N Korea test - failure or fake?

By Penny Spiller
BBC News

North Korea's claim to have successfully detonated a nuclear bomb on 9 October sent shock waves around the world.

But as scientists pore over the blast's data looking for important clues about the North's nuclear programme, one question remains unanswered - was the explosion in fact a failure, or even a fake?

North Korea's claim has raised tensions in the region

Chinese and South Koreans have so far found no evidence of a nuclear test. US officials say an initial investigation picked up traces of radioactivity near the alleged test site, but cautioned that more tests were still needed to be sure.

Doubts about the test's success stem from the size of the blast which seismic monitors around the world picked up in northern North Korea.

Within a few hours, Russia said it was "100% certain" a nuclear test had been carried out and measured it at between five to 15 kilotons.

But South Korea, France and the US all measured it at less than one kiloton, far smaller than the 12.5 kiloton bomb that was dropped on Hiroshima in WWII.

If it was a smaller explosion - and South Korea estimated it at the equivalent of just 550 tons - it would be theoretically possible, though very difficult, for the North to have detonated that amount of conventional explosives underground.

More likely, scientists say, the test did not go to plan.

Most first-generation nuclear devices are between 10 and 20 kilotons because it is the easiest size to build, says James Acton, of the London-based Verification Research, Training and Information Centre (Vertic).

"Both from a technological as well as a political analysis, you would expect North Korea to build a 10-20 kiloton bomb. The fact that it is smaller than that suggests the test was not very successful," he said.

If this was a nuclear explosion, it would be a case of a failed explosion Michele Alliot-Marie French defence minister

Mr Acton said it was very difficult to tell the difference between a nuclear or a conventional explosion based on an earthquake measurement.

"At the moment, you would have to say the evidence of a nuclear test is inconclusive," he said.

France's Defence Minister Michele Alliot-Marie became the highest ranking government figure to express doubts about the test, speaking to French radio on Wednesday.

"Given its weakness, it is hard to say if it was an explosion with a very large amount of conventional explosive or indeed a nuclear explosion," she said.

"In any case, if this was a nuclear explosion, it would be a case of a failed explosion," she said.

Hunt continues

Given this uncertainty, governments and scientists are urgently working to find some proof, either way - and have been carrying out further analysis of the seismic data as well as testing any radioactive material released into the atmosphere.

South Korea received a sophisticated radioactivity detector from Sweden and US, and Japanese monitoring planes based on the island of Okinawa were also believed to be flying in the region.

By the end of the week South Korean and Chinese scientists said they had detected no evidence of radioactivity in air, soil and rainwater tests.

The US on Saturday said traces of radioactive gas had been found in preliminary air tests around the alleged site, but stressed that further investigation was needed.

Officials have warned that the process could take time, and one French nuclear official warned that Monday's nuclear test may never be confirmed.

Xavier Clement of France's Atomic Energy Commission said the seismic data had to be sifted to differentiate the blast from background noise of subterranean movement.

"It is possible that this cannot be done, given the weakness of the signals compared to the background noise," he told the French news agency AFP.

More tests?

Despite these uncertainties, China - North Korea's closest ally - has not given any indication it doubts Pyongyang's claim about the nuclear test, denouncing it in unusually strong words as "brazen".

And two days after its claimed test, North Korea again reiterated that its scientists had "successfully conducted an underground test under secure conditions", and even threatened more tests if the US did not change its "hostile" policy.

Further North Korean tests could be an interesting indicator of the country's intentions and capabilities, the BBC's defence and security correspondent Rob Watson says.

A one-off test may be a sign it has only limited quantities of weapons-grade plutonium and has some way to go on the design of a nuclear weapon.

A second test would answer those doubts, and clear up the uncertainties still lingering from the first.