First,

You should have a look at the batteries of Your calculator.

Pressuse down there is 1000 bars!

Your Question:

both, of course! 1st there will be a sphere of plasma,

then with expansion this will cool down, and then it will collapse like

a cavitation bubble, some hot water will drift along.

But:

This will happen only if the bomb explodes at all!

Any atomic bomb has a conventional explosive load, to compress

the fissible materia. If this is to work, the entire thing had to be

enclosed in a pressure- tight capsule (like the sphere of the Bathyscaph)

Conventional explosives refuse to explode at pressures much higher than

some dozen bars.

Georg