Both the strong force (compressed spring) and the electromagnetic force (the spring). What happens is that there is a fragile balance and occasionally a piece of the plutonium nucleus (alpha particle) breaks free from the nuclear force and is expelled by the electromagnetic force.

Just for clarity, there are four universal forces (in order of increasing strength): gravity, nuclear weak force, electromagnetic force, and nuclear strong force. (The weak force is often implicated in the radioactive decay process, but I don't have a current handle on that.)

Both the strong force (compressed spring) and the electromagnetic force (the spring). What happens is that there is a fragile balance and occasionally a piece of the plutonium nucleus (alpha particle) breaks free from the nuclear force and is expelled by the electromagnetic force.