Page last updated at 15:31 GMT, Tuesday, 26 May 2009 16:31 UK

Factfile: Underground nuclear testing

Nuclear devices are often tested underground to prevent radioactive material released in the explosion reaching the surface and contaminating the environment.

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.


1. Preparation: a hole is drilled, the device set and then the tunnel plugged.
2. Detonation: the device is detonated from the control room.
3. Aftermath: the gas cools and the chamber collapses producing a crater.
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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 test

Seismic shockwaves
Seismic shockwaves from the latest test were recorded in South Korea

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 later, as pressure falls, the chamber collapses in on itself causing subsidence and a crater to appear on the surface.

North Korea

North Korea is believed to have tested nuclear devices in 2006 and 2009 at a site called P'unggye-yok in a remote area in the east of the country, near Kilju.

Satellite images appear to show a number of buildings and earthworks in keeping with other nuclear test sites.

North Korea test site

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