Rupert Cornwell: Big questions remain over little explosion

A million words, mostly angry, have been generated by North Korea's claimed nuclear test on Monday. But most remarkable are the words that have not been spoken. The United States government, with the most sophisticated nuclear technology on earth at its disposal, has still not formally confirmed that a nuclear test took place at all.

In fact officials here do not seriously doubt that a nuclear detonation took place at the site in the remote north-east of the country. But they are astonished at the small size of the blast - less than one kiloton, or 1,000 tons of conventional explosive equivalent. The yield is tiny, a fraction of the estimated 15 kiloton yield of the blast at Hiroshima in 1945, and far less than customary for the debut test of a nuclear power.

So what exactly happened? Two theories are being advanced in Washington. The first, and less widely credited, is that the North Koreans deliberately chose a very small device that would prove that they had become the world's ninth nuclear power, but which sent the message that Pyongyang would use its new technology with the responsibility, and with the utmost concern for the environment. Experts here noted the emphasis in the statement announcing the explosion that no radioactive leakage had occurred from the underground test.

The more widespread belief is that the test may have been a partial failure. The New York Times and The Washington Post quoted a US government official as saying that China - the North's most important ally and neighbour - had been given prior warning that the explosion's yield would be four kilotons. In the event, France and South Korea estimated a yield of below a kiloton, while the American official said it could have been as low as 200 tons of conventional explosive - so small in fact that the precise nature of the blast could not be conclusively established.

A further indication that the test had not lived up to expectations came yesterday from a South Korean newspaper, which reported a Pyongyang diplomat as saying that it had been smaller than anticipated. The test had been "a success" but "smaller in scale than expected", said the diplomat, according to The Hankyoreh newspaper. But he warned: "Success in a small-scale test means that a larger-scale test is also possible." Even so, Monday's outcome has come as something of a comfort here. In essence, North Korea has suffered two weapons setbacks in the space of three months - first the failure in July of a test of the multi-stage Taepodong-2 missile, reputedly with the capability of hitting Alaska or the US West Coast. Less than a minute after launch on 5 July, the rocket crashed into the sea some 200 miles offshore from Japan.

That failure, coupled with the small scale of the test, suggests the direct North Korean nuclear threat may be less than previously believed. Either only part of the plutonium core of the device exploded, or less was used because Pyongyang has less plutonium available. Either way, a North Korean nuclear missile is not the immediate proposition that had been feared.

Independent American experts said the North has produced up to 50kg of separated plutonium. Depending upon the state's technical capability and the desired yield of the bomb, the respected Bulletin of the Atomic Scientists wrote before Monday's claimed blast that Pyongyang could have as few as five weapons or as many as 15. It said: "Ten weapons seems to be a reasonable estimate, with the addition of about one weapon per year."