'According to preliminary information obtained by the special commission [investigating the accident], the reason of the emergency was a short circuit in the power system of the [submarine] that caused a fire in the room for accumulating batteries,' he wrote.
'When the concentration of hydrogen here is over three percent, an explosion followed by a fire occurs.
'This is what happened. The fire developed 'at hurricane speed'. That is why some members of the crew had no time to use special breathing devices.
'The masks were simply melting. And even in these conditions, the badly burned submariners were rescuing each other.'
So that is an interesting explanation. The reason is a short circuit can ignite a small explosion due to hydrogen build-up in the battery well. But there are sensors in a battery well to detect any hydrogen buildup, and a ventilation system to disperse any such build-up. In any case, such an explosion will cause a fire. But not a massive one.
Which makes me think they were using lithium-ion batteries, which might be subject to "thermal runaway". In that case there could indeed by a rapid fire. The USA had one of those on a special forces mini-sub. No injuries, but they had to seal it off and let it burn itself out. Now if the Russians had a lithium battery fire, there very well could have been an explosion and a thermal runaway. However, for such a fire they should have sealed off the compartment and surfaced immediately. There is no way to fight such a fire. Which is why (except for mini-subs) the US Navy continues to use lead-acid batteries.
If the problem occurred the way the Russians claim, crew training is then to blame for the high loss of life. Because the solution for a lithium-ion fire should have been to evacuate and seal the compartment containing the battery well, surface, and then partially flood the compartment with sea water to cool it, while venting the toxic fumes into the atmosphere via the ventilation system.
This is what the submarine probably looks like. The battery well would be in one of the pressure hull spheres. See diagrams.
Trying to do normal firefighting procedures with a lithium-ion fire is impossible. Which is why a properly trained crew would not do so, and instead do it the way I just described above. But since this was a spy sub, most of those on board would not have been properly trained submariners. And so they suffered the consequences.