North Korea announced May 25 that it has successfully conducted its second nuclear test, in defiance of international warnings. Geological authorities in the US, Japan and South Korea reported that the test triggered an earthquake with a magnitude of between 4.5 and 5.3. The tremor emanated from Kilju, the same area where North Korea carried out a test in October 2006. North Korea said that test was a success, but the US and South Korea said the bomb did not detonate fully.

In Seoul, Defense Minister Lee Sang-Hee told a parliamentary hearing that the blast this time appeared to be bigger than the 2006 test. He said a 4.4 magnitude tremor may mean a nuclear detonation of anywhere between one and 20 kilotons. (NYT, May 25)

Twenty kilotons is approximately the force of the bomb the US dropped on Nagasaki, Aug. 9, 1945—as opposed to some 15 kilotons at Hiroshima Aug. 6. A kiloton is a unit of explosive force equal to that of one thousand metric tons of TNT. Contemporary US warheads are measured in megatons: the equivalent of one million metric tons of TNT. (US Energy Department pages)

The largest US explosion was the 15-megaton "Bravo" test at Bikini Atoll, March 1, 1954. (Brookings Institution) The USSR carried out the largest test ever—the 50-megaton "Tsar Bomba" at Mityushikha Bay test range, on Novaya Zemlya Island in the Arctic Sea, Oct. 30, 1961. Today most US and Russian nuclear weapons have yields of around 1 megaton. (FAS Nuclear Weapon Archive, CUNY Nuclear Weapons page)

In 2007, a report prepared by former Defense Department strategist Anthony Cordesman for the Center for Strategic and International Studies (CSIS) claimed Israel is "in possession of 200 nuclear warheads"—including many in the one-megaton range. (Press TV, Dec. 24, 2007)

Israel, like India and Pakistan, is not a signatory to the Non-Proliferation Treaty (NPT). North Korea, which joined the NPT in 1985, formally withdrew in January 2003. Earlier this month, a US official made a rare statement calling on Israel to join the NPT. "Universal adherence to the NPT itself, including by India, Israel, Pakistan and North Korea...remains a fundamental objective of the United States," Assistant Secretary of State Rose Gottemoeller said, speaking at a UN meeting on the treaty. She later declined to say whether Washington planned to pressure Israel to take any new measures to abandon its nuclear program or sign on to the NPT. The comment sparked a firestorm of protest in Israel, but failed to make international headlines. (Reuters, May 25; Jerusalem Post, May 6)

See our last posts on North Korea, the Korean peninsula and nuclear fear.

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