Question

Given two mass (any element, of any mass). Will there be a nuclear explosion or will they totally convert into energy if they are collided with significant amount of speed in empty space?
Or if not that, will the individual mass explode and convert into energy itself since it's fundamental particles are also colliding with each other at high speed inside the mass as the mass is going at super high speed (any limit we may apply hypothetically.)

Answer 1

The answer would be the first case, in which the masses will collide and cause explosion. Since the masses are rigid bodies, and the whole mass is moving at high speed, it also implies that all the particles inside the mass are also moving at the same speed, so there will be no collision between the particles inside the mass.

So, the masses will collide together, making the particles of each other smash leading to an explosion. (It won't be nuclear explosion)

Nuclear explosion happens in a different way, it happens through fission in atoms. ie. unstable nuclei breaks down to release tremendous amounts of energy.