“The Russians used this successfully to stop runaway wells. The idea is to fuse the rock over the well, thus capping it.”
Not sure I’d use the Russians as an example for anything except how to kill or pollute.
Nuclear bombs don’t fuse, they blow up. It won’t melt the rock. It will make lots of little rocks.
Then melt them down into a solid mass.
You might not like to use them as an example, so what. That doesn’t mitigate the facts. They used this method and it worked for them.
We have many years worth of data from underground nuclear testing. You need only 1000 feet of depth for a 1/2 kiloton explosion to be "contained", and they have 12,000 feet of rock between the ocean floor and the oil. I think they have a significant safety margin, but they would probably detonate it 6000' deep to be on the safe side. it's much safer cleaner and cheaper than what's happening now.
The Russian nuke worked because the explosion compressed layers that were plastic (like clay or something) rather than crumbly/brittle like rock. The compression pinched off the well hole. Personally I think they got lucky though.