The Russians have a very long history of operating submarines, having started in 1905. They have also long had problems operating submarines. The US has lost two nuclear subs, both back in the 1960s. I’ve lost count of how many the Russians have lost. The most recent loss was the Kursk, which took 118 sailors with it.
Now comes news that the Russians have had another accident on board a sub. They didn’t lose the boat, but they did lose 20 people. Apparently the fire suppression system discharged unintentionally. When this happens, the freon displaces the oxygen in a compartment. Without oxygen, the fire can’t burn. But people in the compartment can’t breath, either. On US subs, this hazard is mitigated by having air masks and hookups to breathable air. I don’t know how the Russian navy approaches the problem, but they have historically been far more willing to suffer the loss of sailors to accident then our own.