Here are some facts about underground nuclear tests and the international treaties related to them:
HOW DO UNDERGROUND TESTS WORK?
The most common method is to place a test device at the bottom of a vertically drilled hole. Another technique is to place a test device in a horizontal tunnel that leads to a location that is deep enough to contain the blast.
A diagnostic canister is placed in the shaft above the device, it contains instruments to collect data from the blast.
The shaft above the canister is plugged with sand, tar, gravel and epoxy to prevent radioactive materials from escaping.
The different components are lowered into the shaft through an assembly tower which sits at the top.
The shaft is between 200 and 800 metres deep.
THE MOVE FROM ATMOSPHERIC TO UNDERGROUND TESTS
Most nuclear tests moved underground from the early 1960s.
Atmospheric tests were banned by the 1963 Partial Test Ban Treaty (PTBT). Concern over large amounts of the cancer-causing radioactive isotope Strontium-90 being produced during atmospheric nuclear weapons tests in the 1950s and 1960s and dispersed worldwide helped drive the change.
The US carried out its last atmospheric test in 1963. France and China moved their tests underground by 1975-76.
WHAT PROHIBITIONS COVER TESTS?
1963: The Partial Test Ban Treaty (PTBT): Bans nuclear tests in the atmosphere, underwater and in space.
The Soviet Union, the United States and the United Kingdom signed in 1963, the year after the Cuban missile crisis.
1974: The Threshold Ban Treaty: Prohibits underground tests with a yield above 150 kilotons.
Signed in July 1974 by the United States and Russia, the treaty came into force on December 11, 1990.
1996: The Comprehensive Test Ban Treaty (CTBT): Bans all nuclear explosions in all environments, and replaces the PTBT.
France and the UK ratify in 1998, but other signatories, the United States, China, and Israel have not. India, Pakistan, and the Democratic People's Republic of Korea have not signed.
The treaty will not take force until all 44 countries with nuclear power plants sign.
Sources: The Environmental Protection Agency, www.epa.gov, The Comprehensive Test Ban Treaty Organization Preparatory Commission, www.ctbto.org, Global Security, www.atomicarchive.com
(China Daily 10/10/2006 page7)