BBC posted a graphic;
http://news.bbc.co.uk/2/hi/europe/7718604.stm

1. Fire extinguishers switched on in front of submarine for unknown reasons
2. Affected area may have been sealed off, trapping personnel inside
3. Freon gas released by fire extinguishing system to remove oxygen
4. Twenty people die and 21 are injured - reportedly poisoned
The authorities say the submarine's nuclear reactor is not affected, the injured are sent for treatment and the vessel returns to port