does it? God help them.
There are all sorts of things which smell about this.
1.If this was a major Fleet exercise, the underwater area would have been like Picadilly Circus in the rush hour, filled with all sorts of boats sniffing around. Collision quite possible - and probably the loss/damage to the other boat being easy to surpress in the short term. Don't discount the collision theory but the US seems to believe a collision with the bottom - equally possible.
2. Why has the reactor been shut down? If it was still operating, they would have plenty of air etc 'cos they could run the electrolysers, CO2 scrubbers etc. Was the damage such that they either lost the reactor or had to shut it down?
3. Why haven't they tried to escape? A rush escape at their depth (flood up the escape compartment to equalise pressure, open the escape hatch to sea, take a deep breath and breath out all the way to the surface) would be an act of desperation but it has been trialled to 300' and would be better than definitely dying. A tower escape (put the suit on, into the escape tower, flood up the tower to equalise, breathing sub's air throughout, open the hatch and breath normally in the inflated suit to the surface) should be perfectly feasible at their depth.
Perhaps the damage was such that the reactor was put out of action and the escape hatches distorted so much that they couldn't be opened - in which case ikt won't make any difference whose rescue vehicle gets there, it will still take too much time.
Whatever - anybody who has ever served in submarines will shudder at the ultimate nightmare come true.