[Suicide Junkie]

Now, you realize that the squirrel isn't going to explode like that unless it tries to mate with an anti-squirrel.

Then you turn 2Kg of squirrel + almost 0 energy
into
0Kg of mass + 1.8 * 10^17 J of energy

Velocity has nothing to do with it. Nuclear reactions have nothing to do with it either.

The simple fact is that for every Joule of energy that is released, the mass of the system goes down by:
1/C^2

What the 50 megaton thing indicates, is that if you weighed your nuke, set it off, collected all of the remains, you would find that it weighs less than before, and that it weighs about one squirrel-mass less.

The sun loses a couple of kilograms each day, from all the energy it spews into space.