nuclear-news

This method also ensures a degree of secrecy.

The release of radiation from an underground nuclear explosion – an effect known as “venting” – would give away clues to the technical composition and size of a country’s device, and therefore its nuclear capability.

The test site is carefully geologically surveyed to ensure suitability. Such tests usually take place well away from population centres.

The nuclear device is placed into a drilled hole or tunnel usually between 200-800m (650-2,600ft) below the surface, and several metres wide.

A lead-lined canister containing monitoring equipment is lowered into the shaft above the chamber. The hole is then plugged with gravel, sand, gypsum and other fine materials to contain the explosion and fallout underground.

The device is remotely detonated from a surface control bunker. The nuclear explosion vaporises subterranean rock, creating an underground chamber filled with superheated radioactive gas.

As this cools, a pool of molten rock collects at the bottom of the chamber.

Minutes or hours after the blast, as the pressure falls, the chamber collapses in on itself causing subsidence and a crater to appear on the surface.  North Korea’s test……http://www.bbc.com/news/world-asia-35244474