Well, 2 explosions would do it, alright. Nobody's submarines these days can survive the flooding of even one compartment without sinking and from what the Russians are saying, the hull was breached from right foward back to the conning tower. For us, that would mean 2 compartments, including the torpedo compartment which contains the forward escape tower.
I'm afraid it also means that most of the crew would have been killed immediately since I imagine (I hope) all the accommodation areas are forward of the reactor compartment and the only people who would have been aft of the reactor compartment at the time of the accident would have been a small watch of engineers. If it's only a small number of people aft, then there should be enough air for some considerable time.
So far as radiation is concerned, it would seem that the reactor compartment hasn't been breached so far and, so long as that remains the case, any radiation is contained. After all, nobody has yet figured out an acceptable way to decommission nuclear submarines. Every one that has ever been built and is no longer active is alongside somehere (or up a creek) with its reactor permanently shut down and someone keeping some sort of an eye on it. When there is eventually a breach on the Kursk (rust or the collapse of an already weakened reactor compartment bulkhead), I'm no expert but I believe the environmental damage should be relatively small and confined to the local area. Certainly no explosions and fissile material scattered for miles. Sadly, there are already other nuclear boats, both US and Russian, which have gone down without causing environmental disasters.
Dreadful.