Evan Ramstad / Wall Street Journal Online – 2013-02-12 00:22:49

http://online.wsj.com/article/SB10001424127887324880504578298943003246654.html

SEOUL (February 12, 2013) — North Korea appeared to have exploded a nuclear device Tuesday, its third experimental detonation in a long effort to build weapons of mass destruction that the US and other countries consider a serious threat.

An “artificial earthquake” was detected at 11:57 a.m., according to authorities in South Korea who attributed the seismic activity to the northeastern area of North Korea where Pyongyang previously detonated nuclear devices in October 2006 and May 2009.

“It appears that North Korea has conducted a nuclear test,” said Kim Min-seok, spokesman for South Korea’s defense ministry.

North Korea made no immediate statement. Its authoritarian regime gave several explicit warnings since Jan. 24 that it was planning to detonate a nuclear explosive.

The White House had no immediate comment. United Nations officials said late Monday that the UN Security Council would meet at 9 a.m. Eastern time Tuesday to discuss the apparent test.

Diplomats and observers in other countries had considered such an explosion likely as Pyongyang appeared to be repeating a pattern of provocative actions seen before the previous explosions.

China’s earthquake monitoring agency on Tuesday said the seismic activity was a “suspected explosion.” China’s Foreign Ministry couldn’t be immediately reached on Tuesday, a holiday in China.

The US Geological Survey said it detected seismic activity with a magnitude of 5.1, bigger than the seismic activity produced in North Korea’s previous two blasts.

Both of North Korea’s previous detonations were considered small by the standards of nuclear testing in other countries.

The first one in 2006 produced a tremor that measured a magnitude 4.1, and analysts in the US later estimated its explosive force at less than one kiloton. The second one in 2009 produced a seismic tremor of 4.7 and an explosive force of about two to six kilotons, according to estimates by US seismic and geological experts.

Estimates in both cases are constrained by limited information about the geology of the mountains where the explosions took place.

By contrast, the first nuclear bomb tested by the US in 1945 had an explosive force of approximately 18 to 20 kilotons.

North Korea had built up expectations for the explosion for months by launching rockets to test missile technology and then undertaking a propaganda campaign, a pattern seen in both 2006 and 2009.

Last year, it twice tried to shoot rockets into space and succeeded on the second attempt in December, in violation of an international ban of technology related to long-range missiles. After the UN Security Council on Jan. 22 penalized North Korea for the launches, Pyongyang responded with threats to detonate a nuclear device.

On Jan. 24, the National Defense Commission, North Korea’s most powerful body, said “a nuclear test of higher level” would be carried out. It portrayed the test as part of an “all-out action” against the US, which it accused of being the driving force behind the UN penalties.

“Settling accounts with the US needs to be done with force, not with words,” the defense commission said.

With new leaders in South Korea, Japan and China, North Korea’s nuclear test also fits another longtime pattern of a undertaking a provocative act early in the tenure of other political leaders, then following it with steps to smooth relations in hopes of persuading the others to provide financial aid and other assistance.

In addition to the size of Tuesday’s blast, the type of fuel used to create the explosion will be an immediate concern to the US, South Korea and other nations.

But because it takes several days for radioactive isotopes to be detectable in the atmosphere, observers in other countries won’t initially know how North Korea made the explosive unless the country reveals it.

North Korea used reprocessed plutonium in its previous nuclear explosives. But the country in 2010 publicly revealed its effort to develop highly enriched uranium, another material that can trigger a nuclear explosion.

North Korea started developing nuclear weapons in the 1970s, but its work proceeded slowly due to its poverty and international constraints and sanctions. From 1994 to 2002, North Korea formally halted the effort under an agreement with the US

But the agreement fell apart after Washington in 2002 determined Pyongyang had started research on a second means of building nuclear weapons, using enriched uranium rather than the reprocessed plutonium that was explicitly covered in the 1994 pact.

North Korea has long said it needs nuclear weapons to prevent South Korea and its US military ally from invading it, though its conventional weapons have provided a deterrent since the Korean War of the 1950s.

Most outside analysts believe North Korea used its nuclear threat as a negotiating tool to force the US and other countries to provide it with money and security guarantees. It has also bolstered the reputation of its munitions industry, which exports missiles, guns and other conventional weapons to countries like Iraq and Syria.

Since its 2006 explosion, North Korea has repeatedly asked the US and other major countries to recognize it as a nuclear-weapons state and negotiate with it as an equal power.

North Korea has reprocessed enough plutonium from a nuclear power plant to build as many as a dozen small nuclear explosives, outside analysts estimate.

In November 2010, North Korea showed US scientist Siegfried Hecker that it was making progress in developing the skill to use uranium as a fuel for nuclear weapons. Officials gave Mr. Hecker a tour of a laboratory that housed thousands of centrifuges used to enrich uranium to a purity level required for use as weapons fuel.

In-Soo Nam contributed to this article.

A version of this article appeared February 12, 2013, on page A8 in the US edition of The Wall Street Journal, with the headline: Pyongyang Nuclear Test Detected.