The nuclear explosion was used as a last resort after a gas-well blaze lasted for three years. The incredible heat was a main deterrent in bringing the flame under control. The nuclear bomb was put down a drill shaft below many gas impermeable strata. The shock wave of the explosion liquified the rock around it and sealed it off. Nuclear radition was not detected on the surface.
This is not a viable alternative because of the complexity of getting the nuclear device that deep in the ocean and the ecological risks associated with it.