2006 North Korean Nuclear Test - Yield Estimates and Authenticity

Yield Estimates and Authenticity

The low yield of the test initially raised questions as to whether it was a nuclear explosion but detection of airborne radioactive isotopes by a United States military aircraft confirmed that it was a nuclear explosion. The advance warning of the test sent to the Chinese government reportedly said that the planned test yield was to be equivalent to approximately four kilotons in strength, but most outside estimates, based largely on seismic readings, put the yield at much less.

At a meeting with President Putin, Russian Defense Minister Sergei Ivanov stated that "the power of the tests carried out was 5 to 15 kilotons", though this early estimate is much higher than any other international estimate. An early report by the Institute of Geoscience and Mineral Resources (South Korea) said the blast was equivalent to an earthquake registering 3.58 on the Richter scale, which corresponds to the explosion of 100 tons of TNT. This was later revised to at least 800 tons, corresponding to a blast wave of 4.2. The U.S. Geological Survey also estimates the blast wave at 4.2. (Note that 4.2 is considerably more powerful than 3.58 because the Richter scale is a logarithmic scale.)

According to Jane's Defence Weekly, "initial and unconfirmed South Korean reports indicate that the test was a fission device with a yield of 0.55 kT ... The figure of 0.55 kT, however, seems too low given the 4.2 register on the Richter scale. This could suggest – depending upon the geological make-up of the test site – a yield of 2–12 kT."

An official in France's Atomic Energy Commission reported that they estimated the blast was "about or less than a kiloton" and expressed uncertainty about whether or not the blast was actually nuclear. There have been various large planned and unplanned non-nuclear explosions comparable in yield to small nuclear detonations, such as the U.S. "Minor Scale" explosion from 1985, which used conventional explosives to simulate a 4 kiloton detonation. According to the Washington Times anonymous U.S. intelligence sources speculated there "was a seismic event that registered about 4 on the Richter scale, but it still isn't clear if it was a nuclear test. You can get that kind of seismic reading from high explosives." The Wall Street Journal explains that this blast was equivalent to the explosive force of about US$100,000 worth of ammonium nitrate. International experts have said that it will take some time to confirm whether it was a successful nuclear test, as North Korea claimed, or an unsuccessful one ("fizzle"), or perhaps not even a nuclear test at all.

By comparison, the first plutonium core nuclear device tested by the United States (Trinity test) had a yield of 20 kilotons of TNT, and the first nuclear device detonated by India in 1974, though of primitive design, had a yield in the region of 8 kilotons of TNT. If the North Korean nuclear test is less than even a kiloton in yield, it would be a historically small inaugural nuclear test. Even if it were as many as the reported intentional yield of 4 kt it would be the smallest nuclear test ever conducted by a state as a first test. Some advanced nuclear powers have produced very small tactical nuclear weapons in the low-kiloton range, but their development is far more technologically challenging than that of weapons in the 15–20 kiloton range, requiring advanced weapons materials and core geometries.

If the North Korean device was significantly short of its predicted yield, it could be classified as a "fizzle" indicating that some aspect of the nuclear weapon design or material production did not function correctly. In a fizzle the warhead blows itself apart too fast for the nuclear reactions to generate a large amount of energy, or fails to form a supercritical mass for some other reason. A fizzle can result from predetonation, insufficient precision in the explosive lenses used to compress the plutonium core, or impurities in the plutonium itself, among other factors. A fizzle can also result from the use of reactor grade plutonium rather than weapons-grade material.

On October 13, 2006, CNN reported that two U.S. government officials with access to classified information stated that the initial air sampling over North Korea shows no indication of radioactive debris from the event that North Korea says was an underground nuclear test. Some hours later, the report was reversed and stated there was evidence of radiation, though not enough data has been collected yet to be conclusive. The newspaper Hankyoreh reported an unnamed North Korean diplomat had acknowledged that the actual yield was smaller than expected.

On October 16, 2006, the United States government reported that a test had found radioactive gas compatible with a nuclear explosion.

The office of John Negroponte, the US National Intelligence Director confirmed that the size of the explosion was less than 1 kiloton.

The explosion was also recorded worldwide by the global monitoring system operated by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). Two weeks after the detonation, a CTBTO radionuclide monitoring station in northern Canada detected traces of the radioactive noble gas xenon in the air. Backtracking calculations by analysts at the CTBTO indicated that the xenon particles originated from North Korea and that the explosion had been nuclear in nature.