Technically it's just an ordinary explosion. If you look on YouTube or something (I didn't look it up, but I'm pretty sure I'm right) for explosions, the bulk of the explosion moves upward and sort of dissipates in a black cloud of smoke with fire emitting from the base. Nuclear explosions on the other hand do something similar, except the base of the explosion (this time not referring to the surface area) is more narrow, long, and caves inward while the bulk of the explosion is always on the top, forming a mushroom cloud shape. There's often a ring of smoke around the middle of the explosion as well. The surface area of the nuclear explosion is also very wide, as is the top of it.
Technically the explosion shown in Sourc[e]X's button is an ordinary explosion. My guess is it's a grenade or car explosion. Judging by how the fire and smoke seems to sway towards the left and up in the image the way it does with a sort of shape within it, I'd say your best bet be it's a vehicle exploding. Could be a car, truck, or even a tank. IDK. Not enough details to really pinpoint what it is. HOWEVER, when I first saw it, I thought "nuke."