Underwater Explosion - The Dynamics of Very Deep Underwater Nuclear Explosions

The Dynamics of Very Deep Underwater Nuclear Explosions

Unless it breaks the water surface, a very deep underwater explosion (more than about 2,000 ft (610 m)) leaves no trace on the surface but hot, radioactive water rising from below. During such an explosion, the gas bubble over-expands, and then collapses due to the extreme water pressure, and because the amount of water pushed outward increases with the cube of the radius:

Since water is not compressible, moving this much of it out of the way so quickly absorbs a massive amount of energy—all of which comes from the pressure inside the expanding bubble. Eventually, the water pressure outside the bubble causes it to collapse back into a small sphere and then rebound, expanding again. This is repeated several times, but each rebound contains only about 40% of the energy of the previous cycle. At its maximum diameter (during the first oscillation), a very large nuclear bomb exploded in very deep water creates a bubble about a half-mile wide in about one second, and then contracts (which also takes one second).

Blast bubbles from deep nuclear explosions become mere hot water in about six seconds and leave no "regular" bubbles to float up to the surface. This is sooner than blast bubbles from conventional explosives:

This drastic loss of energy between cycles is caused, in part, by the extreme force of a nuclear explosion pushing the bubble wall outward supersonically (faster than the speed of sound in saltwater). This causes Rayleigh–Taylor instability. That is, the smooth inner wall surface becomes turbulent and fractal, with fingers and branches of cold ocean water extending into the bubble. That cold water spray cools the hot gas inside and causes it to condense (and heat the water outside the blast bubble). The bubble becomes less of a sphere and looks more like the Crab Nebula, who's deviation from a smooth surface is also due to Rayleigh–Taylor instability.

As one might expect, large, shallow explosions expand faster than deep, small ones:

Deep explosions have longer oscillations:

The water pressure just outside the bubble varies dramatically:

Note that despite being in direct contact with a nuclear explosion, the water of the expanding bubble wall does not boil. This is because the pressure inside the bubble exceeds (by far) the vapor pressure of ocean water. The water touching the blast can only boil during contraction. This boiling is like evaporation, cooling the bubble wall, and it is another reason that an oscillating blast bubble contains only 40% of the energy it had in the previous cycle.

During these hot gas oscillations, the bubble continually rises ("migrates") for the same reason a mushroom cloud rises: it is less dense. This causes the blast bubble never to be perfectly spherical. Instead, the bottom of the bubble is flatter, and during contraction, it even tends to "reach up" toward the blast center. In the last contraction cycle, the bottom of the bubble touches the top before the sides have fully collapsed, and the bubble becomes a torus in its last second of life. After that, all that remains of a large nuclear explosion is a mass of hot water, slowly rising from the cold depths of the ocean.